Author Topic: What is the smallest antenna that still performs well?  (Read 8794 times)

WhiteHare

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What is the smallest antenna that still performs well?
« on: October 16, 2016, 06:57:15 PM »
As there has been a lot of discussion about antennas lately, I'm wondering if anyone has played around with an inverted F antenna?  I notice in the wikipedia article that there's one used in DECT phones that looks pretty simple and relatively small:

Since DECT phones run around 900Mhz, it would seem to be extremely relevant to those of us running at 915Mhz.  DECT 6.0 is a different frequency altogether (1.9Ghz)

There are also inverted F antenna's that can be traced on PCB, with the advantage being that they're a bit more compact (as in Figure B below) than just straight traces:


[Edit #1: here's a simplified guide I found on how to do a trace antenna:  http://colinkarpfinger.com/blog/2010/the-dropouts-guide-to-antenna-design/
Unfortunately, his example is for 2.4Ghz, so it would have to be adapted somewhat.  It does seem to indicate, though, that I would need a network analyzer, which I can't really cost justify: I could buy a lifetime supply of splatch antennas for the cost of one of one network analyzer.   ::)]

[Edit #2: On the other hand, TI does have a Design Nuide for how to do a 915Mhz Inverted F-antenna:  http://www.ti.com/lit/an/swra228c/swra228c.pdf
and so perhaps that can be more readily adapted.  In fact, it gives the exact dimensions of the antenna trace that would be needed.  Further, it states in the conclusion, "Measurements of reflection show that the center frequency is easily adjusted by trimming the antenna length."  The part that I'm puzzled over though is exactly what all those little discrete components need to be, and their values, between the radio and the antenna. Is there a similar way to wing it when it comes to those?]

[Edit #3: If all else fails, I'm guessing that some kind of folded monopole or meandering monopole antenna should be possible (as briefly outlined in http://www.atmel.com/Images/Atmel-42332-ISM-Band-Antenna-Reference-Design_Application-Note_AT09567.pdf) by making the trace longer than it needs to be and then shortening it a bit at a time until the sweet spot is reached.  It may not be perfect, but perhaps it would work good enough (?). ]

[Edit #4: I just found a Design Note for a chip antenna, and on its face it sounds quite good: PCB footprint is just 8mm x 6mm in sizehttp://www.ti.com/lit/an/swra160b/swra160b.pdf
915Mhz Johanson chip atennas cost $0.92 each on Digikey in quantity 1.  On the other hand, a uSplatch antenna is 9.14mm x 12.7mm but apparently can be mounted vertically if so desired to help keep a small footprint.  The regular splatch antenna is 13.7mm x 27.94mm.  Is a chip antenna the smallest antenna that still performs reasonably well?]
« Last Edit: October 17, 2016, 08:32:24 AM by WhiteHare »

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Re: What is the smallest antenna that still performs well?
« Reply #1 on: October 18, 2016, 05:26:58 PM »
Okay, this may be a bit silly, but this 'antenna' worked for me.



It's two 51ohm resistors in parallel.

It closely matches the radio's 50 ohm load impedance and has a short enough range for me to test signals on the edge of the reception horizon. This way I don't have to separate tranmiter and receiver by hundreds of meters in order to simulate intermittent loss of signal.

Why 2x 51 ohm resistrors in parallel? Isn't that half the value that's required?

Well.. it depends. These particular resistors are metal-film and if you look at how they are made you'll see that they are basically a spiral cut out of a film-cylinder by a laser etch. The end result is that you end up with a resistor that looks like a coil (and behaves like one!). At DC you'll measure the expected normal resistance, but at rf things look very different.

I don't have the plot at hand anymore, but when I put a single 50 ohm metal-film resistor on the VNA I was seeing an impedance of around 100ohms at 433 MHz. This is purely due to the inductor effect which is not negligible with these types of resistors:



So if they were 100ohms at 433MHz, simply adding another one in parallel would half it back to around 50 ohms.

