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Old 06-16-2016, 9:19 AM
jim202 jim202 is offline
Join Date: Mar 2002
Location: New Orleans region
Posts: 2,518

Originally Posted by samiam1234 View Post

I was investigating a 121.5 emergency beacon that was going off near the Oakland Airport. We turned on our helicopters direction finder and followed the arrow until passing over a group of antennas on top of a mountain where the arrow would swing from the 12 o'clock to the 6 o'clock position (as it should). After 3 passes from different directions, we were very confident on the location of the source. We later found out that the signal was actually coming from a boat in a marina back near Oakland. I've been reading about radio repeaters and my onboard DF equipment and cannot figure out why this might have happened.

Has anyone experienced this before?
Does anyone have a theory on why this may have happened?
Is there anything I can do in the aircraft to try and counter future false readings?

I will try and take a wild a** guess on this and say that there might have been a birdie coming from some transmitter there on the mountain. Doing some simple math, which might or might not come up with the correct answer, I have the following frequencies: 30.375 MHz., 40.5 MHz., 486 MHz. I am sure there are others, but these are the direct hits. Now there could be a transmitter on that hill that is just spilling out trash and generating a signal.

You might try listening to the 121.5 frequency with a receiver and see just what it sounds like. This would provide a hint of where it's originating from. The key here if it's on continuously, then that limits the sources. It could be an intermod hit that might involve a mix of two or more transmitters. If this is the case, good luck on locating what is causing it.
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