Author Topic: Small loop antennas @ 433 Mhz  (Read 43236 times)

john4444

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Re: Small loop antennas @ 433 Mhz
« Reply #75 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

 
John AE5HQ

perky

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Re: Small loop antennas @ 433 Mhz
« Reply #76 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.

john4444

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Re: Small loop antennas @ 433 Mhz
« Reply #77 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 " 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
John AE5HQ

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Re: Small loop antennas @ 433 Mhz
« Reply #78 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.

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #79 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

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #80 on: October 21, 2016, 06:06:11 AM »
Second design arrived today:



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.



Joe

perky

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Re: Small loop antennas @ 433 Mhz
« Reply #81 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.
« Last Edit: October 21, 2016, 10:01:43 AM by Felix »

john4444

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Re: Small loop antennas @ 433 Mhz
« Reply #82 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.
John AE5HQ

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #83 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

 
« Last Edit: October 30, 2016, 06:45:09 AM by joelucid »

perky

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Re: Small loop antennas @ 433 Mhz
« Reply #84 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.

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #85 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

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #86 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? 
« Last Edit: October 30, 2016, 09:36:13 PM by WhiteHare »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #87 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%.

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #88 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....   ::)

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #89 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....
« Last Edit: October 31, 2016, 10:20:39 AM by WhiteHare »