[amsat-bb] Re: Re polarity control box - again
domenico.i8cvs at tin.it
Wed Aug 6 15:08:42 PDT 2008
----- Original Message -----
From: "Angus" <angus at young5769.freeserve.co.uk>
To: <amsat-bb at amsat.org>
Sent: Wednesday, August 06, 2008 5:06 PM
Subject: [amsat-bb] Re polarity control box - again
> Hi all, its me again, sorry.
> OK got the 2mtr polarity control box working and very pleased, except for
> one very small item.
> LHCP never seemed to me to be 90 degrees out of phase with RHCP, never had
> the large drop in signal I expected.
> OK time for a bit or research and looked in to my VHF/UHF manual and found
> the pages and guess what, the design we have been using is wrong, or so I
> think, please correct me if I am wrong.
> OK both yagis have a 1/4 wave 75 ohm matching system to allow the system
> back to work with 2 x 50 ohm yagis in parallel, in case anyone forgets, so
> for Vertical both yagis fed in phase, for Horizontal the 135 degrees yagi
> fed with a 1/2 wave of 50 ohm line to delay it by 180 degrees, no problems
> so far.
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 RHCP the 45 degrees yagi is fed with a 1/4 wavelength of 50 ohm delay
> line and again no problems.
> NOW HERE'S THE RUB!
> For LHCP we feed the 135 degrees yagi with a 1/4 wavelength of 50 ohm
> line, woops the system has a 1/2 wavelength of 50 ohm line!
> So instead of feeding as we should we are feeding it to make it simpler
> than it should be. Hence I was never happy that LHCP was never quite 90
> degrees out with RHCP.
What you don't understand is that 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
But 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 the difference of
1/2 -1/4 = 1/4 wavelenght and this difference make 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° that is correct.
> So quite possibly we are going to get some form of strange oval type of
> polarity anyone take a guess at its true nature, I may have a play around
> with one of the antenna programs and see what comes out.
> Am I wrong completely on this or is it the better to have it off a little
> on LHCP to get the other 3 polarities correct?
If you did things like the schematic diagram of G6LVB shows all 4
polarities are completely correct.
> Sorry for boring some of you.
> Gus M0IKB
More information about the AMSAT-BB