Topband: Receive antenna switching

Doug Grant dougk1dg at
Thu Aug 6 15:13:01 PDT 2009

Most control-through-the-coax switches are usually limited to 4 positions,
corresponding to :

1. no control voltage on the coax
2. some positive DC on the coax
3. some negative DC on the coax
4. some AC on the coax

Some diodes and capacitors can decode these states easily. One example of
how to do it is shown at:


The shield of the coax is the common. All the rest is isolated for RF
through a choke.

I recently got a satellite dish installed and the coax they used for the
outside part of the installation is RG6 plus a ground lead, molded together,
but NOT connected to the coax shield. Aha! Another conductor! And I imagine
that stuff is available in long lengths fairly cheap if you know a satellite
TV installer.

Use the same 4 states on that wire (referred to ground) and you can get SP7T

The warnings about critter damage causing water ingress, dc leakage &
subsequent RFI apply to these techniques and any other dc control cables
running around outside...protect your coax and control lines with something
(cheap garden hose or PVC electrical conduit, which is only about 8 cents a
foot), and use RF chokes liberally.


Doug K1DG

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