There you go, dummy load with minimal range (about 3 meters).

If you're planning to make your own dummy load, try to find carbon resistors, as they are not inductive as these metal-film resistors. That way you'll only need 1x 50 ohm resistor.  I think most SMD resistors are still carbon based so they would be a better candidate.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #2 on: October 18, 2016, 06:13:08 PM »
Great post! 

I tried something similar once, but just for receiving:

https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/real-time-rssi-measurement-broken-rfm69cw/msg10960/#msg10960
In retrospect, we probably overlooked the inductive qualities of that resistor that your post so eloquently explains.

Ironically, our goal at the time wasn't to make an antenna but rather to not receive anything at all.  Consequently, Emjay later had me solder tack antenna directly to ground, but even then it still received packets!  In both cases, I was operating in Rx only.

How many ohms should be used at 915Mhz if I want to Tx using a carbon resistor antenna?  Does the length of the wire leads on either side of the resistor significantly affect the range?
« Last Edit: October 18, 2016, 06:30:27 PM by WhiteHare »

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Re: What is the smallest antenna that still performs well?
« Reply #3 on: October 18, 2016, 06:46:15 PM »
Hah, great to see you did the same! :-)

It's amazing that rf is leaked (and picked up) so easily. At home I'm experimenting with RFM12Bs (output power 1mW / 0dBm). If I put a quarter-wave vertical at one end and use the dummy load as pictured, I get a range of easily 30 meters. Whoa!... Heheh..

I would say carbon resistors can just be chosen with the intended value. I don't think they will do much at rf. However, you mention lead length. Yes, that would contribute some, but without a VNA it's hard to say. I would still just use one 50R carbon resistor and go with that. The radio is pretty forgiving. If you want to go smaller, 1206 smd is very easily hand-solderable and will minimise lead length even further.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #4 on: October 18, 2016, 08:09:50 PM »
As luck would have it, I just today received from China a giant sample book of 0805 SMD resistors, all nicely sorted by ohms.  It doesn't have a 50 ohm resistor, but it does have a 47 or a 51 ohm resistor.  It claims both are +-1% accurate.  I could give it a try if you think transmitting would work with one or the other soldered between Tx and GND on the RFM69HW module.  If it worked satisfactorily, then you're right: it probably would be the smallest antenna possible.
« Last Edit: October 18, 2016, 08:12:36 PM by WhiteHare »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #5 on: October 18, 2016, 08:30:57 PM »
As luck would have it, I just today received from China a giant sample book of 0805 SMD resistors, all nicely sorted by ohms.  It doesn't have a 50 ohm resistor, but it does have a 47 or a 51 ohm resistor.  It claims both are +-1% accurate.  I could give it a try if you think transmitting would work with one or the other soldered between Tx and GND on the RFM69HW module.  If it worked satisfactorily, then you're right: it probably would be the smallest antenna possible.

BTW, which range is it that you got?  Your first post said 3 meters, but your second post said 30 meters.

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Re: What is the smallest antenna that still performs well?
« Reply #6 on: October 18, 2016, 09:54:50 PM »
If I use the dummy loads on both the transmitter and receiver I get about 3 meters.

If I change one of them to be a proper antenna the range jumps to about 30 meters (and go through a couple wooden/fibro walls).

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #7 on: October 18, 2016, 10:53:03 PM »
If I use the dummy loads on both the transmitter and receiver I get about 3 meters.

If I change one of them to be a proper antenna the range jumps to about 30 meters (and go through a couple wooden/fibro walls).

Thanks for the clarification.  In that case, this sounds very promising!  Hopefully, upgrading from quarter wave antenna to a dipole antenna on the one transceiver and keeping the resistor antenna on the other transceiver will prove more than adequate as a wireless link in a home environment.  I'll give it a try with an SMD resistor antenna and afterward post how it went.  I'll use a 51 ohms SMD, since that's closer to 50 ohms than 47 ohms is.

