LowPowerLab Forum

Hardware support => RF - Range - Antennas - RFM69 library => Topic started by: joelucid on May 23, 2016, 03:52:58 PM

Title: Small loop antennas @ 433 Mhz
Post by: joelucid on May 23, 2016, 03:52:58 PM
As you know from this thread (https://lowpowerlab.com/forum/index.php/topic,1898.0.html) I've been toying with small antennas and in particular the small loop antenna. I think it holds a lot of promise for good efficiency at very small form factors.

Well now - after purchasing a small vector network analyzer - I have had the first successes today. The end result is ok, not great yet. But I think there is still a lot of room for improvement.

I went for an inductively coupled antenna, where a small non-resonant loop drives a larger resonant one:

(http://i.imgur.com/OgNehnq.jpg)
(http://i.imgur.com/9BGWJTL.jpg)
(http://i.imgur.com/IRcvceM.jpg)

This approach allows fixing the resistive impedance mismatch between the 50 Ohm radio module and the very low resistance resonant loop.

For the resonant loop I used 10mm copper tape. I left a gap which I first covered with adhesive tesa tape. I then covered the tesa film with a small piece of copper tape, effectively creating a capacitor:

(http://i.imgur.com/StWVcWG.jpg)

It takes a lot of cutting and measuring to get the resonance of the resulting LC circuit just right onto 433 Mhz.

It's very possible to reach a return loss of -16dB here, which is very respectable. However getting the frequency right is not easy. The antenna bandwidth is just very small:

(http://i.imgur.com/Jun9zqD.jpg)

Lots of things change a bit when you disconnect the analyzer, so I think I need to do some further tuning based on RSSI values. Still - I already get reception all throughout the apartment with this setup. The RSSI is about maybe 15 lower than what I'd see with a Moteino with wire antenna. I suspect we can do much better than this.

Also this antenna still has a diameter of 4cm and I think one could go down a bit there to 3-3.5cm. However one problem is that the 2cm coin cell does block the magnetic field in a portion of the antenna. So going smaller loses gain in this setup even faster than with small loops by themselves (which already is with A^2). Possible solutions are going with the cr1632, which leaves more space for the loop - potentially even mounting the board perpendicular to the loop axis.

Still wanted to share this as a promising direction.

Joe 
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 23, 2016, 08:02:45 PM
Very impressed with the direction you're heading. You mentioned that loop antenna's are very narrow banded and I'm wondering how much of the design can be easily reproduced by others. The antenna's performance may also be very much influenced by what's inside (and metallic objects just outside) the loop.

Comparing your design with common HF designs I noticed that your inner loop has a relatively large circumference. Also, the tuning capacitor would have to be tiiiiny and may already be detuned by changes in temperature or humidity.

One big benefit to the loops is that they are much better on signal to noise ratio, so a design well worth pursuing..

I look forward to further posts as I've been toying around with making dipoles on pcb's just so that they're reproducable and on a rigid material. Oh, well.. back to 4NEC2.. :-)
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 24, 2016, 03:07:12 AM
Quote
You mentioned that loop antenna's are very narrow banded and I'm wondering how much of the design can be easily reproduced by others.

I agree. Kind of hard to tune it without VNA. On the other hand once I have a working design I could see others build it based on exact specifications. Maybe I would have to laser-cut the cap tape so it is just right.

BTW, I measured a monopole on the same Tiny board yesterday, too. And it behaves very similarly when you make it very short. It's a well know fact of antenna design that small efficient antennas are very narrow band. To really make them work well they need be tuned.

Loops are less susceptible to the environment since they are magnetic antennas. Which is one of the reasons why I think you'd rather want a tuned loop than a tuned stub.

Quote
Comparing your design with common HF designs I noticed that your inner loop has a relatively large circumference.

Yeah, typically one uses 1/5th of the greater loop size. Now whether I have to adjust for the object inside the loop - I'm not clear. I think probably not though. I do think that having the mote inside the loop will affect efficiency significantly. The changing magnetic field will induce eddy currents in the ground plane and the coin cell. Which is why I would prefer to mount the pcb vertically - but even with cr1632 that's 1.6 cm of height, yuck.

A big question is whether it's better to have a smaller loop with pcb outside or the larger one as I've done it now. The upside of the current design is that you use every square mm of area to maximize loop size. But eddy currents are going to be larger inside because of the larger field strength. It will require more testing.

Quote
Also, the tuning capacitor would have to be tiiiiny and may already be detuned by changes in temperature or humidity.


Luckily plastic film capacitors are among the most temperature stable, so hopefully at least on that front we should be good.

Being fixed on one frequency brings a lot of disadvantages obviously: No frequency hopping, no switching bands for listen mode or OTA etc. There are now some low loss digitally tunable capacitors (http://www.psemi.com/products/digitally-tunable-capacitors-dtc for example). But they are super expensive and still too high ESR for the low resistance resonant loop. So I just don't think they will work.

I also think these lastertrim caps are kind of cool (http://www.johansontechnology.com/lasertrim-rf-tuning). Although probably a bit too involved for a hobby setup and frankly also not high q enough.

You can always add some resistance to make the antenna more broad band. I think that's what Felix has done in his design. Unfortunately you pay for it in efficiency.

Quote
I look forward to further posts as I've been toying around with making dipoles on pcb's just so that they're reproducable and on a rigid material. Oh, well.. back to 4NEC2.. :-)

Definitely report on your results here! I use a dipole on the gateway and it works great. But I've also thought about making it on a PCB for manufacturability and as you say reproducibility. The antenna will probably become much smaller due to the dielectric constant of the pcb.

Quote
Comparing your design with common HF designs

BTW, that's a key point: consensus is that small loops are inefficient. Yet it is a antenna that HAM's build with an efficiency approaching that of a dipole in HF. Radiation resistance is proportional to A^2 / l^4, so as long as proportions are the same relative to wavelength it should not change in UHF. Only problem is skin-effect and proximity effect. They do cause losses that grow with sqrt( f ). This is why I think a good sized copper tape has a better chance of success than a pcb trace.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: luisgcu on May 24, 2016, 07:17:56 PM
As you know from this thread (https://lowpowerlab.com/forum/index.php/topic,1898.0.html) I've been toying with small antennas and in particular the small loop antenna. I think it holds a lot of promise for good efficiency at very small form factors.

Well now - after purchasing a small vector network analyzer - I have had the first successes today. The end result is ok, not great yet. But I think there is still a lot of room for improvement.

I went for an inductively coupled antenna, where a small non-resonant loop drives a larger resonant one:

***IMAGE(s) FROM ORIGINAL POST***

This approach allows fixing the resistive impedance mismatch between the 50 Ohm radio module and the very low resistance resonant loop.

For the resonant loop I used 10mm copper tape. I left a gap which I first covered with adhesive tesa tape. I then covered the tesa film with a small piece of copper tape, effectively creating a capacitor:

***IMAGE(s) FROM ORIGINAL POST***

It takes a lot of cutting and measuring to get the resonance of the resulting LC circuit just right onto 433 Mhz.

It's very possible to reach a return loss of -16dB here, which is very respectable. However getting the frequency right is not easy. The antenna bandwidth is just very small:

***IMAGE(s) FROM ORIGINAL POST***

Lots of things change a bit when you disconnect the analyzer, so I think I need to do some further tuning based on RSSI values. Still - I already get reception all throughout the apartment with this setup. The RSSI is about maybe 15 lower than what I'd see with a Moteino with wire antenna. I suspect we can do much better than this.

Also this antenna still has a diameter of 4cm and I think one could go down a bit there to 3-3.5cm. However one problem is that the 2cm coin cell does block the magnetic field in a portion of the antenna. So going smaller loses gain in this setup even faster than with small loops by themselves (which already is with A^2). Possible solutions are going with the cr1632, which leaves more space for the loop - potentially even mounting the board perpendicular to the loop axis.

Still wanted to share this as a promising direction.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: emjay on May 25, 2016, 02:00:10 AM
@luisgcu,

Out of courtesy to the author, IMHO the source of your download should be attributed.
http://www.hoperf.com/upload/rf/ANTENNAS_MODULE.pdf
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on May 25, 2016, 01:56:39 PM
With or without the source, thank you both for posting it.  It nicely consolidates a lot of practical information.I
Title: Re: Small loop antennas @ 433 Mhz
Post by: luisgcu on May 25, 2016, 09:16:30 PM
@luisgcu,

Out of courtesy to the author, IMHO the source of your download should be attributed.
http://www.hoperf.com/upload/rf/ANTENNAS_MODULE.pdf
thanks for refreshing my mind.. I  just simply couldn't remember from  where  I got that file.. ;)
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 25, 2016, 10:56:04 PM
Quote from: joelucid
Quote
"The antenna will probably become much smaller due to the dielectric constant of the pcb."

