I belong to a small group of transmitter hunters here on Long Island. My Elmer originally got me involved by having me navigate for him, and as I learned the technique, I graduated to running my own vehicle along with my better half, KD2CHE. The last few hunts, she has been driving while I watch the signal strength and maps.
Most of the hunters use doppler equipment, from various vendors. Jill and I have had great luck just using our maps, and a good assortment of receivers. We watch the signal strength on my best receiver, a Kenwood TR-9000, and then keep two HTs set to different levels of sensitivity. I hang the HTs in locations so that I can hear them come on. One with no antenna at all, and the other with a mismatched stubby antenna. This combination has gotten us to the bunny box before many of the doppler-equipped cars.
Friday night we had a hunt. The box was to be hidden within 3.5 miles from the Republic Airport in Farmingdale, NY. The box itself is managed by my Elmer, WA2CDL. It’s an old Radio Shack HTX-202, controlled by a custom PIC controller, powered by an external rechargeable battery, all contained in a surplus ammo box. We set out in our new Kia Soul, with me in the back seat. I folded half the back seat so as to have a kind of desk next to me. There I had the TR-9000 and a Dell laptop with the RTL-SDR setup, and Google Maps. Internet was provided by a hotspot on my T-Mobile HD7. We waited.
Unfortunately no one could hear the transmitter. It was mentioned that it might be operating on low power, since it had not been charged. So we drove around till we heard it. Each time it transmitted (30 seconds on, 30 seconds off), we would get a signal, and as we drove in the direction I though it would be in, the signal got weaker. As I plotted all of the vectors around the area where we heard the signal, it seemed that it was simply nowhere. Every time we chased the signal, it got weaker.
Eventually we were given a hint, and once found, I have to admire the location (about 15 feet off of a trail, in a nature preserve, up in a tree). Only some of the doppler-equipped cars were able to locate the box. As it turned out, my eternally fading signal was caused by the low battery. WA2CDL did some measuring after the hunt and told me that it was starting out transmitting just over a Watt, and finishing the 30 second transmission with about 250 Milliwatts. That explained why we always seemed to be heading away from it.
In practical use, what does this mean? If we were trying to find a lost hiker, for example, who had a radio with a dying battery, we would need to take the fading signal into account. I think if I had known that the signal was fading out each time we heard it due to the battery running low, I would have altered my technique. I think if we had simply moved at one minute intervals and marked the signal strength at each location just for the beginning of the transmission, I would have been able to plot the location. I guess I need to ask what the charge on the bunny box battery is in advance next time, or stay put for a couple of transmissions, and see if the signal fades.
How did the RTL-SDR do? Well, if we had steady signal, or if I had known to compensate, it would have been fantastic. I could see the signal before we could hear it, and signal strength was more visual. The battery quit before the end of the hunt though, because of the difficulty in finding the box. I hope to try it again.