[amsat-bb] Re polarity control box - again
angus at young5769.freeserve.co.uk
Wed Aug 6 08:06:55 PDT 2008
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 is fed with a 1/2 wave of 50 ohm line to delay it by 180 degrees, no problems so far.
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.
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?
Sorry for boring some of you.
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