Correct. With a pcb antenna, one half of the dielectic is the surrounding air, the other half will be the pcb substrate, so the pcb only partially affects the physical length of the antenna elements. After reading various articles on the matter I noticed that the dielectric constant of FR-4 (the common substrate for PCB's) can depend on:
 - pcb materials used (changes between manufacturers)
 - frequency of operation
 - thickness of the pcb (as a percentage of wavelength)

The design of the antenna also affects how to work out inter-element spacing (think yagi's and log-periodics). It's a puzzle with a lot of variables.

Dielectric constants at DC for FR-4 I've seen can range from 4.2 to 4.6 (velocity factor around 0.48). When you get into the GHz range this number can drop to 3.9 (Vf around 0.51) (i.e. requiring a physically longer antenna). They're not hugely different but it all matters when you're trying to design something that is flexible in terms of how to make one yet also performs well.

I am awaiting my first batch of dipole-PCBs from OSH Park. Because I want this antenna design to be accessible to the general public I kept it as simple as possible so it can be ordered as-is from most pcb shops (although I would recommend to stay with OSH Park to make sure the materials are not a factor to be worried about).

My design uses:
- 1 Oz copper
- 2-layer board
- 1.6mm thick pcb
- FR-4 pcb material
- 1mm wide copper antenna traces with solder resist on top
- SMA PCB edge connector pads (or just solder coax to it)

PCB dimensions: 1.6mm x 12mm x length (where 'length' will be determined by the target frequency, somewhere around 30cm for a 433MHz antenna).
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 26, 2016, 03:35:27 AM
Quote
My design uses:
- 1 Oz copper
- 2-layer board
- 1.6mm thick pcb
- FR-4 pcb material
- 1mm wide copper antenna traces with solder resist on top
- SMA PCB edge connector pads (or just solder coax to it)

Sounds nice! Are you planning to add a balun to the design at some point? That might improve the efficiency of the antenna a bit further esp. with our rfm69 radios and with longer transmission lines.
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 26, 2016, 05:47:43 AM
I haven't got any data yet on impedance figures but if it behaves like a normal dipole it should be fine without a balun. Even at 1.5:1 SWR (73 ohm to 50 ohm mismatch) it would only be 4% reflected power and I'm not really losing sleep over that. Same goes for monopoles (quarter-wave verticals), their mismatch also produces around 1.5:1 (36 ohms into 50 ohms) and should also not worry most people. Okay, there's only 100mW to go around, now you get 96mW, big deal. The greatest concern should be lossy transmission lines at near-GHz frequencies and insertion losses.

One design I have been thinking about is to do a folded dipole and then we'd need the balun for sure. The folded dipole could go all on one copper surface, or it could be going through the pcb using through-hole pads (not sure if the distance between the elements is going to be large enough then). I like the folded dipole design as it has a wider bandwidth than the normal dipole and allows for much better matching to a 4:1 balun by varying element spacing and thickness. Although intriguing, this is waay down my list, hehe..

Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 26, 2016, 07:26:14 AM
Quote
Even at 1.5:1 SWR (73 ohm to 50 ohm mismatch) it would only be 4% reflected power and I'm not really losing sleep over that.

I was thinking a 1:1 balun in order to decouple the dipole from the feedline, not to transform impedance. HAM's seem to swear by that setup.
Title: Re: Small loop antennas @ 433 Mhz
Post by: TomWS on May 26, 2016, 08:46:44 AM
I am awaiting my first batch of dipole-PCBs from OSH Park. Because I want this antenna design to be accessible to the general public I kept it as simple as possible so it can be ordered as-is from most pcb shops (although I would recommend to stay with OSH Park to make sure the materials are not a factor to be worried about).
How much is the dielectric constant affected by the purple solder mask?  ;)

I'm looking forward to your results.

Tom
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 26, 2016, 07:52:31 PM

Quote
1:1 balun in order to decouple the dipole from the feedline

One could add a small binocular ferrite (mini-circuits has some nice ones) to the design but I feel that this is largely overkill with such small power levels. It adds additional losses and may try to solve a non-existing problem. Sure, itís probably not going to win the beauty contest but there are practically no reports on the forum about locked up or erratically behaving atmegas which leads me to believe that itís going to be just fine.

Iím always impressed by the level of success people are getting with antenna contraptions that in my view violate all design rules, but hey.. if those work and people donít report any significant issues, then Iím not too worried about using a properly designed dipole without a balun.

Itís different at HF where hams commonly use 100W and more. Adding even a slightly lossy balun there to protect the expensive radio equipment makes much more sense.


Quote
How much is the dielectric constant affected by the purple solder mask?

Iíve read that the Er of common solder mask is around 3.5 Ė 4.0 but as can be seen on the below picture itís not a layer that is applied equally in all areas. I expect it will make the antenna physically somewhat shorter but the layer is relatively thin. OSH Park lists an Er of 4.6 @1MHz for their FR-4, but doesnít list it for the solder mask. Iíll ask them and post the answer here.

(http://www.eurocircuits.com/clientmedia/ecImage/pages/sm-solder-mask/solder%20mask%20layer.jpg)

Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 27, 2016, 04:11:27 AM
Quote
there are practically no reports on the forum about locked up or erratically behaving atmegas which leads me to believe that itís going to be just fine.

Oh that wasn't my concern at all. I was hoping to improve primarily antenna rx performance and in particular noise levels. Receiver noise reduction has been a recurring theme for me. When I researched and measured it, it seemed a lot came in via the antenna.

Then you read success stories like this: http://www.eham.net/ehamforum/smf/index.php?topic=65621.15 and wonder if that applies to our circuits as well. Of course the full recommended package in HAM land is a balun at the receiver, then shielded transmission line and another balun at the dipole.

Definitely not yet my area of expertise though. Just getting started in antenna land. But I do have cocoaNEC installed and running. And I'm starting to like my miniVNA Tiny  ;)
Title: Re: Small loop antennas @ 433 Mhz
Post by: TomWS on May 27, 2016, 08:29:07 AM

Quote
How much is the dielectric constant affected by the purple solder mask?

Iíve read that the Er of common solder mask is around 3.5 Ė 4.0 but as can be seen on the below picture itís not a layer that is applied equally in all areas. I expect it will make the antenna physically somewhat shorter but the layer is relatively thin. OSH Park lists an Er of 4.6 @1MHz for their FR-4, but doesnít list it for the solder mask. Iíll ask them and post the answer here.

(http://www.eurocircuits.com/clientmedia/ecImage/pages/sm-solder-mask/solder%20mask%20layer.jpg)
Wow, thanks for the thorough answer!  My question was facetious, wondering whether the purple color made a difference or not, but I do appreciate having the answer. 

Thanks again,
Tom
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 27, 2016, 08:59:22 PM
Quote
I was hoping to improve primarily antenna rx performance and in particular noise levels.

Yeah, loops are the way to go then. I live on a block that is insanely close to a 'crossroads' of three overhead power lines. 160m and 80m are pretty much useless to me. For 40m and 20m I have put up full-wave loop antennas, about 10m (30ft) above ground. It gives me a chance to work some dx. Loops are not as loud as a dipole but the SNR is just much better.

Congrats on your VNA purchase. They're a little pricey but it gives you so much insight into what is happening. The sign of the reactance, nice frequency sweep graphs, all at your finger tips. I'm impressed they go up to 3GHz. I don't have one myself but a fellow ham is often happy to help me out with his VNWA-3 from SDR-kits.


Quote
wondering whether the purple color made a difference or not

I did take it seriously as small tweaks can sometimes have significant implications. "hey, wouldn't it be cool if the lacquer would be purple!!?". It would be odd if pcb manufacturers didn't think of this, but I did get a reply back from OSH Park yesterday.

Quote
The mask web is the minimum amount of web that should be left between pads or traces of exposed copper on the top or bottom of the board. This is mostly relevant between the pins of parts with small packages. The mask thickness of 1mil is the vertical thickness of the mask between the air and the copper or substrate below it.

We don't have the dieletric of the purple mask at hand, but we get a lot of folks ordering RF-capable boards and it seems like the mask dieletric is such a small adjustment to the substrate dieletric constant that it can safely be ignored.
(Source: OSH Park, 28 May 2016)

Now we know!  :)
 
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 30, 2016, 04:52:54 AM
Pretty interesting result on the antenna front:

As you know I've been getting huge improvements using a l/2 dipole over a l/4 wire over ground (https://lowpowerlab.com/forum/index.php/topic,1389.msg13730.html#msg13730). I built this antenna just using 2x 17.3mm wire, no tuning.

Well now that I'm the proud owner of a minivna tiny I build a new one starting with slightly too long wires and the clipping to get the resonance right onto 433 Mhz. I ended up with 30.8cm length - quite a bit shorter than the theoretical numbers. This is likely due to the wire insulation I kept on the antenna wires.

