RSSI Direction Finding Problems

Seitekful

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Feb 9, 2020
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Well I have reached the limits of my wisdom and now I seek advice among the professionals here in the Forums.

Let me quickly describe the project I'm working on: The goal is that a LoRa Receiver can detect the direction of addressed LoRa Signals (home in on the Transmitter basically).

My current Setup (See Picture 1) cosists of two breadboard-mounted RFM95's (868MHz) and two Arduinos (I use OLED displays to show RSSI and potentiometers to set TX-Power), powered by powerbanks each. To detect the transmitter direction I wanted to use the RSSI-reading in combination with an directional antenna/shielded antenna. The final setup should be much smaller (matchbox size). I tested the setup with two quarter wavelength antennas with great success (9 miles on flat ground through an light forest).

For testing I would place the transmitter (lets call her ALICE) on chest height in a light bush with the omnidirectional quarter Wavelength Wire antenna. She sends a addressed signal to the receiver (called BOB), which picks up that Message, takes the RSSI and sends an Reply to ALICE. The RSSI of Bobs message is then send back to BOB. On Bobs screen I then can read out the RSSI of Alices Message and the RSSI of the Message Bob send to Alice. I thought that by using a directional antenna on BOB or shielding BOBs antenna I could see a significant spike of RSSI in a certain direction.

I tested Loop-Antennas and V-Antennas with no success. Then I though that I could shield 3/4 of an omnidirectional Antenna with copper plate it would kinda create a directional beam when sending or a "sensitive" direction when receiving (which just shows how little I know about RF). I have used a couple of different shield shapes and sizes. It was very confusing, sometimes I got the desired result, and sometimes I would get the opposite (better RSSI with hole facing other way). Most of the time the signal just got worse in all directions, sometimes better when I touched the shielding sometimes even worse when touching...

So here are my some of questions:


1.) could the breadboard setup with all these cables and stuff be causing problems and reflecting waves? Would remote mounting the antenna with coax cable help?

2.) Is the shielding even working when not connected to a ground? Is grounding even possible on battery powered devices. (I have some ceramic capacitors to connect in_between ground and shield, would that help?

3.) I can't get my head around the near-field interactions... Does a shield so close to the antenna even work? The wave is much bigger than the hole...

4.) Is there a way to almost completely shield of some part of incoming transmissions? Like a really super strong shield would make it possible to detect nulls... Is it helpful to have multiple layers of plate (isolated to each other) or static charge the plate?? How could a possible shield be looking like?

5.) Any ideas on a Highly directional antenna in that small size? Yagis would be to big, can I just shrink them? I don't care for overall gain loss if I get a good directivity... Horn antennas? I heard the are only useful for >3GHz.


Any help and suggestions would be greatly appreachiated! Also please feel free to ask any questions
Thank you!

PS: I read up on the subject with: "Small-Aperture Radio Direction-Finding" by Herdon H. Jenkins and "Electromagentic Theory and Design" by P.A. Chatterton. The first one is very good but all about Amplitude/Phase/Time detection and the later is very technical and i was not able to comprehend the shielding Applications fully...


Picture 1:
(the two Breadboards)
IMG_20200209_123050.jpg

Picture 2:
(helical antenna in the RFM95)
IMG_20200209_123138.jpg

Picture 3:
(Used groundplate, not sure if effective at all)
IMG_20200209_125857.jpg

Picture 4:
(Shielded antenna with insulation between)
IMG_20200209_125950.jpg

Picture 5:
(Some shields i tested, the big plate was sometimes formed to diffrent shapes)
IMG_20200209_145037.jpg
 

jonwienke

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The setup you're using will not give you any directionality. The shield needs to be 1/4-wavelength ftom the vertical to work. Putting it right next to the vertical element like you did will just bork reception in all directions. Or better yet, do a parabolic curve on your shield with the antenna at the focal point.
 

Seitekful

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Feb 9, 2020
Messages
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@jonwienke Thank you for your helpful insight. So in my case, due to size limitations, it would not be possible to shield 270° and leave a 90° slit. But the other way around could work, by having a small shield which would block the signal when pointing to the source, correct? What would be an appropriate surface area/shape to perform such shielding, do you know?
Also would maybe a diffrent antenna shape result in smaller distance for shields?

@WA0CBW Also thank you Bill for pointing that out, you are correct. I tried to find an open field with little manmade structures around.
 

prcguy

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You can't make a directional antenna that would give you any better than maybe +/- 15 degrees of directionality if you could call it that and the point of peak signal will be very broad. This would be with a Yagi or corner reflector with 2 wavelength long sides, both very large.

Direction finding usually does not go for peak signal but a null instead because its easy to make an antenna with a deep sharp null. The problem with a null is you need good signal strength so you can still see some of the target signal deep into the null to make good use of its narrow beam when nulling.

Another approach would be multiple 1/4 wave antennas and you rapidly switch between them and use timing and other stuff but that gets complicated real fast.
 

jonwienke

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Also would maybe a diffrent antenna shape result in smaller distance for shields?
No. Distance is dictated by wavelength. You can coil up an antenna element to make it fit in a smaller area, but you can't coil up the space between elements.
 

StaticDischarge

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Back in the day I used a loop to find and "fix" ole "CB Savage"... Was fun, stealth and I learned a lot about direction finding/triangulation without having the expense of the multiple antenna "doppler" equipment as pcrguy is mentioning..
Anyway, I did a little research and came up with this: FHL-UHF loop which may be of some interest to you?? well at least a good read if nothing else...
 
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