Signal Overload from Outside Sources
Recently I ran into a problem that current and future search pilots need to be aware of and guard against. On an airborne electronic search I found that when you are flying within a few miles of broadcast antennas, the transmitted power can overwhelm the circuitry of the DF unit, making it appear that there is a carrier-only signal on 121.5 MHz or any other channel you are DFing. 1811 MSL Sutro Tower in San Francisco is a particularly bad case, as it has FOURTEEN broadcast stations transmitting from it! http://www.jimprice.com/sutro/
Of course, one obvious clue that something was wrong was the fact that from some directions, the DF pointed straight at the tower. I came to the erroneous conclusion that the signal was being induced in, and re-radiated from, the tower. This belief was reinforced by the fact that there were a couple of other points over the city where it was possible to circle a spot and have the needle stay on the same side of the airplane. However the ground team was unable to acquire a definite signal.
The next day I was discussing this with the Group 2 DC, and he told me a more definitive test for whether a signal is real, or if it is just a strong signal overloading the DF. If you switch to another frequency and you still see the same indication on the signal strength meter, that is good evidence that the unit is responding to an overload situation, and not a signal on the frequency of interest.
On a subsequent flight over the city, I was able to apply this test and found that it was indeed an overload situation.
If the above test shows a signal only on the frequency you are DFing, there is still a theoretical possibility that it is a false signal. I am told that DF units have their sensitivity maximized on 121.5, so that under certain conditions the above test might show the signal is real when it is not. A way to test for this would be to check to see if there is any quieting of the noise on an aircraft radio with the squelch open. Aircraft radios are generally not designed to favor one frequency over another, and they typically have better unwanted signal rejection than a DF unit, so they may be able to reject interference that affects the DF. In particular, with the squelch open, if there is a real signal on 121.5, you should be able to change to another frequency and hear a difference in the noise level. If you can't, then there's probably nothing there.
Note that the above comments only apply to carrier-only signals. If you are hearing sweep tones, that is good evidence that the signal is real.
This story submitted by Richard Palm, 1LT, CAP
This page of the CAP Emergency Services Resources™ website was last updated 07/01/2008
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