But now the surprise:

This new dipole receives around 9 dBm higher RSSI levels than the old one. Which was already so much (16 dBm) better than the wire antenna. Speaking from an antenna perspective we (or certainly I) have really been living in the stone age.

(http://i.imgur.com/dnk25Wm.jpg)

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: Felix on May 30, 2016, 09:12:25 AM
Joe,
Nice results!
I assume you use this with your own tiny nodes? Did you try this on a Moteino?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 30, 2016, 09:37:00 AM
Quote
I assume you use this with your own tiny nodes? Did you try this on a Moteino?

This was actually on my esp8266/rfm69hcw gateway. I haven't tried with Moteino's yet. On my tiny nodes I'm not considering dipoles for reasons that should be obvious from the picture  ;)
Title: Re: Small loop antennas @ 433 Mhz
Post by: Felix on May 30, 2016, 10:03:46 AM
I thought maybe you are using it for buried nodes or a gateway node. But perhaps I totally forgot you are using a esp gateway.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 30, 2016, 10:11:03 AM
Quote
I thought maybe you are using it for buried nodes or a gateway node.

Buried nodes are special because the changing moisture content of the soil causes wavelength to change very drastically. So an underground antenna needs to be very wide-band. I have a couple of ideas there but nothing actually tried out yet.

But yeah, I think for the gateway dipoles are a great option. Having a big and efficient antenna at the gateway gives you more room to compromise on space or otherwise constrained motes.
Title: Re: Small loop antennas @ 433 Mhz
Post by: john4444 on May 30, 2016, 02:58:45 PM
Hi Joe, My Moteinos are all in the 915-Mhz band. I investigated loop antennas but quickly got discouraged because of the small physical size and measurement accuracy needed to construct the antenna. In addition, I suspected that the tiny capacitance value required to tune the antenna would be very difficult to maintain since it is sensitive to objects in the local environment.
However, your experience at 433-Mhz is causing me to re-examine the issue.

I suspect it may be feasible to isolate the tuning cap is such a way that the loop its self could be sealed against moisture. If this idea works as expected, then buried Moteinos would be able to provide normal range and signal level using magnetic loop antennas.

I'm going to need some 433-Mhz Moteinos just so I can play around with them.
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 30, 2016, 06:03:56 PM
Great to read you're seeing such an improvement.

Quote
Speaking from an antenna perspective we (or certainly I) have really been living in the stone age.

Looks like the VNA has opened your eyes..  :)

What length and type of feedline are you using?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 31, 2016, 02:16:16 AM
Quote
If this idea works as expected, then buried Moteinos would be able to provide normal range and signal level using magnetic loop antennas.

I think that's an excellent point and one I hadn't considered. In a small loop the only part affected by the varying permittivity of the soil is the capacitor. If that is fully shielded eg by copper tape one could indeed end up with a narrow-band antenna that works independent of the dielectric properties of the immediate environment.

I'm particularly excited about this as all the efficient wide-band antennas I've studied require a prohibitively large antenna at 433 MHz. I had already resigned to use 868 MHz for that very reason - hardly a good idea otherwise because of the much larger signal attenuation in soil at higher frequencies. 
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 31, 2016, 02:18:46 AM
Quote
What length and type of feedline are you using?

I'm using coax, maybe 30cm. Since I don't use a balun the feedline will modify the antenna characteristics so I've measured it with feedline.
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on May 31, 2016, 05:04:35 AM
Quote
I'm using coax, maybe 30cm.

Reason I'm asking is that this is probably the right time and place to insert a little something about (coax) feedline lengths and how they can pick up radiation from the antenna. Maybe not so much for those in this thread but for everyone else who has questions about how to connect the antenna to the radio this may be of interest.

For those unfamiliar with the relationship between feedline length and how it picks up radiation from the antenna, here's an example that relates to the above dipole antenna.

Apart from the coax' internal dielectric, velocity factor and standing wave ratio, what can become an issue is a phenomenon called 'RF in the shack', relating to 'hot' radio equipment and feedlines that will act as an extension of the radiating antenna and can cause all sorts of nasties (resetting microcontrollers, chip memory corruption, component failure, etc..).

The main reason these 'common mode' currents appear is because of asymmetry of the antenna and using a length of feedline that is resonant at the frequency of operation.

(http://i.imgur.com/fLtRO9y.jpg)

In this case it's not so much the internal dielectric that's responsible for determining how long one physical wavelength in coax is for picking up antenna currents, but the outside braid and its surrounding insulation material. I think I've seen between 0.80 and 0.95 as a velocity factors.

Some sources:
The Dipole and the Coax, L.B. Cebik (https://www.antennex.com/w4rnl/col0606/amod100.html)
My Feedline Tunes My Antenna (http://www.rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/27%20-%20RF%20FEEDLINE/RF%20Feedline%20-%20My%20Feedline%20Tunes%20My%20Antenna%20%28By%20Byron%20Goodman%20W1DX%20QST%20195603%29.pdf)
Common Mode Current, W8JI (http://www.w8ji.com/common_mode_current.htm)


Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 31, 2016, 09:41:08 AM
Another update from Joe Lucid's Antenna Lab:

I built a new loop antenna today, with a diameter of 3.5cm. I tuned it using the VNA, but afterwards fine tuned using RSSI values. I then took a moteino and the th mote with loop antenna on a shootout through the house:

(http://i.imgur.com/suJkdas.jpg)

The Moteino is equipped with a rfm69hw, the th mote with a rfm69w. I set both to full power and measured RSSI values at the receiver (an espgw with dipole antenna). Thus I was transmitting with 7dBm more power on the Moteino than the loop.

Not surprisingly RSSI values in the same room with the receiver differed drastically by orientation reflecting the linear polarization of both antennas in perpendicular directions. But starting in the next room there wasn't much difference between the orientations.

Overall the Moteino produced RSSI values between 9dBm and 3dBm better than the loop, depending on the orientation of the Moteino (rotation in its main plane) and inline with the 7dBm higher tx power. The loop produced very omnidirectional radiation across rotations around its axis.

From this data the loop performed just a bit better than the moteino with less variation - much better than my first attempt and pretty respectable given the form factor. That's not to say one can be content with these results as the dipole measurements have shown that significant gains can be had over the monopole over insufficient ground plane approach.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on May 31, 2016, 09:47:20 AM
Quote
If that is fully shielded eg by copper tape one could indeed end up with a narrow-band antenna that works independent of the dielectric properties of the immediate environment.

Today while measuring the new antenna I introduced "hand effect" by positioning my hand right next to the capacitor. Interestingly there was absolutely no effect on frequency, but some decrease in return loss and broadening in bandwidth.

I'm not sure but it felt like this antenna was more robust against influences from the environment than the first one. Which might well be due to its smaller size and thus more magnetic nature.
Title: Re: Small loop antennas @ 433 Mhz
Post by: TomWS on May 31, 2016, 10:09:48 AM
I built a new loop antenna today, with a diameter of 3.5cm.
Pretty good that it can fit inside a 1.5" PVC pipe (I can use it on my Stream Level sensor, maybe my Landscape lights, and one other landscape related item but I can't remember what it is...).  Now, can you get it to 3cm so I can put it on the wall with the rest of my art work?

By the way, what's that thing hanging off of the Moteino?  It doesn't look like any coin cell I've ever seen!   ;D

Tom
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on May 31, 2016, 10:30:45 AM
How essential is the vna turning out to be? 
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 01, 2016, 03:28:29 AM
Quote
How essential is the vna turning out to be?

It's definitely very useful. Much better to see return loss or SWR over a full frequency sweep than to fiddle with RSSI measurements. That said with our small motes you do have Heisenberg phenomena where the measuring apparatus influences the results. So it seems useful to still do some RSSI based fine tuning afterwards.

Some of this might be due to my still very primitive use of the VNA and superficial understanding of the RF world.
Title: Re: Small loop antennas @ 433 Mhz
Post by: john4444 on June 01, 2016, 12:29:58 PM
How essential is the vna turning out to be? 
Without that type of tool to "see" what is happening, it is like trying to build something "blind".
Not that it cannot be done but it becomes very difficult and tedious.
Quote
you do have Heisenberg phenomena where the measuring apparatus influences the results.
Again, very true.
It can be very difficult to duplicate what someone else has already done.
Sometimes, I have trouble even duplicating what I have done with the same setup.
I think that plays a roll in the "black magic" image of radio.

Thank you Felix for providing a low-cost and stable medium to work with.
Not everyone is able to get this combination of components to work as well as you have done.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 01, 2016, 01:29:18 PM
How essential is the vna turning out to be? 
Without that type of tool to "see" what is happening, it is like trying to build something "blind".
Not that it cannot be done but it becomes very difficult and tedious.

