The techs have a full time job as QRM detectors (not their official title) - they were familiar with ham radio and have experience in tracking down pirate FM stations. We had an interesting conversation as we waited for the clock to hit 1400Z - the time I had coordinated the shutdown of the radar.
|Antenna input removed from receiver; spec-an now attached to the N-cable|
I had imagined that, with the radar down, they would bring out some sort of receiver with a directional antenna and find the direction to the source of the QRM. I had assumed that the source was more local than it turned out to be and that finding it would be accomplished with the equipment they'd brought with them.
And it was, but the first step in the procedure left me with a "why didn't I think of that?" feeling.
Once the radar transmitter was shut down, the FCC techs told me that they needed to attach their spectrum analyzer (an H600 RFHawk, an analyzer made specifically for tracking down illegal transmissions) to the radar's parabolic antenna. I patched them in to the proper place and they set the spec-an to sweep a 30 MHz wide swath of frequencies centered on the radar's operating frequency.
The result was essentially a C-band receiver with a panadapter/waterfall.
|Radar's antenna can be controlled via this terminal|
Per their request, I commanded the antenna to slowly rotate, half a degree/second. We watched the spec-an's display as the antenna approached the two bearings of interest and, sure enough, there were the pips that indicated reception of the devices operating on our frequency.
|The QRM (yellow)|
I stopped the antenna on each bearing and the FCC guys replaced the spec-an with a data analyzer that decoded the SSID and other info.
|And off they go...|
They then set out with a 5GHz feedhorn mounted to the top of their SUV and, two hours later, had located the offending transmitter - 47 miles away. The device has an output of only 50mW but our antenna has 50 dB gain and an antenna beamwidth of only half a degree. Compared to return echoes normally received by radars, this produced a potent signal.
All in all, a fun day.
If you've read this far, you may be interested in further info on this topic found here.