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|The e-mail & photos below are from John Kemper, W6JN,
who has been doing DF since the 1940s.
Retired from the FCC and FAA, he is now a consultant to FAA,
suppling custom DF equipment to FAA, FCC and other federal agencies.
His system includes yagi and loop antennas from Arrow Antenna.
Last Thursday, I gave the Fox Hunt presentation to the South Bay Amateur Radio Club SBARC W6SBA. It went very well. I think you might be interested in some of the happenings and the results, so heres a brief bit of the half-hour speech and demo.
After explaining all about dipoles, loops and yagis, I set up a demo to illustrate the points I had made.
I selected 5 persons at random from the audience, and arranged them in a close-order line. I gave one a small 144 MHz 50 milliwatt xmtr. I asked them all to fold their hands in front of them, and then turn their backs to me. This was so they all looked the same from the back. With my back turned so I could not see, I asked that one of the 5 hold the xmtr in his (her) hands, so that from the back, all would look the same with hands folded at their waists.
CConnecting the yagi to my DF system and turning around to face their backs, I was at the back of the meeting room, about 35 or so away. I asked the fox to start transmitting. Since the audience could not see the analog S-meter I use on the DF, I put the mode into BFO so all could hear the sound of the carrier. Using the yagi at the back of the room, I could only tell that the xmtr was ahead of me +/- 15 degrees or so. I slowly advanced toward the group until I got within about 10 feet. At that point, I could only tell that it was one of the five, since the nose was so broad. Much closer, and the yagi was of no help in spotting the actual fox it was just too broad.
I then repeated the procedure, using the DF and the UHF loop, since I was so close, and it added attenuation. But I had complete control of the receiver, down to zero, if I wanted it. I then walked slowly toward the group until I was just over an arms length from the line of people.
I placed the DF and the loop so the plane of the loop was parallel to the line of people. As I walked down the line, I had a deep and perfect null at the man with the white jacket. I did it twice so the audience could hear what I was hearing and seeing. I then asked him to turn around. He had the xmtr in his hands, and the audience burst out in applause. The loop had proved its great superiority over the yagi for close-in work.
Ive attached two pix that one of the members took during the demo. The first with the yagi is where I am about 20 away from the people, and the indication isnt getting much narrower as I approached them. The second is about 2 seconds before I asked the fox to turn around, as I was standing in a total and smooth null when I was in the middle of his back.