Posts Tagged ‘direction finding’
As I’ve mentioned before, KD2CHE and I belong to a local transmitter hunting group. We get together one or two times a month. One member will hide the box and give us a general area, which is usually a radius from a point (i.e. 2 miles from the intersection of routes 1 and 2 in Anytown, NY). We are the only ones without doppler equipment, yet we almost always find the transmitter. Here’s some pointers for those of you that enjoy a bunny hunt every now and then, but may not think you can participate without sophisticated direction-finding equipment, or for those of you with dopplers that want to refine your techniques.
First of all, when I know the area, I print a map from the computer, and draw a circle for the hunting area. Then, using either our knowledge of the area, or a combination of Google Maps, and Bing Maps, I find a large building that I can drive all the way around, as close to the center as possible. I mark the spot on the map as our starting point. KD2CHE drives, while I navigate. When the time comes to leave for the location, I load up the equipment:
- The mobile in the car, for communicating with the other hunters, an Icom IC-207H
- An older Kenwood all-mode 2 meter mobile, with an RF-gain control, attached to an OPEK micro mag mount antenna
- A smartphone with Google Maps, or a laptop with a broadband connection
- Bright flash-lite
- 1 HT set to the third harmonic of the hunt frequency, usually my Icom IC-T90A with a good antenna
- 1 HT set to the hunt frequency with the antenna off, usually my Baofeng UV3R MkII
- 1 HT set to the hunt frequency for KD2CHE to use out of the car
Once the transmitter is activated, KD2CHE drives slowly around the building at the starting point, stopping when the transmitter stops, and starting up again when it comes back (the one we use is usually 30 seconds on / 30 seconds off). I mark the points in our loop where the signal was strongest and weakest, drawing a line with an arrow to get our initial vector. Then we navigate in that direction, using Google Maps as a guide. As the signal changes I mark the observed strength on the map. When we reach the point where the signal strength goes back down, we determine the high point, and KD2CHE drives as close to perpendicular to the original route as she can.
During this process, the RF gain control on the Kenwood comes in handy. The box puts out a full 5 Watts, so as you get close to it, an un-attenuated receiver becomes useless. For the signal readings with the RF gain all the way down, I write an ‘A’ in front of the s-meter reading on the map. As the strength goes up again, we keep an eye on the 2 HTs. When we’re within 1 or 2 blocks, one or both of them will become active. The third harmonic will usually not work until you are almost on top of the transmitter, which comes in handy. This is when we start looking for a good spot. In many cases there will be a public park, or area of some sort nearby, and the rest of the hunt is done on foot with the HTs, and the flash-lite if necessary.
Body shielding will get you a direction to walk in. Hold a radio that is getting a weak signal (the IC-T90A has a fixed-level attenuator I can activate) close in to your chest and slowly turn around, and make note again of the weak and strong points in the circle. Keep in mind though that sometimes, a good hider will put the box in a location that creates reflections and ghosts. Sometimes you just need to use logic, or in the case of our last hunt, KD2CHE found the box simply by looking, while everyone else was wandering around the woods with Yagis and other fancy equipment.
Of course it helps to know the area, and to have some insight into where people like to hide things. One of our hiders frequently hides in places he discovers while hiking. Another likes to hide in very unique, and sometimes questionable places. Once or twice we’ve had to explain to the authorities what we were doing.
I’m convinced that a doppler might enhance our abilities, but I’m afraid of relying on it too much. We actually have one, but it needs some work. We’ll see.
73! Neil W2NDG