[amsat-bb] re polarity control help needed

Angus angus at young5769.freeserve.co.uk
Wed Jul 23 02:47:09 PDT 2008

Hi all, I have built a simple polarity control box to allow V, H, RHCP & LHCP off my 6 element homebrew crossed yagi.
The yagi had an SWR of 1.1 to 1 on 145mhz on the 45degree and 135degree elements, so they are fine.
The control box is a copy off the G6LVB website and the VHF/UHF manual 4th edition.
The unit is basically 2 x 1/4waves of 75ohm coax to use as a matching unit and then relays to switch in or out 1/4 wave and a 1/2wavelength of 50ohm coax to give the different polarities.
My small problem is although the SWR through all the modes is under 1.5 to 1 I would like to bring it down a little, but is it worth the effort?
V polarity 144mhz 1.2 to 1, 145mhz 1.3 to 1, 146mhz 1.5 to 1
H polarity 144mhz 1.5 to 1, 145mhz 1.3 to 1, 146mhz 1.2 to 1
RHCP      144mhz 1.1 to 1, 145mhz 1.3 to 1, 146mhz 1.5 to 1
LHCP       144mhz 1.5 to 1, 145mhz 1.3 to 1, 146mhz 1.1 to 

Am I worrying over nothing or should I go and alter the unit to give a slightly better result, my head says 'leave lt alone or you will muck it up'.
But I am also thinking a little shorter cable might be all it needs?
Does anyone know by looking at the figures what cable needs to be altered (longer/shorter).
Thanks for any advice
Gus M0IKB 

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