Yes, no doubt it would be a helpful tool.  However, not having a vna, I've lately been wondering what the alternatives might be.  I'm thinking that with an RTL-SDR, one can perhaps check the radiated frequency well enough to trim one's antenna by doing a progression of snips and seeing if you're getting closer or further away from the desired frequency.  Also, assuming adequate sampling,  RSSI gives you some idea of the radiated power, so I'm guessing that trying a wide enough range of pi-network impedance matching fixes should help identify what sort of adjustment might be needed by noticing which one produces the highest RSSI.  Then repeat that process with a narrower range of adjustments, and continue iterating until satisfied.  Without investing in a vna, is there a better way?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 01, 2016, 06:08:31 PM
Quote
I'm thinking that with an RTL-SDR, one can perhaps check the radiated frequency well enough to trim one's antenna by doing a progression of snips and seeing if you're getting closer or further away from the desired frequency.

Not really. You will still radiate at the same frequency, just with lower efficiency if the antenna isn't matched correctly. What you'd have to do is send at different frequencies and see how much power is received. But that will also be influenced by the characteristics of the receiving antenna which will also be resonant at some frequency.

Quote
Also, assuming adequate sampling,  RSSI gives you some idea of the radiated power, so I'm guessing that trying a wide enough range of pi-network impedance matching fixes should help identify what sort of adjustment might be needed by noticing which one produces the highest RSSI.

I think that works for one dimensional changes only. You can cut a dipole antenna to size that way. But if you need to tune 3 components I just don't see how you do it just looking at RSSI.

One should be grateful that VNA's can finally be bought at reasonable prices. It's hard enough to get a grip on RF, so I think it's worth starting with a tool set that gives you a fighting chance.
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on June 01, 2016, 07:10:13 PM
Even though every application is different I think we could eventually come up with building instructions for a handful of common types of antennas that are compatible with the Moteino, but are designed with different performance factors in mind. Provided all these designs use the Moteino/RFM69 we can take several variables out of the equation and offer antenna designs that have been proven to work. No need for everyone to re-invent the wheel every time.

For instance:

Short range antennas
For solutions that use the loss of signal to signal an out-of-range condition that can be used for locking doors, asset tracking, man-overboard scenarios, etc..). Range 1-10m. Type of antenna: a 50 ohm resistor, small loop on a pcb, bit of wire.

Medium range antennas
Range around 50-100m. Type of antenna: small loop on a pcb or helicals with the pcb as the groundplane. Suitable for Moteinos in a plastic enclosure.

Long range antennas
Range several km. Yagiís, quarter wave verticals and dipoles fed with proper feedline. Suitable for Moteinos in metal enclosure.

Itís great to see that Felix already has made a model available with a pcb antenna and I think this would suit many. What I couldnít find are the schematics, dimensions and performance graphs/swr curves for it. This could be an ideal starting point to bring all these various designs together and improve upon it as we go.

Not sure if a forum-post-structure is good enough to keep track of which designs work best as sometimes it can be hard to find what youíre looking for.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 01, 2016, 11:55:43 PM
Quote
I thought maybe you are using it for buried nodes or a gateway node.

Buried nodes are special because the changing moisture content of the soil causes wavelength to change very drastically.

For a soil moisture sensor, that sounds like a feature more than a problem.  Maybe a soil moisture sensor could be literally nothing more than a transmitter.  i.e. If it were kept narrowband, then perhaps the receiver could infer the soil moisture content by the change of frequency in the received signal?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 02, 2016, 12:45:19 AM
Quote
the receiver could infer the soil moisture content by the change of frequency in the received signal?

It's not the frequency that changes but the wavelength in soil. The receiver would only see a massively weaker signal. Whether that's because of soil moisture of someone having parked a car in the path would be impossible to tell even if the receiver could still hear the signal.

Also if I remember correctly Rf attenuation in soil depends significantly on conductivity so you'd be back to the missing robustness against salinity changes one hopes to overcome with a high frequency capacitive sensor.
Title: Re: Small loop antennas @ 433 Mhz
Post by: Felix on June 02, 2016, 02:01:59 PM
Thank you Felix for providing a low-cost and stable medium to work with.
Not everyone is able to get this combination of components to work as well as you have done.

That felt good, thank you :)
In addition we have this nice little self sustained RF/R&D ecosystem here. So that makes me happy that it's not just some dull support forum with angry and confused customers.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 02, 2016, 04:30:47 PM
Quote
the receiver could infer the soil moisture content by the change of frequency in the received signal?

It's not the frequency that changes but the wavelength in soil.

That statement seems to contradict itself.  Either that, or there's a disconnect in my understanding.  Suppose the transmitter is outside the soil but the receiver is buried under the soil, and assume a narrowband transmission.  Setting aside the issue of attenuation, you seem to be saying that the wavelength in the soil changes depending on the moisture content. Am I understanding that right? If so, wouldn't that imply a change in frequency?   If so, does being buried in the soil somehow prevent the receiver from detecting that change in frequency?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 02, 2016, 04:53:35 PM
F = c / l. F stays the same, c decreases and l decreases in wet soil.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 02, 2016, 05:22:48 PM
So, it's basic physics then.   For doing radio, it's certainly convenient that frequency remains preserved and only the speed of propagation and wavelength vary.  Aside from the attenuation, I guess that makes it all pretty much transparent as far as an RFM69 is concerned.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 02, 2016, 05:48:34 PM
Yeah. If you think about it how could it be any different. You wiggle on one end its got to wiggle at the same frequency on the other as long as both points of observation are stationary. Otherwise where would the wiggles go? You've never seen light change color as is passes through a lense have you? Of course if you move away from the source you do get a redshift due to Doppler effect.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 03, 2016, 03:49:58 AM
Quote
However, not having a vna, I've lately been wondering what the alternatives might be.

This may not be what you're looking for, but I just tested the linx splatch antenna yesterday (http://www.digikey.com/product-detail/en/linx-technologies-inc/ANT-433-SP/ANT-433-SP-ND/1679578). It claims to be very stable, no tuning required. And it really is, it's working very well even though I didn't even connect all pads and there's no matching.

It should make for a great travel companion:

(http://i.imgur.com/yB9gWme.jpg)
(http://i.imgur.com/olPs7ex.jpg)
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 03, 2016, 06:14:52 AM
Continuing work on my travel kit. This one isn't nearly as efficient as the bigger one above. But given I only want to be able to do software dev on the road this is much better than the wire. Given Felix doesn't have a loop for 433mhz yet.

(http://i.imgur.com/iQxgzEd.jpg)

It's called the micro splatch (http://www.digikey.com/en/product-highlight/l/linx-technologies/microsplatch-chip-antennas).
Title: Re: Small loop antennas @ 433 Mhz
Post by: Felix on June 03, 2016, 07:22:32 AM
Thanks, I will try this out. It's pricey though for what it is.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 03, 2016, 07:45:33 AM
Quote
Thanks, I will try this out. It's pricey though for what it is.

True. Performance was a bit better than with wire though which I found impressive. Maybe we should open one up and see if you can do it as trace. I bet it's a short stub loaded with a small loop.
Title: Re: Small loop antennas @ 433 Mhz
Post by: Felix on June 03, 2016, 08:35:50 AM
Apparently they have a patent (pending) so that might not go so well :)
Title: Re: Small loop antennas @ 433 Mhz
Post by: TomWS on June 03, 2016, 08:40:49 AM
Quote
However, not having a vna, I've lately been wondering what the alternatives might be.

This may not be what you're looking for, but I just tested the linx splatch antenna yesterday (http://www.digikey.com/product-detail/en/linx-technologies-inc/ANT-433-SP/ANT-433-SP-ND/1679578). It claims to be very stable, no tuning required. And it really is, it's working very well even though I didn't even connect all pads and there's no matching.

It should make for a great travel companion:

(http://i.imgur.com/yB9gWme.jpg)
This reminded me of the predecessor of my Dust Collector project (https://lowpowerlab.com/forum/index.php/topic,918.0.html).  I used the Splatch antenna on the controller and the remotes.  They worked well enough for that application although I don't have any metrics on performance.

Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 03, 2016, 09:06:57 AM
Quote
Apparently they have a patent (pending) so that might not go so well :)

Well pending doesn't mean granted and granted doesn't mean that there aren't good ideas in there that don't relate to the patent. BTW if there is a patent pending I couldn't find it.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 04, 2016, 09:27:54 AM
Hmmm..  Looking at the datasheet for the uSplatch and splatch antennas.  If I'm reading them right, they both seem to assume the existence of a proper ground plane: 
https://www.linxtechnologies.com/resources/data-guides/ant-916-usp.pdf
https://www.linxtechnologies.com/resources/data-guides/ant-916-sp.pdf

I wonder: would it work to simply attach a sufficiently large uniform antenna ground plane to the back of the moteino with, say, double sided tape so that the splatch has a sufficient counterpoise?  Just solder GND to it with a wire?  It's so simple it's probably worth a try.  I've been meaning to try it underneath just the RFM69 module (execuval already did a very small one to good effect in a different context, so I thought I'd try something bigger), but maybe the RFM69 doesn't need to be separated like that for it to work.  However, lacking the proper measurement equipment, I'm reticent to mess with it, as it might require a lot of blind experimentation to get anywhere with it.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 04, 2016, 11:49:10 AM
Quote
If I'm reading them right, they both seem to assume the existence of a proper ground plane

They do require a ground plane, but not a full one. A 433 mhz monopole would need at least a 36cm diameter ground plane disk to be considered fully grounded.