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Re: What is the smallest antenna that still performs well?
« Reply #8 on: October 19, 2016, 06:56:37 AM »
Quote
I'll use a 51 ohms SMD

Yeah, that should work just fine. You may get a lot better performance as a dummy load that way (read: almost no range) if you can minimise the lead length.

On the topic of metal film resistors.. If you still want to make a dummy load with metal film resistors you can certainly do that but you'll have to parallel as many as you can. Reason being that when you put these metal film resistors in parallel their total inductance goes down. A common recipe for a home-brew 5W dummy load is to parallel 20x 1k/0.25W resistors.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #9 on: October 19, 2016, 07:54:23 AM »
Quote
I'll use a 51 ohms SMD

Yeah, that should work just fine. You may get a lot better performance as a dummy load that way (read: almost no range) if you can minimise the lead length.


But wait, here I'm now trying to maximize range, not get almost no range.  So, for that purpose, are you saying I would be better off with the thru-hole carbon resistors, since they do have some lead length (unlike the SMD)?

[Edit: looking into it more deeply, if it's the thru-hole type, then apparently what I would need is the "carbon composition" type resistor.  The carbon film resistors would have the same helical spiral as the metal film resistors you highlighted.  It seems they also have lower tolerances (more like 10% maybe).]
« Last Edit: October 19, 2016, 08:31:08 AM by WhiteHare »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #10 on: October 19, 2016, 10:20:16 AM »
Success!  I replaced the wire antenna on a Moteino with a 51 ohm SMD resistor (see attached photo), and it transmits fine.  A quick eyeballing and comparison of RSSI figures as compared to a typical monopole wire antenna suggests about a 22dB penalty, give or take.  Upgrading to a dipole antenna on a gateway would offset much of that, but not all of it.

Considering the minuscule footprint and minuscule cost, this seems like it could be a winner for use in low-cost ultra-compact wireless nodes.   :)
« Last Edit: October 19, 2016, 10:26:20 AM by WhiteHare »

Felix

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Re: What is the smallest antenna that still performs well?
« Reply #11 on: October 19, 2016, 11:27:22 AM »
@WhiteHare You just invented the world's smallest antenna, patent it before someone else does :D
Why didn't you just solder the resistor straight between the RFM module pads?

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #12 on: October 19, 2016, 11:55:43 AM »

Why didn't you just solder the resistor straight between the RFM module pads?

Would that work even better?  I'm wide open and welcome any and all suggestions. 

« Last Edit: October 19, 2016, 12:17:26 PM by WhiteHare »

Felix

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Re: What is the smallest antenna that still performs well?
« Reply #13 on: October 19, 2016, 12:08:49 PM »

Why didn't you just solder the resistor straight between the RFM module pads?

Would that work even better?  I'm wide open and welcome any and all suggestions.
Hmm, technically it should but perhaps the extra "brace" to the pad and ANT pin hole would act as a divider and produce some reflection? Maybe it's too short to really matter. But I was more hinting at soldering directly between the RFM pinholes to avoid the solder avalanche.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #14 on: October 19, 2016, 12:16:03 PM »
Hopefully others will now try this, and we can pool our datapoints to collectively figure out what works best.   :)

[Edit: I'm still confused from the earlier thread discussion as to whether longer leads are good or bad, so this was just a compromise for a first try it out.

I suppose this "SMD resistor antenna" could be especially practical on a LoRa module.  That thing has so much potential link budget that for a home environment you could easily workaround the antenna impairment and still have rock solid wireless links everywhere in the home. ]

In any event, I look forward to comparing a uSplatch antenna to it.  I should think a uSplatch would pretty much necessarily outperform it, because otherwise who would buy them?

