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? results: 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 regards Gus M0IKB
----- Original Message ----- From: "Angus" angus@young5769.freeserve.co.uk To: amsat-bb@amsat.org Sent: Wednesday, July 23, 2008 11:47 AM Subject: [amsat-bb] re polarity control help needed
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?
results: 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 regards Gus M0IKB
Hi Gus, M0IKB
For the theory on how a four polarity switch works read please the article "Switching Four Polarizations on a 70 cm Crossed yagi" Part-1 publisched in the AMSAT Journal March/April 2007 www.amsat.org
For building the switcher read Part-2 in the issue May/June 2007
By the way the SWR that you got in 2 meters is excellent and I suggest you do not alter (longher/shorter) any cable of the original designe. Changing the lenght of the delay lines or of any other cable in the switcher will change the phase of the component signals changing the actual circularity of the RHCP and LHCP becoming elliptical. In addition the actual 45 and 135 degrees linear polarizations will shift to different angles.
Best 73" de
i8CVS Domenico
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