Amplitude Comparison Monopulse with Pluto SDR

[mikeb1279]

Hi All,

Just thought I would share a project I have been fiddling with in case it’s interesting to anyone:

The TL;DR version: I’m building an amplitude monopulse system to try and be able to do an azimuth track on my DJI Mini 2 Drone.



I followed along with the video above by Jon Kraft at Analog systems, and thought it would be interesting to create a version that used amplitude monopulse, as opposed to phase comparison. I have set a goal to be able to get an azimuth track on the signal of my DJI Mini 2 from a range of 100+ metres in a low clutter/interference environment. It resulted in this:



And before anyone points it out – yes, I know the pieces of MDF the antennas are mounted on are different sizes. They were pieces of scrap and as long as the parts that mattered were square and the mounting holes were in the right places I was just going to roll with it. 

Now, important ‘parameters’ for an amplitude monopulse system:

Antenna angle offset/beam squint: 10 degrees - for an antenna with a HPBW of 80 degrees, this is a low offset. It is only 10 degrees as it was designed for an antenna with a HPBW of 50 degrees.
Antennas: Helical antennas. Modelled as Gaussian radiation pattern.
Frequency of Operation: We are tracking a drone, so will be between 2.4 and 2.5ghz

The antenna’s are just Helical Wifi antenna’s from an Aliexpress seller. I used the HP beamwidth supplied by the seller to start with (it was 50 degrees, which is why offset angle is only 10 degrees), but when I got strange results I went and ran it through a helical antenna calculator based on the ARRL Antenna Book. From this, I got a HP beamwidth of between 75 and 80 degrees which appeared to be much closer when I tested it. Unfortunately the wider beamwidth and lower gain mean a much flatter Gaussian curve, which makes the angular resolution much less stable. None the less, it should still at least be close - even if it isn’t stable.

Code will go up on my github sometime in the next couple of days here, because I don't think pasting it into EEVblog will work so well:

https://github.com/mikeb127

I used Grok to generate some of the code for the SDR - mostly just to see how much of it Grok got right. It did a reasonable job, but had to have a few goes at it and I still had to edit it in the end.

Test results so far:

I ran some tests with a HackRF One as the ‘signal’ at 2.395 GHz a few metres away from the monopulse assembly(all I could manage to get inside the house). Angles were as expected after changing HPBW but quite noisy/jittery. I don’t think the range will end up being quite what I was after (if I had to guess, I would say probably no more than 75 metres will work) – primarily due to what I think is the wide beam-width of the antennas.

Hopefully in the next fortnight or so, I will get to testing it with the DJI Mini 2 and get some more definitive test results.

Once I have it working as desired, I would like to try using different antennas - particularly wideband conical spiral antennas that are used in ARM seekers/EW systems (something similar to the image below). This would allow tracking of both 2.4ghz and 5.8ghz signals by simply changing the code to tune the SDR across different frequencies as opposed to any of the hardware:



If you managed to get this far, thanks for reading and hoping this triggers a discussion on microwaves and SDRs.

[mikeb1279]

Code is up:

https://github.com/mikeb127/amplitude_monopulse