[RFI] Link-coupled loop - more
Roger Parsons
ve3zi at yahoo.com
Thu Apr 3 09:50:28 EDT 2014
Larry:
There is no magic sense circuit design of which I am aware. My first DF receiver used a DF96 1.5V filament valve as the sense amplifier. My next used a germanium transistor which worked fine until I got too close to the hidden station's antenna and all went quiet. I've updated a bit since then.
Most locally received signals have a significant vertical polarisation component, and so the loop and sense whip can work well. The problem comes when there is a predominant horizontal component because this can skew both sense and bearing, as whilst a loop will respond to horizontally polarised signals it has no directivity in that case. A small movement will generally allow better results when one is fairly close to the target. In my experience, the real problem arises when trying to DF skywave signals, and even more so where ground wave and skywave are of similar strength.
There is a magic DF antenna, invented during WW1 by Frank Adcock. However, it would be difficult (but perhaps not impossible) to make an 80m one small enough to be carried around. Even better is the Wullenweber antenna, but that is definitely not remotely portable.
A thousand DF hunts is quite a lot - once a week summer and winter for nearly twenty years. I am not, and was not, saying that it is impossible to DF a source to a particular building. I was saying, and am saying, that accidental radiators usually couple into random conductors and that it is very easy indeed to be misled.
73 Roger
VE3ZI
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