In the spirit of KISS, my near-term goal is to fit everything, including 2x AAA batteries, into a 4x AAA battery holder case like this:

https://www.aliexpress.com/item/New-Plastic-4-AAA-Battery-Srorage-Box-Holder-4pcs-AAA-Batteries-Case-with-a-Lid-ON/32579574841.html?spm=2114.30010308.3.2.l7ZbuM&ws_ab_test=searchweb0_0,searchweb201602_5_10065_10068_10069_10017_10080_10082_10081_10060_10061_10062_10056_10055_10037_10054_10059_10032_10078_10079_10077_10073_10070_421_420_10052_10053_10050_10051,searchweb201603_4&btsid=53ae9427-799a-435b-9575-d6f3fba7ec42

I hope to fit a wireless node inside the case along with 2x AAA batteries positioned to produce 3V.  The case already has an exposed switch to turn it on/off. Total case size is relatively small, around: 6.2 x 4.8 x 1.5cm / 2.4" x 1.9" x 0.6"(LWH)  I think it will have enough space to fit a uSplatch in there, but if not this SMD resistor antenna is my fallback.  There's a good chance the new Moteino with the PCB antenna might fit nicely as well.
« Last Edit: October 19, 2016, 03:21:37 PM by Felix »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #15 on: October 19, 2016, 05:36:26 PM »
I didn't realize until just now that a 3V CR2477 has around 1000mah capacity, which is more than most AAA's.  So, I may a tabbed one and go that route:


That would keep the pressure on to keep the antenna as small as possible, so its more and more likely I'll end up using an SMD resistor antenna after all.   :) 

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Re: What is the smallest antenna that still performs well?
« Reply #16 on: October 19, 2016, 05:39:20 PM »
Quote
I'm now trying to maximize range, not get almost no range.  So, for that purpose, are you saying I would be better off with the thru-hole carbon resistors, since they do have some lead length (unlike the SMD)?

Correct, I think it's the small lead length on the through-hole resistor that still acts as a radiator. Using SMD would make the radiation effects much smaller.

If you look at proper dummy loads, you'll see there's no 'leads' to speak of; it's all encapsulated:


50-ohm SMA dummy load


Quote
The carbon film resistors would have the same helical spiral as the metal film resistors you highlighted.

I'm not sure where the carbon comes into play with 'carbon film' resistors, but it's the spiraled laser-cut 'film' bit that turns the resistor into an inductor.

Quote
51 ohm SMD resistor ..., and it transmits fine.

Don't forget that by simply matching the impedances you have also maximised the output power. This is why it's generally a good idea to look at impedance matching after you have a resonant antenna.. :-)

Quote
I suppose this "SMD resistor antenna" could be especially practical on a LoRa module.  That thing has so much potential link budget that for a home environment you could easily workaround the antenna impairment and still have rock solid wireless links everywhere in the home. ]

Don't expect these dummy loads to be a very good antenna though. Their radiation pattern is chaotic at best and may vary greatly from device to device. But, yeah, if you have a strong enough transmitter and a decent antenna for the base station you may very well use this technique to shrink the footprint of your motes around the house. You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

Quote
my near-term goal is to fit everything, including 2x AAA batteries, into a 4x AAA battery holder case like this

Just one word of caution. I've heard that black colour in plastics is often achieved by adding carbon to the material. This could adversely influence your signal strength.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #17 on: October 19, 2016, 06:31:38 PM »
You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

Well, if it's only one or two cm, then perhaps it makes sense to add that as a trace to the PCB?  It wouldn't require much PCB real-estate.  Or is that venturing too far in the direction of then needing to do impedance matching?  I just very recently started designing my own PCB's and ordering them from osh-park, so I could add antenna traces easily enough, but I'm not equipped to do impedance matching.
« Last Edit: October 19, 2016, 06:36:33 PM by WhiteHare »

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Re: What is the smallest antenna that still performs well?
« Reply #18 on: October 19, 2016, 08:36:41 PM »
There are some PCB-trace impedance calculators on the net that will give you an idea what width tracks you'd need (and separation from underlying GND tracks) to come up with 50 ohms impedance.

