A Small modification to my EMTECH ZM-2 ATU

With the current bad conditions the need to run 80m during the day for SOTA activations has caused me to visit antenna options that would allow me to run 80 with a “minimum” amount of wire.

My normal SOTA / WWFF antenna is an end fed of around 25m with a 6m counterpoise, this is then matched to 50 ohm using an external L Match. This configuration is fine on 40m and up, but is too close to a quarter wave length on 80m to match well.

Whilst not writing off that configuration I considered other options that may work on 80m with out the need for 40m length of wire. The Doublet seemed a good choice, certainly it is an antenna that others sing the praise of, so after some research and the use a program to calculate the impedance at the bottom of the feed line I settled with a flat top of 26m and a 6m feed line. The short length of feeder was selected to allow use on a Squid pole without the need to keep it off the ground.



To match this I use one of the EM-Tech ZM-2 atu. The Z Match worked fine providing a match across the bands of interest, 80m to 20m. However I wanted to have the option to operate with the feeders shorted together, to run the antenna as a Flat Top loaded against ground radials. Whilst the EM-Tech Z Match has a “grounding option” it switches one leg to ground rather than shorting the feeder together.

ZM-2 with modifed ground switch

ZM-2 with modified ground switch

Looking at the circuit I felt this would be an easy modification to make by swapping the ground Switch for a DPDT type and a small re-wiring.


Original and Modified input circuits

The modified right angle bnc connector provides the alternate connection for the Ground wires

I have not done any real on air testing with the modified unit, but it certainly seems to work. Whilst I am happy with the doublet I still believe it is simpler to erect a end fed wire when on a park or a peak, but of course the balanced nature of the doublet has its own advantages.

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