[amsat-bb] Re: re Polarity control box - again

i8cvs domenico.i8cvs at tin.it
Wed Aug 6 18:46:08 PDT 2008

----- Original Message -----
From: "Angus" <angus at young5769.freeserve.co.uk>
To: <amsat-bb at amsat.org>
Sent: Wednesday, August 06, 2008 7:10 PM
Subject: [amsat-bb] re Polarity control box - again

> Hi Mak, thanks for reply.
> The problems occur when we go and try and make the polarity control unit
remote controlled. I have a fully working manual unit here in the shack and
is 100% fine.
> However the remote unit I built use's relays to add in extra lengths of
50ohm coax as a delay line (a 1/4 wave on the -45degree yagi, and a
1/2wavelength on the +45degree yagi.)
> This system I have seen used by very many operators and also some
comercial units, however with just the 2 delay lines the LHCP will never
really be LHCP, it needs an extra set of relays to switch in the other
1/4wave delay line.
> I was just wondering if anyone else had seen this or have I just
misunderstood the circuit somehow.
> Any thoughts would be appreaciated.
> regards
> Gus M0IKB

Hi Gus M0IKB

In a previous message dated 23 july you writes:

"The control box is a copy off the G6LVB website and the VHF/UHF manua
l 4th edition"

For this reason I visited the G6LVB site at


And I found the polarization switching diagram absolutely correct.

For LHCP the -45° yagi is fed with a 1/4 wavelenght of 50 ohm delay
line because RL1 is ON and RL2 is OFF

For RHCP the +45° yagi is fed with a 1/2 wavelenght of 50 ohm delay
line  because RL1 is ON and RL2 is ON

What you missed is that when RL1 and RL2 are both ON for RHCP then
the 1/4 wavelengt delay line is still in circuit in series with the -45°
yagi so that the resultant delay line in series with the +45° yagi for RHCP
is made by the difference of 1/2 -1/4 = 1/4 wavelenght and this difference
generates a resultant phase shift of 90° in series with the other +45° yagi.

In other words for LHCP only a 1/4 wavelength delay line is in series with
the -45° yagi and for RHCP only a 1/4 wavelenght delay line is in series
with the +45° yagi so that between LHCP and RHCP there is a phase difference
of 90° and that is true only if the -45° and the +45° yagi are mounted in
the same plane over the boom and if the coax line between RL1 and the -45°
yagi have the same lenght of the coax line between RL2 and the +45° yagi.

The above circuit described by G6LVB was first designed by H. Stoll,
DF7SO and published in VHF COMMUNICATIONS  1/1980 pages

The main advantage of this circuit is that if all 4 coaxial relays are of
the same type i.e. they have the same physical lenght when cutting the
coax cable delay lines it is not necessary to take the lenght of each relay
into account.

I have used the above designe in 2 meters for several years beginning with
OSCAR-10 but with minor changes I have applied the same concept
to a 70 cm crossed yagi KLM  435-40CX as described in the article
" Switching Four Polarizations on a 70 cm Crossed Yagi"
AMSAT Journal  march/april 2007 Part-1 and may/june 2007 Part-2

73" de

i8CVS Domenico

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