Something like this:
https://www.eeweb.com/toolbox/microstrip-impedance

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #19 on: October 19, 2016, 09:12:39 PM »
Thanks!  Maybe it's worth a shot:  it doesn't have to be perfect;  it just has to be better than a dummy load "antenna".   ;)

Felix

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Re: What is the smallest antenna that still performs well?
« Reply #20 on: October 20, 2016, 12:57:49 PM »
RE this 1Ah CR2477:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129
IMHO the cost is prohibitive given the capacity.



How about the smaller 620mAh battery, at less than half the cost:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2450-H1AN/P662-ND/2404067

These have a variable influence on the small antenna performance since the board+antenna (most we've seen in the forum) is sandwiched somewhere in between the RFM and this type of battery. Would be great to see a compare between such batteries.

john4444

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Re: What is the smallest antenna that still performs well?
« Reply #21 on: October 20, 2016, 02:49:19 PM »
@ WhiteHare, in general, what kind of usable range do you experience using the SMD resistor for an antenna?
I'm curious to learn how the RSSI level compares to the wire antenna at various distances.
I have all of my Moteinos in use at the moment or I would try it my self.
John AE5HQ

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #22 on: October 20, 2016, 02:51:30 PM »
RE this 1Ah CR2477:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129
IMHO the cost is prohibitive given the capacity.



How about the smaller 620mAh battery, at less than half the cost:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2450-H1AN/P662-ND/2404067

These have a variable influence on the small antenna performance since the board+antenna (most we've seen in the forum) is sandwiched somewhere in between the RFM and this type of battery. Would be great to see a compare between such batteries.
Thanks for the links.  Those are all good points you make.  I simply looked on Wikipedia to find the button cell with the most mah, and apparently it's the CR2477.  So, there's that, and also the hope that it won't have as much voltage droop as the thinner button cell that Joe and Tom have been experimenting with and commenting about.  Not sure if that's a realistic hope or not though.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #23 on: October 20, 2016, 03:28:47 PM »
@ WhiteHare, in general, what kind of usable range do you experience using the SMD resistor for an antenna?
I'm curious to learn how the RSSI level compares to the wire antenna at various distances.
I have all of my Moteinos in use at the moment or I would try it my self.

I'll give you the range in my house, but it wouldn't necessarily translate to yours.  That's why I expressed it as the amount of RSSI impairment, which should be about the same, and which you could interpret for your own environment.

That said, I live in two story, roughly 4,000 sf house made using commonplace wood stick construction.  If I put a regular moteino gateway with quarter wave antenna in a corner of the lower floor and I then put the impaired node (i.e. the one running the dummy load resistive antenna pictured above) in the opposite diagonal corner on the second story (which I assume to be a worst case scenario, or close to it), I do receive packets, but with a lot of losses.  I suspect I could improve or worsen that by how I orientate the node, but that's what I got just from plunking it down without any thought.  Also, those results are from running Felix's generic gateway and node sketches.  If I were to change the firmware to use a lower OTA bitrate and narrower bandwidth, and/or possibly increase the transmit power (I'm forgetting now what Felix chose as the default power Tx level), I'm sure it would do better.  Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.

[Edit: In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it. ]
« Last Edit: October 20, 2016, 05:19:59 PM by WhiteHare »

john4444

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Re: What is the smallest antenna that still performs well?
« Reply #24 on: October 20, 2016, 05:34:17 PM »
@ WhiteHare, You are right that my experience would be different than yours.
Ideally a person would have two units side-by side for the comparison.
But, your last comment is especially interesting to me.
Quote
In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it.
For short range or initial testing - the resistor sounds like a good trick.
John AE5HQ

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Re: What is the smallest antenna that still performs well?
« Reply #25 on: October 20, 2016, 05:58:50 PM »
Here we go, my 50th post on this site.. :-)

Quote
I do receive packets

Fantastic news! Real-world data! :-) I really think this may be an eye opener for people who really want to miniaturise their motes and don't have a lot of distance to cover.

Quote
Even as is, though, it's doing far better than an NRF24L01+ module would.

And that would have everything to with those NRF24L01+ modules working on 2.4GHz.

