Foxhunt

Yesterday, RMHAM hosted their first big ARDF foxhunt in a while. I’ve run two foxhunts for high school students before, but this is the first city-scale foxhunt I’ve participated in.

In addition to the traditional tools of the trade, an HT, yagi and offset attenuator, I was using something special: a KrakenSDR. The receiver consists essentially of five RTL-SDR dongles (five sets of R820T2 and RTL2832U chips) tied to a signle clock source, which lets it calculate the direction of arrival (DOA) of signals. Instead of jumping out of the car and spinning around with the yagi, I can just drive around and let the Kraken sample every few seconds. Even if each sample is not quite as accurate, the Kraken can take many more samples and average them to get a more accurate result.

Yesterday morning as the event started, I felt good about my chances of being successful. When the fox started transmitting, I was able to get some pings and start driving generally towards it. I wish I’d had a separeate driver with me so I could focus on the Kraken, but as it was, I sort of noticed the Kraken’s predicted target was jumping around a little bit as the samples came in. After a couple of miles of driving I turned on the turn-by-turn navigation in the Kraken app and started following it. The pings kept getting stronger, and I was getting closer. Eventually I turned into a suburban neighborhood that was a rabbit-warren of criss-crossing roads; if I had been using the yagi I would have had a very hard time. But the Kraken knew where the fox was and the navigation was spot-on.

The fox was located at a house that was only obvious because of the antennas on the roof. I parked, rang the doorbell, and sure enough, this was the place! I was the first one to find the fox. John W0VG and Mark N0OHJ hadn’t even finished getting their coffee yet. I was pretty proud of myself.

Admittedly, this was designed to be an easy foxhunt. The fox was transmitting at 20W from a pretty high location, without any tricky terrain. Robert N0CFM talked about some of the foxhunts he’s run, and one in particular where he was transmitting from a low canyon and the signal was bouncing off a rock formation outside the search boundary. That would be a lot harder to find even with the Kraken. Still, have to start somewhere!