[amsat-bb] Re: re polarity control help needed

[i8cvs]

----- Original Message -----
From: "Angus" <angus at young5769.freeserve.co.uk>
To: <amsat-bb at 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