The lower you go in frequency the higher the level of signal penetration trough objects like wood, chipboard, stone, etc.. As nice as small antennas for 915MHz may be I will stick to 433MHz just for that reason (well.. it also has something to do with the legality of it all of course).

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #26 on: October 20, 2016, 06:46:10 PM »
I do think there's a role it might play.  For instance, my wife has a lot of potted plants, and I'd be interested in making some sensors to monitor each one (i.e. each potted plant gets its own wireless sensor node).  In this case, she would want the sensors to be as small and discrete as possible, and to make any sense it would need to be inexpensive.  So, that might mean running from a button cell and having no visible antenna.  Maybe there are better ways, but a resistor antenna might possibly be a good fit for that.  If it turns out that range is an issue, I could always add another cheap wireless gateway that's within range, but not visible (perhaps in a closet or cabinet or something like that).

Just one word of caution. I've heard that black colour in plastics is often achieved by adding carbon to the material. This could adversely influence your signal strength.

I hadn't heard before that carbon might be a problem, but after reading your warning, I'll try it both with and without the cover.

My inspiration for the idea came from this:
http://johan.kanflo.com/the-aaduino/
so if it were a monster problem, I'm guessing he would have run into it and written about it, but... maybe not.

You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

Yes,  I wouldn't be surprised if it led to at least some improvement.
« Last Edit: October 21, 2016, 09:33:45 AM by Felix »

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Re: What is the smallest antenna that still performs well?
« Reply #27 on: October 20, 2016, 09:52:45 PM »
AAduino..., and I thought it had something to do with monitoring alcohol consumption before clicking the link, hahaha.. ;-)

But yeah, what a nifty idea..

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #28 on: October 20, 2016, 10:45:29 PM »
I'll give you the range in my house, but it wouldn't necessarily translate to yours.  That's why I expressed it as the amount of RSSI impairment, which should be about the same, and which you could interpret for your own environment.

That said, I live in two story, roughly 4,000 sf house made using commonplace wood stick construction.  If I put a regular moteino gateway with quarter wave antenna in a corner of the lower floor and I then put the impaired node (i.e. the one running the dummy load resistive antenna pictured above) in the opposite diagonal corner on the second story (which I assume to be a worst case scenario, or close to it), I do receive packets, but with a lot of losses.  I suspect I could improve or worsen that by how I orientate the node, but that's what I got just from plunking it down without any thought.  Also, those results are from running Felix's generic gateway and node sketches.  If I were to change the firmware to use a lower OTA bitrate and narrower bandwidth, and/or possibly increase the transmit power (I'm forgetting now what Felix chose as the default power Tx level), I'm sure it would do better.  Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.

[Edit: In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it. ]

I just now re-did the above experiment, but this time using a uSplatch antenna in a vertical orientation.  Wow, what a difference!  This time it looked as though the gateway received every packet the node sent, even in the above "worst case" scenario.  The reason was clear:  received RSSI at the gateway looked to be about 26dB better than with just the SMD resistor antenna.  Because of the uSplatch Felix's demo sketches had plenty of link budget to work within.

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Re: What is the smallest antenna that still performs well?
« Reply #29 on: October 21, 2016, 09:41:15 AM »
I just now re-did the above experiment, but this time using a uSplatch antenna in a vertical orientation.  Wow, what a difference!  This time it looked as though the gateway received every packet the node sent, even in the above "worst case" scenario.  The reason was clear:  received RSSI at the gateway looked to be about 26dB better than with just the SMD resistor antenna.  Because of the uSplatch Felix's demo sketches had plenty of link budget to work within.
Very nice, I will make a note to buy and try this uSplatch. Was this test at 915mhz?

Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.
Aww, that's why I love sub-ghz FSK :), it excels where 2.4ghz fails.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #30 on: October 21, 2016, 10:01:46 AM »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #31 on: October 28, 2016, 01:24:53 PM »
Regarding the SMD resistor antenna, one caution is that section 3.3.7 High Power Settings of the datasheet says, "The Duty Cycle of transmission at +20dBm is limited to 1%, with a maximum VSWR of 3:1 at antenna port, over the standard operating range [-40;+85C]. For any other operating condition, contact your Semtech representative."

