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Re: DFing Terrestrial QRM

At 11:16 PM 9/21/2007, Robert Bruninga wrote:

>Average Signal Strength is a very significant indicator that can
>be used to home in on a signal.  Combined with local terrain
>knowledge, a little driving and chris-crossing can quickly give
>knowledge of the signal profile and allow the driver to just
>keep narrowing down on the signal and with enough time, arrive
>at the door.

While I can do this quite well (and we used to have CB foxhunts that 
worked on this very principle), there are some inexpensive add ons 
that can make DFing that much easier, such as an attenuator box, 
which has variable attenuation, selectable by switches on the front 
panel.  The attenuator will allow you to get in closer without 
saturating your S meter.  Quite cheap to put together out of a metal 
box  and a bunch of metal film resisters.  The other is a small 
Yagi.  The VK3VT 3 element design has been proven over 30 years and 
works well, with a good pattern.

The biggest danger of hunting with an omni is chasing reflections, 
especially in built up or hilly areas.  Even with a directional 
antenna, this sometimes confuses the experts. :)

>When you get to MAX signal and the mobile Smeter is max'ed, then
>you need your HT sitting on the seat with its ruber duck becomes
>the benchmark.  Once it is full quieting, you can usually remove
>its antenna and stick in shorter and shorter paper clip as you
>get closer and closer.  Eventually the HT will hear the signal
>with no antenna.  Now it is time to get out of the car and walk.
>Holding the HT on your chest.  You have a NULL out your back.
>Use it to get walking directions and by now you are within yards
>of the source.

Strangely enough, this has never really worked for me, much to my 
surprise.  Dunno why.

73 de VK3JED

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