NEC-LIST: DF for Search and Rescue

Dan:

You got some good advice about looking into the ham literature. My
philosophy is to try to use something that they are carrying anyhow
rather than adding to the weight burden of the searchers.

I did DF experiments in the jungle in Thailand in 1967. At VHF, when
you are in the forest, the angle of arrival you get with a loop's null
is equally distributed in angle, on the average. The signal appears to
arrive from the direction of the strongest local scatterer, and that
varies as one proceeds through the forest looking for the source. I
worked this problem to help a friend who had put radio collars on
monkeys, when this biological monitoring using RF was in its
infancy. I advised him to work on the average power, using a 3-element
Yagi-Uda array that he could carry relatively easily. When you get to
any clearing, the direction of arrival of the power (average Poynting
vector) is the best thing to go for, but you can use the loop in the
clearing and get a better bearing from the null (when there are no
nearby trees to mess you up with a false bearing.

I also worked the downed-pilot beacon problem, and built a XELEDOP
full-scale pattern measurement system to measure the jungle loss and
pattern effects for the UHF beacon. When I researched the problem, I
found that the real problem was that the pilots were adjusting the
squelch in the aircraft's receiver to avoid the background noise and
the low-level beacon signals could not break squelch. My
recommendation was that one of the air crew monitor the channel with
the squelch disabled, and consider the situation if they were the
downed pilot (for added motivation to listen to the noisy channel). As
a result, I never flew the UHF XELEDOP even though it would have been
interesting to do so and get more data in this band to check out our
models.

Now for the specifics:

1) The simplest thing they can do is use a handie talkie with a
vertical whip (vertical when the set is sitting up). There is a null
off the direction of the whip. This is because the dot product of the
arriving E field and the vector effective length of the whip is
zero. The actual null depth is about 20 dB or so in practice, but this
produces a noticable change in audio output. For FM, this shows up in
a decrease in quieting, and for AM it is just a reduction in audio
volume. While crude, this method is theoretically sound and works in
practice. AND, it uses gear they are already packing. 2) There are
other ways to go. Look into the "fox hunting" articles in QST, etc.

A few to check out include:

a) O.G. Villard, Jr., G.H. Hagn, and J.M. Lomasney, QST, Vol. LXXVIII,
No. 8, pp. 33-35, August 1994.

b) O.G. Villard, Jr., G.H. Hagn, and J.M. Lomasney, IEEE Antennas and
Propagation Magazine, Vol. 36, No. 5, pp. 82-85, October 1994.

c) O.G. Villard, Jr., G.H. Hagn, and J.M. Lomasney, IEEE Trans.
Broadcasting, Vol. 38, No. 4, pp. 243-251, December 1992.

There may be another article on VHF, but these will get you started on
what Mike Villard (W6QYT) and Jim Lomasney (WA6NIL) and I (xWA6...)
did in this area earlier in the decade. All of the HF (and MF)
concepts work at VHF. The cascaded loops were used by the FCC in
Baltimore (Mroz) on the 2 meter band, for example.

For the application you described, the KISS advice really
applies. Dont make them carry anything more than they have to. The
"off the tip of the whip" method is the best in my opinion--at least
for a ground SAR mission.

Hope this helps.

George
Received on Fri Apr 30 1999 - 13:53:03 EDT