[Edit: Also, looking now at the RFM69 library I notice that the transmit power settings are fully maxed out by default if using an RFM69HW. ]
« Last Edit: October 28, 2016, 01:56:32 PM by WhiteHare »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #32 on: November 23, 2016, 10:51:51 AM »
RE this 1Ah CR2477:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129
IMHO the cost is prohibitive given the capacity.



How about the smaller 620mAh battery, at less than half the cost:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2450-H1AN/P662-ND/2404067

These have a variable influence on the small antenna performance since the board+antenna (most we've seen in the forum) is sandwiched somewhere in between the RFM and this type of battery. Would be great to see a compare between such batteries.

I had ordered some tabbed CR2477 batteries from China, but when they arrived, they weren't tabbed.   :o  So, I took your suggestion and ordered three tabbed CR2450's from Digikey.  Shortly afterward, I got a call from Digikey saying they can't ship them by regular mail because their lithium batteries contain, uh, lithium.   ::)   So, I ordered them anyways, but now it's costing me about $10 in FedEx ground delivery charges, or about 3x what it would have been if USPS didn't prohibit shipment of lithium coincell batteries.  Seems ridiculous to me at many levels, but there you have it.  Well, whenever they do finally arrive, at least this time I expect they will actually be tabbed.
« Last Edit: November 23, 2016, 11:01:33 AM by WhiteHare »

joelucid

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Re: What is the smallest antenna that still performs well?
« Reply #33 on: November 23, 2016, 01:20:28 PM »
I use exactly these batteries - the panasonic 2032 and 2450 with tabs. They work well. For shipments to Germany digikey offers free Fedex express shipping starting at $70 or so. So I tend to just add stuff in to qualify for the free shipping (which is otherwise outrageously expensive.)

Joe

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Re: What is the smallest antenna that still performs well?
« Reply #34 on: February 03, 2017, 04:25:51 PM »
This is pretty neat! For projects not too far away, this SMD resistor hack somehow 'magically' works. Nice find, WhiteHare!

I found this SMD chip antenna: http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey0915AT43A0026E which are about $1 each. It has some mounting considerations for PCB in its datasheet (screenshots attached if link clicking is too much of a hassle). Doesn't seem too complicated to work with, but without a network analyzer, pretty much designing in the blind. Has anyone tested performance of such chip antennas and this design?

I still think wire antennas are the cheapest solution that's most reliable, but when people start thinking about enclosures and stuff, alternatives like this may help, as long as it doesn't break the bank and add too much to the design.


WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #35 on: February 03, 2017, 07:27:26 PM »
Cool!  Looks like that chip antenna would be worth a try.

perky

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Re: What is the smallest antenna that still performs well?
« Reply #36 on: February 04, 2017, 05:49:06 AM »
Here's an interesting document. They are using 'crossed dipoles' on credit cards with random meandering to reduce the size. I'm a little confused as these appear to be fed from the same source, but they're labelled circularly polarized which normally requires one of the pairs to be phase delayed. Anyhoo the omnidirectional radiation plots seem too good to be true...

http://cearl.ee.psu.edu/projects/assets/Project3/Project3_3/Stochastic%20Antennas1.pdf

Mark.
« Last Edit: February 04, 2017, 05:53:56 AM by perky »

perky

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Re: What is the smallest antenna that still performs well?
« Reply #37 on: February 04, 2017, 06:09:27 AM »
And another interesting article about using two Splatch antennae to create a composite circularly polarized antenna:
http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1062&context=eesp

Mark.