Quote
would it work to simply attach a sufficiently large uniform antenna ground plane to the back of the moteino with, say, double sided tape so that the splatch has a sufficient counterpoise?

In my setup the ground plane on the Moteino serves as antenna ground plane. But yes, adding an uninterrupted ground plane maybe even of the recommended dimensions should certainly improve things. If you put the antenna on top you should also put the ground plane on top to shield components and pcb.

As I said though even the very ad-hoc mounting I used already provided better reception than the wire antenna in my measurement which I found quite sufficient for this purpose (travel kit).
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 05, 2016, 01:26:22 PM
As I said though even the very ad-hoc mounting I used already provided better reception than the wire antenna in my measurement ...
You mean better eve than the typical straight piece of wire?  Or the loop of wire you've been working on recently (the OP)?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 05, 2016, 03:45:29 PM
Quote
You mean better eve than the typical straight piece of wire?  Or the loop of wire you've been working on recently (the OP)?

I directly compared a 443mhz Moteino with 17,3 cm wire against the modded Moteino with uSplatch. The uSplatch performed consistently better.
Title: Re: Small loop antennas @ 433 Mhz
Post by: TomWS on June 05, 2016, 03:54:07 PM
I directly compared a 443mhz Moteino with 17,3 cm wire against the modded Moteino with uSplatch. The uSplatch performed consistently better.
Is this with the uSplatch perpendicular to the Moteino plane or parallel with it.  I think your photo showed perpendicular but I think Linx specs parallel (ground plane to the pad edge of the antenna).

Tom
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on June 05, 2016, 04:22:21 PM
Quote
Is this with the uSplatch perpendicular to the Moteino plane or parallel with it.  I think your photo showed perpendicular but I think Linx specs parallel (ground plane to the pad edge of the antenna).

Yeah it's with the perpendicular mount that I did just to try it out. I also only connected two of the three pads - one to the antenna out and the other to the gnd pin opposite of the ftdi header. I'm very aware that's not the recommended mount so the results surprised me quite a bit.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on June 05, 2016, 06:52:02 PM
Quote
You mean better eve than the typical straight piece of wire?  Or the loop of wire you've been working on recently (the OP)?

I directly compared a 443mhz Moteino with 17,3 cm wire against the modded Moteino with uSplatch. The uSplatch performed consistently better.

Wow, that ranks as a really important find then, and the fact that it's so compact makes it even more awesome.   8)

So, I just ordered a couple to try out.   Hopefully I'll get them by next weekend.

Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on July 04, 2016, 09:37:03 AM
I tried the 915Mhz uSplatch antenna on a node. Looking at the RSSI, it's noticeably more directional, but with a dipole gateway I'm not worrying about it.

I haven't yet tried comparing it to a helical.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on July 04, 2016, 03:11:10 PM
Did you also find it performed on average better than the wire?
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on September 03, 2016, 08:00:25 PM
As mentioned a few posts back I have finally completed testing and uploaded my dipole-on-pcb boards to OSH park.

See related post on this forum for full details:
https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/easy-pcb-dipoles-for-433-868-and-915mhz/ (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/easy-pcb-dipoles-for-433-868-and-915mhz/)
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 14, 2016, 06:22:48 PM
@JoeLucid
I'd like to modify my custom temp-rh node's pcb to use a splatch and send it to the fab sometime soon. Looking at the ANT-915-USP datasheet (https://www.linxtechnologies.com/resources/data-guides/ant-916-usp.pdf), I notice it is recommending it be connected to a rather long and wide PCB (3.145" x 1.53") PCB with what looks like a dedicated ground plane.  Have you found the micro splatch to be especially sensitive to those dimensions?  If so, then I might try modifying the bigger experimental node's PCB instead (see attached).

Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 14, 2016, 10:55:25 PM
Meh, it was easy enough to add the micro-splatch pads and traces to the small experimental node, so I just now did it and sent it off to the fab.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 15, 2016, 05:41:24 AM
All monopole type antennas suffer immensely with small ground planes. You'll get much better results with larger ones. But I had fairly good results with a Moteino and splatch - albeit with a connected 9v battery which adds to ground plane size. That's comparing to a Moteino with l/4 monopole which obviously suffers from the same limitations.

Just try it out - I think you'll be surprised by how well they work.

I'm waiting for 3 different trial boards for a th mote with loop antenna. When I converge on a design I'll share here.

Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 15, 2016, 09:22:19 AM
I think from now on I'm going to add pads and traces for a Splatch to everything that's experimental.  In the (unlikely) event it doesn't cut the mustard, I'll just un-install and solder a regular wire antenna directly to the RFM69.  In this manner, the option will exist to utilize it if desired, and along the way I'll collect datapoints on where it's effective and where it isn't.  The added PCB board real estate to accommodate its pads is small.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 16, 2016, 12:34:53 PM
As to the board layout for the micro Splatch, the datasheet refers to a "50 ohm microstrip (0.21" minimum)" which appears to be connected to the antenna pad of the micro Splatch.  Can anyone here shed light on what such a microstrip is?  Is it a trace of some kind, or a distinct discrete component, or....?
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on October 16, 2016, 01:43:22 PM
It's a trace on a PCB with a ground plane beneath forming a transmission line. The trace and the ground plane means the trace has a capacitance per unit length with the ground plane and inductance per unit length, the very characteristics of a transmission line. The ratio of the width of the trace to the distance between the trace and the ground plane defines its characteristic impedance. For a two layer FR4 type PCB and 1.6mm thick the width of the trace needs to be about 3mm (i.e. roughly 2:1 ratio) to get 50 ohms.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 16, 2016, 01:44:05 PM
Can anyone here shed light on what such a microstrip is?  Is it a trace of some kind, or a distinct discrete component, or....?

It's a trace of a certain width on top of a ground plane. Think of it as a pcb implementation of a 50 ohm coax cable. The width depends on the pcb parameters like thickness, dielectric constant etc. Google and you'll find plenty of calculators which will give you the width based on those parameters.

Now practically speaking you'll want the antenna directly on the ant contact of the radio. So you shouldn't need to worry about a 50 ohm micro strip from module to splatch.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 16, 2016, 02:10:47 PM
Here's the top layer (see attached) from another version of experimental node that I sent into the fab last night.  I made wider both the antenna and ground traces that connect the RFM69 to the splatch, which I positioned to be very near the pins of the RFM69.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 16, 2016, 02:49:53 PM
I did it a little differently for the top layer on the splatch version of the bigger experimental sensor (shown in the photo above).  I include that here, since it might be easier to understand how it relates (see attached).  In total I now have sent three different experimental nodes (small, medium, and large) to that fab that have been revised to include PCB traces and pads for connecting up a micro splatch.  On the big experimental node (shown in the attachment to this post), you can see them at the very bottom.  The datasheet doesn't mention anything about the minimum width for the ground traces, but I beefed some of them up a bit.  I didn't use a copper pour on the earlier experimental nodes, and even the small one seems to be working just fine without it.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 16, 2016, 03:54:23 PM
For the small node, I only beefed up the antenna trace and left the ground traces at normal thickness.  So, this way, perhaps I'll see whether the ground trace thickness makes any difference to the splatch.  Not sure whether the right angle on the ground trace will make any difference or not (?).  In general, I've tried to avoid 90 degree trace angles by using a couple 45's instead of a 90.  I didn't catch this 90 before shipping it to the fab, but with more carefully considered routing the trace angle could have been handled with a single 45 instead of the 90.
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on October 16, 2016, 05:07:45 PM
The 90 degree thing will make very little difference, only up in the several GHz region which your radio should not be outputting as this would be spurious. I wouldn't worry about it.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 16, 2016, 07:19:20 PM
@perky :  Thanks.  Good to know!

@JoeLucid: I'm forgetting now, but have you found the larger splatch to perform a lot better than the micro splatch?  I'm assuming it does, because otherwise why would Linx make it or people buy it?  Presently I have 3 of the microsplatches and 3 of the larger splatches that I got from Digikey after you first started posting about them.  I'm debating whether to cook up a batch of experimental nodes with pads for connecting to the larger splatch, but thought I'd touch base with you first to see which of the two you now prefer most based on whatever testing you've performed to date.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 18, 2016, 02:55:54 PM
See attached layout.  I refined the layout to do two things:
1.  Give it a ground plane, and
2.  Have one design which equally accommodates either the Splatch or the uSplatch.

