----- Original Message ----- From: "Angus" angus@young5769.freeserve.co.uk To: amsat-bb@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
http://www.g6lvb.com/remotepolarization.htm
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 33-34-35
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