Topband: Adjusting termination resistance

John Harden, D.M.D. jhdmd at
Wed Dec 28 15:17:22 EST 2005

I use W8JI's technique for my K9AY loop system (homebrew). I use an SWR
analyzer feeding a small amount of RF into the system, and adjust the
termination resistance SWR for minimum at 1.830.


I have a remotely adjustable termination resistance setup using a
Perkin-Elmer Vactrol device. On the K9AY control box in the shack I have a
precision/multi-turn potentiometer that varies the voltage to the LED in the
Vactrol device (located in the relay box outside). There is a 120 ohm
resistor between the Vactrol and ground. There are 3 1N4003 diodes in series
that are in parallel with the Vactrol. This sets the photoresistor at a
minimum value of 54 ohms.


The 54 ohm value corresponds to 30 mA through the LED. At 0 mA the
photoresistor is >10K ohms. It is easy to sit in the shack and adjust the
termination resistance while watching the SWR analyzer injecting RF into the
system. I admit it would be better to have the reading right at the antenna
feedpoint. However we're below 2 MHz.


See the following URL:




John, W4NU


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