There are some copper islands left over after the ground pour.  Is it best practice to leave them, remove them, or it doesn't matter?  Anyone know?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 20, 2016, 07:43:16 AM
Got my first test board today.

Here's the design:

(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/2b061d71a5209592d67c58b9eb9c3761.png)
(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/acdc2f370ecef48b326cba356b40e8d6.png)

Turns out I could get a perfect match with 2.8 pF implemented using 1x 1 pF + 1x 1.8 pF. Note that the small loop could be a little smaller to get even closer to 50 Ohm.


(http://i.imgur.com/94sDOnv.jpg)

Performance is very similar to the l/4 monopole.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 20, 2016, 09:29:55 AM
@JoeLucid
That's truly very beautiful to be behold.   :)  Sorry if my x-ray vision isn't as good as it used to be, but looking at the images you posted it appears that the small round loop is a pathway whereby ANT connects to GND, but with a capacitor in parallel, bridging the open gap in the loop.  The large square loop is completely standalone (not connected to anything else by traces), with  2 capacitors plus something else( ?) closing what would otherwise be an open loop.  Is that what's going on?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 20, 2016, 09:58:46 AM
Quote
Sorry if my x-ray vision isn't as good as it used to be, but looking at the images you posted it appears that the small round loop is a pathway whereby ANT connects to GND, but with a capacitor in parallel, bridging the open gap in the loop.  The large square loop is completely standalone (not connected to anything else by traces), with  2 capacitors plus something else( ?) closing what would otherwise be an open loop.  Is that what's going on?

The small loop actually doesn't have a cap in parallel. I only created those two pads when I created the antenna as component in eagle so I could connect the radio to it. Ideally they would not be blank.

The large loop is completely standalone, correct, only inductively coupled to the small one. By choosing the relative sizes you can match the real impedance once the large loop is tuned for resonance. The large loop has 3 components. Two are for capacitors allowing finer tuning than possible with standard sizes. One is for a resistor to broaden bandwidth where needed (e.g. if you can't get a good enough match or if you need to do frequency hopping).

I have two more designs coming: one exactly like this but with a large loop that's duplicated on the other side of the board for less resistive loss. The other one has a round loop around the coin cell instead of to the side, allowing for a larger circumference with hopefully larger radiation loss and more aesthetic form factor.

Additionally I think the smaller loop should rather be flatter and closer to the large loop for better radiation since that will induce a larger current in the large loop. I expect to keep improving this design with the unfortunately large turnaround times caused by pcb manufacturing.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 20, 2016, 10:25:29 AM
Here are the other two designs:

(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/b41a76761e304c7c231d2b41dc686b96.png)
(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/22c0b9f073585722de369fac0bd97c13.png)

and

(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/ca00e22214c7bd86d51477f485d277a3.png)
(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/9a0ef2055f10044fd7ae3f60ffa826a2.png)

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 20, 2016, 12:24:14 PM
A couple of things I'm wondering:

1.

Note that the small loop could be a little smaller to get even closer to 50 Ohm.


Are you saying that the small loop is (or could be made to be) 50 Ohm?  i.e. in a sense, the small loop is acting like the dummy load resistor antenna on the other thread?  If so, that would be a lucky find for me, as it would probably serve that purpose far better than I could approximate with an SMD and just a guess as to lead length (re: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/inverted-f-antenna/msg15865/#msg15865)

2.  Since the overall footprint is bigger than what you already had previoiusly with a splatch antenna, is the motivation for this approach primarily better omnidirectionality?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 20, 2016, 12:48:49 PM
Quote
Are you saying that the small loop is (or could be made to be) 50 Ohm?

Only together with the large loop. This is like a transformer which transforms the tiny impedance of the large loop up to 50 ohm.

Quote
If so, that would be a lucky find for me, as it would probably serve that purpose far better than I could approximate with an SMD and just a guess as to lead length (re: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/inverted-f-antenna/msg15865/#msg15865)

Why anyone would want to build an antenna that consists exclusively of a dissipative element is frankly beyond me. I think one would want to match impedance without resistive elements.

Quote
2.  Since the overall footprint is bigger than what you already had previoiusly with a splatch antenna, is the motivation for this approach primarily better omnidirectionality?

I think the small loop can perform better than a splatch with a super tiny ground plane. I'm particularly hopeful for the round design. Also as you say the loop is very omnidirectional - which the splatch is probably not unless you use it vertically oriented.

The current design is about as large as a Moteino - which is not bigger than a Moteino with splatch.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: john4444 on October 20, 2016, 02:38:16 PM
@ Joe, Very nice work and documentation.

Persons not into antenna design may be unaware of the advantages of small loop-antennas.
Loop-antennas are noted for being very 'quiet' due to their very narrow bandwidth.
This means that they do not pick up out of band signals competing with the signal of interest.
In addition, loop-antennas are sensitive to the magnetic field as opposed to the
electrostatic field of the radio signal. This characteristic means loop-antennas are not de-tuned
when they are close to adjacent objects as most common antennas experience.
I'm looking forward to seeing how well they operate when buried (at shallow depths).

I do have an unverified concern that because they are so small and have such narrow bandwidth
that they may shift out of the desired operating frequency due to expansion/contraction from temperature changes.

I think you are on the right path. Please keep up the good work.
I'm expecting your loop-antennas may become a valuable alternative to the vertical wire.

Good Luck, John

 
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on October 20, 2016, 02:53:43 PM
Here's a thought, fractal antennae have wide bandwidth. Is there a way to create the loop trace using cross-hatching rather than simple tracks that makes it look like a whole load of different length loops superimposed on one another?
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: john4444 on October 20, 2016, 05:22:48 PM
@ Perky, Yes, fractal loop-antennas do provide wider bandwidth.
However, for construction reasons it is easier to make the width of the antenna element wider.
For example, loop-antennas do not have to be thin as in wire but they can also be made thicker
using tubing without changing the overall outer diameter.

I've made a few loops for 150-MHz using Ĺ" copper pipe and all were very critical to tune properly.
For that reason they do not make good general coverage antennas but excel at a fixed frequency.
In addition, I found it necessary to use high quality mica capacitors to tune the loop.
Ceramic caps were the worst for stability and consistency of the various caps I tried.
From that experience, I am kind of surprised that Joe is having such success.

It is probably from his skill/experience. (but don't tell him that)

John
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on October 20, 2016, 07:20:38 PM
Awesome designs Joe! Really looking nicely and it looks like you paid a lot of detail to component positioning and board layout. I especially like the round one..

Quote from: joelucid
perfect match with 2.8 pF implemented using 1x 1 pF + 1x 1.8 pF

Whoa, that.. is.. tiny...!  :o

You might even roll your own capacitor on the next revision by putting PCB tracks close together and trim to length.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 21, 2016, 03:06:22 AM
Thanks for the great feedback, guys. A couple of comments:

Quote
I do have an unverified concern that because they are so small and have such narrow bandwidth
that they may shift out of the desired operating frequency due to expansion/contraction from temperature changes.

The thermal expansion coefficient of copper is 17 ppm per C. So I don't think that is a problem. (http://hyperphysics.phy-astr.gsu.edu/hbase/tables/thexp.html).

Quote
Ceramic caps were the worst for stability and consistency of the various caps I tried.

Yeah, that is a huge issue. I can't find caps with tolerances much below 1%. Maybe 0.5%. But that is clearly MUCH too high and would kill a high q design.

Johanson has some air trimmers with great specs - but they are huge for what's being considered here (http://www.johansonmfg.com/media/pdf/Air-Capacitor.pdf). Still might be worth a test.

I clearly got very lucky with the great match I got from the 2.8pF. The smallest increment is 0.1pF, or around 3%!

Quote
Is there a way to create the loop trace using cross-hatching rather than simple tracks that makes it look like a whole load of different length loops superimposed on one another?

Quote
However, for construction reasons it is easier to make the width of the antenna element wider.
For example, loop-antennas do not have to be thin as in wire but they can also be made thicker
using tubing without changing the overall outer diameter.

If that were true I would expect a much larger bandwidth than I'm seeing based on the 2mm trace width I'm using. I think the reason it doesn't work is the proximity effect (https://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)) which causes current to flow primarily on the smallest loop.

Now what might work is to have one loop on the front and one on the back (or even 4 using a 4 layer board) and slightly offset the resonant frequencies. On the other hand one gives up return loss on each resonant frequency in return.

Maybe the trace width is not that important due to the proximity effect and one could instead do multiple thinner loops that only meet at the capacitors. That would create multiple resonances.

