Author Topic: Small loop antennas @ 433 Mhz  (Read 36509 times)

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #30 on: June 01, 2016, 06:08:31 PM »
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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.

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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.

captcha

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Re: Small loop antennas @ 433 Mhz
« Reply #31 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.

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #32 on: June 01, 2016, 11:55:43 PM »
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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?

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #33 on: June 02, 2016, 12:45:19 AM »
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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.

Felix

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Re: Small loop antennas @ 433 Mhz
« Reply #34 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.

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #35 on: June 02, 2016, 04:30:47 PM »
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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?
« Last Edit: June 02, 2016, 04:46:49 PM by WhiteHare »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #36 on: June 02, 2016, 04:53:35 PM »
F = c / l. F stays the same, c decreases and l decreases in wet soil.
« Last Edit: June 02, 2016, 04:55:47 PM by joelucid »

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #37 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.

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #38 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.

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #39 on: June 03, 2016, 03:49:58 AM »
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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:



joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #40 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.



It's called the micro splatch (http://www.digikey.com/en/product-highlight/l/linx-technologies/microsplatch-chip-antennas).
« Last Edit: June 03, 2016, 06:16:46 AM by Joe Lucid »

Felix

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Re: Small loop antennas @ 433 Mhz
« Reply #41 on: June 03, 2016, 07:22:32 AM »
Thanks, I will try this out. It's pricey though for what it is.

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #42 on: June 03, 2016, 07:45:33 AM »
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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.

Felix

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Re: Small loop antennas @ 433 Mhz
« Reply #43 on: June 03, 2016, 08:35:50 AM »
Apparently they have a patent (pending) so that might not go so well :)

TomWS

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Re: Small loop antennas @ 433 Mhz
« Reply #44 on: June 03, 2016, 08:40:49 AM »
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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:


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.