Topband: control voltage noise in K9AY loop

Pete Smith n4zr at
Wed Oct 10 17:39:27 EDT 2007

I have just finished homebrewing a 4-way switched K9AY loop.  I used an old 
Heathkit cable-less antenna switch, model HD-1481, as the controller.  On 
the antenna end I used Gary's circuit except for switching to 24 volt 
relays and using a binocular transformer instead of the trifilat 
autotransformer in the original design.

The problem is that I have serious hum modulation in each of the powered 
positions of the direction switch (+/- 24 DC and 24 AC).  The antenna shows 
good directivity, so I'm fairly sure the basic circuit is wired correctly, 
and the relay switching is working.

To isolate the receiving antenna from the 24 volts, the HD-1481 relay box 
uses a 1 MH RF choke and .02 uf blocking capacitors.  The original K9AY 
design just uses .01 capacitors at the antenna end.  Should I plan on 
adding a similar RF choke at the antenna unit, or is there someplace else I 
should be looking for the problem.

73, Pete N4ZR
The World HF Contest Station Database
Full details on 3300 contest stations
Updated 5/5/07

More information about the Topband mailing list