QRZ Logbook

Tuesday, June 15, 2010

Doppler-gangers

Due to the nature of the subject, I've omitted names and callsigns.

Over the last few weeks we've had several individuals causing malicious interference across several repeaters here in East Tennessee. One was caught a few weeks ago, and turned out to be a non-ham. Of the few others that have appeared, one has been very clever in hiding his (or her) signal quite well, but a few have been able to pick them up. While we have a few leads as to who the suspects are, we haven't singled any people out (yet!).

So, last night on my way home, I checked in to the local SKYWARN net and listened in to the gang wrapping up the net as the weather abated. However, as I was checking on some weather approaching from Nashville, we suddenly heard a dear carrier, covering up all signals.

My immediate reaction was that the NWS station had left the mic stuck in the "transmit" position. However, the NWS station chimed in on another repeater that they were not the cause. So the next natural suggestion was that we had a QRMer.

However, something told me it wasn't intentional. The normal interference we had heard was someone scratching a mic, retransmitting signals from HF, or the occasional bodily function. This was just silence. However, a couple of other hams were convinced it was intentional, because they'd been getting QRM recently.

I stepped up my drive home and arrived and put my home station into action. I swung the beams south and immediately got a spike in my signal strength (about S-7) to the southeast, towards Pigeon Forge. We'd had a couple of leads of QRMers from that area, so I started to think we had a bead on the signal from that general direction. However, another ham advised they had a strong signal near McGee-Tyson Airport, directly south of me. I swung my beams due south, and the signal shot to S-9, almost full quieting! Now, all I needed was another ham with a beam to triangulate a general location.

Unfortunately, no one else with a beam was listening, or wasn't hearing the signal.

I got on the repeater's output frequency and sent out a simplex transmission that we were actively searching for the signal. The repeater owner was listening and managed to shut the repeater off through a control link on another band, preventing damage to his repeater.

A DFer with a doppler unit was in the area, so I advised he head towards the airport. While the DFers were doing their manhunt, I got on the repeater's output frequency again and advised that I had the signal to the south and requested input from users who could hear the signal and give a direction or bearing. Unfortunately, no one was giving beam headings or any signal report. I gave out my phone number and a couple of hams called and gave me some signal strength reports, but again, not from a beam, just what they picked up on their HTs.

About 30 minutes into the situation, we new this was not intentional. No one with common sense would risk their equipment just to harass other hams, especially during a SKYWARN net. 3-4 DFers got in on the hunt, and they had bearings near the airport, but further south. As they got closer to the transmitter, I began looking up hams in the QRZ database who might be in the area. I would ask for street names to try and see if there was any hams who lived on particular streets. After they gave information on one street, I called out a ham's callsign and address, and they sped towards that position. Another ham knew they person and called him on the phone. It turned out he was not the QRMer. The DFer's briefly stayed, using the time to get another bearing before continuing on.

Suddenly, the signal developed noise. it had now been transmitting for over an hour and I figured the radio was beginning to overheat. It went from S-9 to S-7 on my radio's signal strength meter. Some thought the drop in signal was an indication it was mobile, but I didn't hear "picket fencing", indicative of a mobile station on the move.

As the DFers moved in closer, the signal suddenly dropped out. Was the radio dead? The hunters made comments that the radio must've died, but suddenly it reappeared, then cut out again! Suspicion arose. Was this intentional, and the guy was messing with us? Was he monitoring us and shutting off the transmitter the closer the hunters were getting?

Suddenly, it came on again. Then off, and on...it appeared to me that the transmitter was shutting itself off due to thermal protection. We knew we had to find it before the radio was permanently damaged, or worse, started a fire!

We were approaching 2 hours, when suddenly one of the DFers picked up a strong signal near the house of a well-known ham. We finally had our culprit.

The ham had gone out to tend to some business and had just plugged in an ICOM radio he had just acquired. His wife was home and allowed one of the DFers in to check the equipment, and the radio was finally turned off. It turned out the mic had a short in the wiring, and was shorted to transmit.

The radio itself was warm, but not terribly hot. The power supply, however, was able to fry an egg, from what I was told.

It was a successful "hunt" and with the terrain and occasional "kerchunk" from hams trying to figure out why the repeater wasn't on, the DFers were able to resolve the situation in good time.

It was a great exercise in direction finding. Hopefully the lessons learned will make the hunters better when it comes time to locate the actual QRMers on the repeaters.