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Re: What is the smallest antenna that still performs well?
« Reply #38 on: February 04, 2017, 07:10:36 AM »
Interesting approach to let genetic algorithms tune the bending optimizing for both size and symmetrical radiation.  Shame that is is still 43mm x 43mm in the end that is pretty gigantic compared to a mote and still gigantic compared to 2xAAA (approximately 47mm x 20mm).  I need something 10mm x 10mm for my idea though I'm not too worried if it is directional.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #39 on: February 04, 2017, 07:35:53 AM »

I found this SMD chip antenna: http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey0915AT43A0026E which are about $1 each. It has some mounting considerations for PCB in its datasheet (screenshots attached if link clicking is too much of a hassle).

By the way, I just now noticed that a very similar looking chip antenna is what's being used on some of the amplified NRF24L01's:




Also, something new to look forward to: "zero clearance" chip antennas that can mount directly above or below the metal layer on a PCB.  So far I've only seen it at 2.4Ghz (http://www.mouser.com/new/Johanson/johanson-2450A-mini-chip-antenna/), but maybe there exists a zero clearance chip antenna for 915mhz too?
« Last Edit: February 04, 2017, 11:15:57 AM by WhiteHare »

joelucid

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Re: What is the smallest antenna that still performs well?
« Reply #40 on: February 04, 2017, 08:42:37 AM »
Quote
Anyhoo the omnidirectional radiation plots seem too good to be true..

50% size reduction using meandering with no impact on gain at all does indeed sound too good to be true in my mind.

Then again check this out: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.936.8142&rep=rep1&type=pdf

80% effiency at 430 MHz using a 5.6 x 2.5 cm meander dipole. Scale down and use at 2.8 x 1.25 cm at 860 MHz? Sounds like magic - it has got to do with those ants.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #41 on: March 24, 2017, 01:28:17 PM »
This is pretty neat! For projects not too far away, this SMD resistor hack somehow 'magically' works. Nice find, WhiteHare!

I found this SMD chip antenna: http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkey0915AT43A0026E which are about $1 each. It has some mounting considerations for PCB in its datasheet (screenshots attached if link clicking is too much of a hassle). Doesn't seem too complicated to work with, but without a network analyzer, pretty much designing in the blind. Has anyone tested performance of such chip antennas and this design?

I still think wire antennas are the cheapest solution that's most reliable, but when people start thinking about enclosures and stuff, alternatives like this may help, as long as it doesn't break the bank and add too much to the design.

I was just about to buy some of these SMD chip antennas, but a closer reading of the datasheet revealed that it requires some detailed PCB layout, including a 50ohm feedline on the PCB (which I have no idea how to do). 

Has anyone tried it?

perky

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Re: What is the smallest antenna that still performs well?
« Reply #42 on: March 24, 2017, 03:37:52 PM »
Not tried it.

A transmission line like a coax has capacitance per unit length and inductance per unit length. It's characteristic impedance depends on those values. A similar construction can be done on a PCB by tracking a trace above a solid ground plane. Inductance per unit length is set by the track width, and capacitance per unit length is set by the distance between the track and the ground plane below it, the dielectric constant of the PCB, and the width of the track.

It just so happens to get 50R impedance with a 2 layer sided FR4 PCB you need the ratio between the width and the PCB thickness to be around 2:1, so for 1.6mm thick PCB you'll need a track 3.2mm wide. 50R is far easier to do with multilayer PCBs because the separation distances between signal and ground plane are much smaller, meaning sensible width tracks.

Mark.
« Last Edit: March 24, 2017, 03:39:58 PM by perky »

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #43 on: March 25, 2017, 07:45:29 AM »
Thanks!  That info might prove useful in the case of a Splatch antenna, where the feedline is assumed to be on one side and the ground plane is assumed to cover the opposite side of the PCB.  In the case of the Johanson Technology antenna, though, the DS  seems to assume that the feedline is co-planar with the ground plane--I guess so various inductors/capacitors can be bridging between the two.  Hmm..  All in all, this does seem to re-affirm that the Splatch is comparatively easy.
« Last Edit: March 25, 2017, 07:47:39 AM by WhiteHare »