I think it would be best if one could just really precisely tune to the correct frequency.

Quote
You might even roll your own capacitor on the next revision by putting PCB tracks close together and trim to length.

I think this is a great idea. http://www.vhfcomm.co.uk/interdigital%20capacitors.pdf has a good case study. Note that the capacitance of their 4 finger 4.25 x 2.75 mm cap is 0.3 pF. It's tiny!

Now they use 0.25mm finger spacing and 0.5mm finger width. OSHPark can do 0.15mm spacing and trace width. Squeeze 10 mini fingers together and you might get to around 1.5pF. Do that on both sides and you're there. Something to try out.

Now such a cap would make the antenna very sensitive to detuning so it would need to be shielded. But this is a good direction to explore.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 21, 2016, 06:06:11 AM
Second design arrived today:

(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/b41a76761e304c7c231d2b41dc686b96.png)

It needed 3.3 pF to match and the match isn't as good. Note that return loss is much less but the antenna has a broader bandwidth. Maybe that's Perky's "fractal antenna" at work, where the current can take multiple paths switching between board sides.

The return loss is a disappointment. Now I wish I had done the circular design as two independent loops - well maybe the larger loop size will bail me out.

(http://i.imgur.com/PYJ0wv4.jpg)

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on October 21, 2016, 09:52:21 AM
Interesting stuff. I found this about fractal patch antennae which is also quite interesting:
http://www.sciencedirect.com/science/article/pii/S2215098615000026
Mark.

Quote
Base shape and first three iterations of modified square fractal antenna with parasitic patches.
(http://ars.els-cdn.com/content/image/1-s2.0-S2215098615000026-gr4.jpg)
Title: Re: Small loop antennas @ 433 Mhz
Post by: john4444 on October 21, 2016, 12:29:38 PM
I'm admiring your efforts Joe.
Quote
Quote
I do have an unverified concern that because they are so small and have such narrow bandwidth
that they may shift out of the desired operating frequency due to expansion/contraction from temperature changes.

The thermal expansion coefficient of copper is 17 ppm per C. So I don't think that is a problem. (http://hyperphysics.phy-astr.gsu.edu/hbase/tables/thexp.html).
I was really thinking about expansion of the fiberglass circuit board.
But, reality over rides speculation. If it isn't an issue, then it is not an issue.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 30, 2016, 06:37:28 AM
I got the circular design yesterday and tried to match it. Two observations:

A) This one I couldn't get to 433 mhz with my set of tuning caps at all. I needed to add a 7k resistor to get it to work - obviously not good for efficiency. Part of the problem is that this larger loop only needs around 1.6 pF to match and at this magnitude granularity and tolerance are just bad in caps.

B) Return loss was around -8 db for a cap only match at around 440 mhz. So this antenna suffers from the via related loss as well. The simple loop works better than a two sided one with vias.

Given results so far I think there are basically two overall directions: (a) a low q loop which is pretty inefficient, but implementable with standard parts or (b) a high q loop which needs to be custom tuned.

The possibility (b) of a high q loop with efficiency close to a dipole is what originally drew me to this antenna design. I see three options to still get there:

1) Use a high q air piston trimmer like Johanson's 5801 (http://www.farnell.com/datasheets/35551.pdf) and custom tune each Mote.

I think this should enable efficiencies higher than 50% but it's expensive, large and prone to detuning after environmental changes. I ordered a few of these trimmers to try out. This is probably more to measure potential rather than as viable volume circuit.

2) Use a high q ceramic capacitor with a parallel varactor tuner.

This might give you 30% efficiency. Varactors are voltage controlled capacitors. They don't have a high enough q by themselves. But if you pair them with a fixed high q cap it could be good enough.

Unfortunately this approach invites all kinds of complexity: need a stable voltage to control varactor -> regulator required. Varactors are very temperature dependent -> need to retune frequently and/or compensate for temp changes.

3) Use a high q ceramic capacitor and tune the gateway.

Motes could continue to be simple if the tuning responsibility can be shifted to the gateway. This is possible: my gateway has a dipole which has a fairly wide bandwidth. The gateway could slice time into rx windows dedicated separately to all Motes. It would measure out the best frequency for each Mote and listen during its window at that frequency. The Motes would need a 32khz crystal to keep track of time.

I think this works in the US on 915Mhz where there's enough bandwidth available. It doesn't work in Europe where the 433 and 868 bands are fairly narrow.

Unfortunately I don't really like any of these options very much. Anybody with better ideas?

Joe

 
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on October 30, 2016, 07:51:48 AM
This is good work Joe. What we really need is a low Q antenna that is close to resonance across the desired band, I'm still convinced there might be a fractal patch approach out there somewhere. Something like this:
https://piers.org/piersproceedings/download.php?file=cGllcnMyMDExU3V6aG91fDRBNF8xMjYyLnBkZnwxMTA0MTkwMjIzMDA
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 30, 2016, 10:21:51 AM
Quote
What we really need is a low Q antenna that is close to resonance across the desired band

Yeah - if only we could - that would be perfect. But fundamentally you have to pick two out of (1) high bandwidth, (2) high efficiency, (3) small size in antenna design.

An antenna that is electrically as small as the loop in my examples has a minuscule radiation resistance. You need quite wide traces due to skin effect to get dissipative resistance to be lower than that. The German example I posted early in this thread used a copper ring of 1cm width for ~90% efficiency @ 433 mhz! As soon as you provide other resonances to get more wide-band there's less copper for the first resonance.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 30, 2016, 11:40:34 AM
Part of the problem is that this larger loop only needs around 1.6 pF to match and at this magnitude granularity and tolerance are just bad in caps.


Would making your own pF capacitor using PCB traces, as someone recently suggested, have the required granularity?  Or, if not, maybe it could be adjusted to the proper capacitance by deliberately making it too capacitive and then scraping off trace with an exacto knife? 
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on October 31, 2016, 04:21:23 AM
Quote
Would making your own pF capacitor using PCB traces, as someone recently suggested, have the required granularity?  Or, if not, maybe it could be adjusted to the proper capacitance by deliberately making it too capacitive and then scraping off trace with an exacto knife?

The issue with these interdigital finger caps is they have fairly high losses. Not even close to ultra low esr ceramic caps or air trimmers. But I do think that could be an interesting middle ground: at least you could precisely tune this loop and the resistance of the cap would broaden bw potentially enough to deal with environmental change.

I've seen estimates for Q for a finger cap of 40 - 400. At 40 the cap would have a resistance of about 6 Ohm. At 400 that'd be 0.6 Ohm. Which might bring efficiency down to 5-30%.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 31, 2016, 07:31:49 AM
Meanwhile, I'm liking the uSplatch form factor a lot.  Thanks for suggesting it!   :)  Attached is a photo of it in vivo, which gives a sense of the scale. 

Now I need to find a better way to plug in a TH sensor without having it stick out like a sore thumb....   ::)
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 31, 2016, 10:07:14 AM
For those who haven't seen these antennas before, attached is a similar photo but incorporating a regular Splatch antenna.  Compared to a uSplatch, it's huge, but on an absolute measure it's still not bad.

It would be interesting to make these antenna's "pluggable".  That way, as the experimental node hardware continues to evolve, they could be de-mounted easily and plugged into the newest design iteration.  I'm already doing that now with the RFM69 and TH modules, as you can see.  Not sure if doing that would create complications for the RF, but I may just give it a try to find out....
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on October 31, 2016, 11:24:52 AM
In any event, it looks as though the small TH experimental node will now fit nicely inside a very small case (attached photo) which already has a bunch of vents.  With a tall gangling monopole antenna sticking out, it just wouldn't look as streamlined.  I haven't attached the TH sensor yet because I've run out of them, but I'm sure it will fit if I solder it directly.  Now I just need to figure out the best way to wire it for power.  I have some tabbed coincells on order from China, so that's one possibility.

That's the epilog.  I'll stop hijacking this thread now....   ;)

[Edit: P.S. I think the lesson here, at least for me, is to start with the case I want to use and work backward from that.  Otherwise, one can over-optimize for size and it won't really matter. Finding appropriate cases with good aesthetics isn't particularly easy. ]
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on November 01, 2016, 12:32:57 PM
By the way, I just now did an off-the-cuff, very unscientific, test to get a rough gauge on the various antenna performances.  Comparing wire monopole, uSplatch, and Splatch antennas.  Before starting the test, I thought the monopole would be the clear winner, and the Splatch would handily beat the uSplatch.  Honestly, though, I didn't see a clear winner or loser.  For any one of them, the RSSI depends a lot on the vagaries of how it's orientated or situated.  So, with that in mind, I'd say they're all in about the same ballpark, give or take.  There are possibly better configurations that I haven't yet tested (such as with the Splatch in a vertical configuration), but those are the early preliminary results with the units I've already made (photos above).
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on November 05, 2016, 10:35:40 AM
Hey, I've just come across this article. Seems you can use the free NEC-2 software for antenna design, this normally only uses air dielectric, but you can fudge it to use FR4 dielectric with some simple hacks. They've come up with an inverted L type 915MHz design that apparently performs better than a monopole and is only 1.25" square:
http://www.edn.com/design/communications-networking/4332858/Small-915-MHz-antenna-beats-monopole
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 06, 2016, 01:33:29 AM
Quote
Hey, I've just come across this article.

