Project Whack a Mole Part 2

I’ve been steadily working on this project so here is an update. You might like to review Part 1 which describes how this direction finding system works.

The good news is it works with real off-air radio signals! I could detect repeatable phase angles using two antennas with an RF signal, first in my office using a signal generator, then with a real signal from a local repeater. However the experimental set up was delicate and the software slow and cumbersome. So I’ve put some time into making the system easier to use and more robust.

New RF Head

I’ve built a new RF Head based on a NE602 active mixer:


The 32 kHz LO is on the RHS of the photo. Here is the saga of getting the 32kHz oscillator to run.

The mixer has an impedance of about 3000 ohms across it’s balanced inputs and outputs so I’ve coupled the 50 ohm signals with a single turn loop to make some sort of impedance match. The tuned circuits also give some selectivity. This is important as I am afraid the untuned HackRF front end will collapse with overload when I poke a real antenna up above the Adelaide Plains and it can see every signal on the VHF and UHF spectrum.

Antenna 1 (A1) is coupled using a tapped tuned circuit, and with the mixer output forms a 3 winding transformer. Overall gain for the A1 and A2 signals is about -6dB which is OK. The carrier feed through from the A2 mixer is 14dB down. Need to make sure this carrier feed through stays well down on A1 which is on the same frequency. Otherwise the DSP breaks – it assumes there is no carrier feed through. In practice the levels of A1 and A2 will bob about due to multipath, so some attenuation of A2 relative to A1 is a good idea.

Real Time-ish Software

I refactored the df_mixer.m Octave code to make it run faster and make repeated system calls to hackrf_transfer. So now it runs real time (ish); grabs a second of samples, does the DSP foo, plots, then repeats about once every 2 seconds. Much easier to see whats going on now, here it is working with a FM signal:

You can “view image” on your browser for a larger image. I really like my “propeller plot”. It’s a polar histogram of the angles the DSP comes up with. It has two “blades” due to the 180 degree ambiguity of the system. The propellor gets fatter with low SNR as there is more uncertainty, and thinner with higher SNR. It simultaneously tells me the angle and the quality of the angle. I think that’s a neat innovation.

Note the “Rx signal at SDR Input” plot. The signals we want are centered on 48kHz (A1), 16 and 80kHz (A2 mixer products). Above 80kHz you can see the higher order mixer products, more on that below.

Reflections

As per Part 1 the first step is a bench test. I used my sig gen to supply a test signal which I split and fed into A1 and A2. By adding a small length of transmission line (38mm of SMA adapters screwed together), I could induce known amounts of phase shift.

Only I was getting dud results, 10 degrees one way then 30 the other when I swapped the 38mm segment from A1 to A2. It should be symmetrical, same phase difference but opposite.

I thought about the A1 and A2 ports. It’s unlikely they are 50 ohms with my crude matching system. Maybe this is causing some mysterious reflections that are messing up the phase at each port? Wild guess but I inserted some 10dB SMA attenuators into A1 and A2 and it started working! I measured +/- 30 +/-1 degrees as I swapped the 38mm segment. Plugging 38mm into my spreadsheet the expected phase shift is 30.03 degrees. Yayyyyyyy…..

So I need to add some built-in termination impedance for each port, like a 6dB “pad”. Why are they called “pads” BTW?

The near-real time software and propeller plot made it really easy to see what was going on and I could see and avoid any silly errors. Visualisation helps.

Potential Problems

I can see some potential problems with this mixer based method for direction finding:

  1. If the spectrum is “busy” and other nearby channels are in use the mixer will plonk them right on top of our signals. Oh dear.
  2. The mixer has high order output products – at multiples of the LO (32, 64, 96 ….. kHz) away from the input frequency. So any strong signal some distance away could potentially be mixed into our pass band. A strong BPF and resonant antennas might help. Yet to see if this is a real problem.

Next Steps

Anyway, onward and upwards. I’ll add some “pads” to A1 and A2, then assemble the RF head with a couple of antennas so I can mount the whole thing outdoors on a mast.

Mark has given me a small beacon transmitter that I will use for local testing, before trying it on a repeater. If I get repeatable repeater-bearings (lol) I will take the system to mountain overlooking the city and see if it blows up with strong signals. Gold star if I can pull bearings off the repeater input as that’s where our elusive mole lives.

2 thoughts on “Project Whack a Mole Part 2”

  1. I think the pad term comes from acoustic insulation. You pad a wall with, for example, rockwool batts to attentuate the sound.

  2. My memory has it that a PAD was a fixed dB passive attenuator.

    If it wasn’t fixed, it wasn’t a PAD. Thus, you had a Step Attenuator box with toggle switches to switch in and out various 3 dB, and 10 dB PAD’s. Probably Marconi said to his assistant “Just pad the input of that amplifier with 6 dB and ship it!”

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