----- Original Message ----- From: "Angus" [email protected] To: [email protected] 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 is 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
http://www.g6lvb.com/remotepolarization.htm
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. regards Gus M0IKB
73" de
i8CVS Domenico