Looks interesting. They don't mention it but I'd assume you need a full sized ground plane for this antenna though. So the miniaturization only benefits one half of the antenna.
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on November 06, 2016, 08:45:32 AM
Yes, it's an inverted L folded monopole so I assume would require a ground plane. I was more interested in the fact you can simulate these with NEC-2 (4NEC2) and the FR4 hack. Pity there's no download for the NEC-2 design files...
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 22, 2016, 03:43:36 PM
Guys, check out these very cool air trimmers I got today:

(http://i.imgur.com/bpx8oco.jpg)

Here's the design I just threw together for it:

(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/01992d424f40b35e64a0b41992a0b093.png)
(https://644db4de3505c40a0444-327723bce298e3ff5813fb42baeefbaa.ssl.cf1.rackcdn.com/015215edc15569115bc3ff56f9c0f7e7.png)

It's the Johanson 5851 (http://www.farnell.com/datasheets/35551.pdf) with a Q of >7500 at 100 Mhz. This thing should fly! I'll report back when I have the boards.

I paid like $7 each for the trimmers - so this is definitely for the enthusiast  ;)

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: tve on November 23, 2016, 02:05:38 AM
I don't quite understand the uSplatch. Isn't a 50Ohm resistor across antenna and gnd pins essentially a 0dBi point source antenna? And the uSplatch has max gain of 0.3dBi, is that worth it? (Note that a 50Ohm RF resistor costs about the same as the uSplatch but is a lot smaller.) I'm sure I'm missing something :-)
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on November 23, 2016, 09:59:12 AM
I don't quite understand the uSplatch. Isn't a 50Ohm resistor across antenna and gnd pins essentially a 0dBi point source antenna? And the uSplatch has max gain of 0.3dBi, is that worth it? (Note that a 50Ohm RF resistor costs about the same as the uSplatch but is a lot smaller.) I'm sure I'm missing something :-)
I've tried it both ways, and simply put: the uSplatch is more efficient at converting electrical energy into RF energy at the target frequency.  If range isn't a concern, then I think an argument can be made for using a resistor antenna.  Likewise, if space isn't a concern, then a simple wire antenna like the standard issue Moteino uses remains a good choice.

If Joe succeeds in his experiment, then it should outperform the uSplatch, and that would be great.  I certainly do admire his tenacity in pursuing it, and I look forward to hearing how it turns out.
Title: Re: Small loop antennas @ 433 Mhz
Post by: tve on November 23, 2016, 11:39:08 AM
Quote
I've tried it both ways

Ah, you compared with a 50 Ohm RF chip resistor soldered across ant and gnd on the rfm69 module? By RF resistor I mean something like http://www.digikey.com/product-detail/en/vishay-thin-film/FC0603E50R0BTBST1/FC0603-50BFCT-ND/1769793
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on November 23, 2016, 01:43:45 PM
Dummy resistor loads will only 'leak' some radiation, but it's tiny compared to a proper resonating antenna with a reasonable radiation length. Remember with a dummy load practically all the energy goes into resistive heating of the resistor rather than being transmitted, and it is physically small.
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 23, 2016, 01:50:51 PM
Quote
Dummy resistor loads will only 'leak' some radiation, but it's tiny compared to a proper resonating antenna with a reasonable radiation length. Remember with a dummy load practically all the energy goes into resistive heating of the resistor rather than being transmitted, and it is physically small.

Right. That's a long way of saying that they don't make any sense  ;)
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on November 23, 2016, 04:11:25 PM
Quote from: joelucid
they don't make any sense

Oh, I beg to differ here.

A purely resistive 50 ohm dummy load would have superior impedance matching to the radio over any of the commercial or home brew antennas for these radios. If I'm bench-testing between two or more radios the resistor is also a far more compact solution and more than adequate to test comms in close quarters.

With an in-line rf power meter, dummy loads are also the best option to verify the transmitter's output power.

They have their place, but they finish last when you're after an antenna system to achieve long-range comms.
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 23, 2016, 04:32:43 PM
Quote
A purely resistive 50 ohm dummy load would have superior impedance matching to the radio over any of the commercial or home brew antennas for these radios.

But what's the point? Sure 100% of tx power gets transmitted to the antenna and ... burned off as heat. No matter how bad your match with a real antenna - you'll always come out better than with a resistor. 100% of 0 is still 0.

But yeah they have their place - as long as its not sending or receiving rf  ;)
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on November 23, 2016, 04:58:27 PM
Oh, I beg to differ here.

A purely resistive 50 ohm dummy load would have superior impedance matching to the radio over any of the commercial or home brew antennas for these radios. If I'm bench-testing between two or more radios the resistor is also a far more compact solution and more than adequate to test comms in close quarters.

With an in-line rf power meter, dummy loads are also the best option to verify the transmitter's output power.

They have their place, but they finish last when you're after an antenna system to achieve long-range comms.

The resistor will have a finite size, also some inductance and capaitance. It'll act like a very, very low efficiency antenna with a very, very high loss as almost all of the power is converted into heat. The radios are very sensitive nowadays, so it will pick up and transmit some signals but its gain will be through the floor. You might on the bench be able to simulate receiving from a long distant transmitter,or it being a transmitter that is a long way away from its receiver when the devices are right next to each other, but for any other practical purpose they are useless as antennae.
Mark.
Title: Re: Small loop antennas @ 433 Mhz
Post by: captcha on November 23, 2016, 05:35:08 PM
Yep, couldn't agree more.

Instead of dummy load, I think a more appropriate name would be dummy antenna because as loads these resistors are quite effective.. :-)
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 25, 2016, 01:47:38 PM
While I'm waiting for the new round board with drills for the air trimmers and two independent loops I built a fully assembled version of the first two boards but with the trimmer instead of fixed caps:

(http://i.imgur.com/iOIUh4Y.jpg)

This works really well:

I wrote a little utility which sends packets at different frequencies across the spectrum and use the gateway to capture RSSI. That way I can adjust the trimmer without even attaching the VNA and with a fully live setup.

Both boards work well, close to the performance of Moteinos, Tinos etc. The rectangular model has smaller bandwidth than the round design. It might be the vias in the round one that degrade its Q and thus its performance. That had been a problem also when I measured it with the VNA. But it's still as good overall because the larger loop means more radiation resistance.

What's best is - and this is what I wanted to test with these boards - these loops are really incredibly stable to environmental changes. I've had them on wood, on the floor, on the balcony, in hand etc with very, very minor shift in resonant frequency.

For me this proves that these high Q loops are viable without active tuning if you use air trimmers.

Joe
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on November 26, 2016, 08:37:32 AM
@joelucid
nice!  Notionally speaking, could you use something like the following instead of the manual tuning variable capacitor?  http://www.mouser.com/ProductDetail/IXYS/NCD2100MTR/?qs=npTsUczJOtOwQXlqwUd7kA%3D%3D&gclid=Cj0KEQiAguXBBRCE_pbQ5reuq8MBEiQANji2LVV0erIGf3g5t223BxZMYxFCKXWrrXXcT5ZdMpY_Zi4aAoT28P8HAQ
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 26, 2016, 08:46:59 AM
Not really - these digitally adjustable ones have fairly low Q's and are fairly unstable. So efficiency would be worse and you'd have to have some auto-tuning mechanism to keep the resonance right in the face of temp changes.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on November 26, 2016, 07:04:38 PM
After you have a few of the same type assembled and tuned, I'd be curious to know whether the variable capacitors end up having the same settings or not.
Title: Re: Small loop antennas @ 433 Mhz
Post by: WhiteHare on November 27, 2016, 08:28:23 AM
Insead of a switched capacitor ladder, what if it was just a capacitor ladder where you cut traces (or perhaps burned out fuses) to zero in on the correct capacitance?  Would that also have inadequate Q?
Title: Re: Small loop antennas @ 433 Mhz
Post by: joelucid on November 27, 2016, 08:46:24 AM
Unfortunately from what I've read not very high Q either.
Title: Re: Small loop antennas @ 433 Mhz
Post by: perky on November 27, 2016, 09:48:25 AM
@WhiteHare Those digital switch capacitors unfortunately don't work well at UHF, the one you mentioned has large increase in capacitance even at 500MHz. The internals appear to become capacitive at higher frequencies.
Mark.