[amsat-bb] Polarization Issues
clintbradford at mac.com
Wed Oct 20 17:06:37 PDT 2010
>From my in box ... I will forward all replies to him! Thank you. -Clint
A note on my activity and my station.
I started working SAT the same way I did CW: 3QSOs, 2SATs in a month, a couple of months in a year... Then the wind suggested to me to reduce (2x2x11VHF, 4x2x20UHF) the antenna system and I am actually putting up:
- VHF, 2x11 at 45/135deg, lambda spaced, hybrid ring power divider, 0.25lambda delay section (for CP) added on one of the two identical length cables;
- UHF (will be mounted "next week"), 2x20 at 45/135deg, 2xlambda spaced, 2x0.25lambda air dielectric (vf=1) power divider, delay section (for CP) not yet determined;
- SHF, 20" dish, offset, 4 turns helical feeder, rirectly connected to low noise (German) converter; only rx and I am looking to sell it;
The tower atop the roof is 3m high, 1' triangular, with rotor mast offsetted aside for easy mounting: that is the reason my spacing on 435MHz is so wide, I need the antennas to be far from the tower as the tower is wide. Single h/mast on el rotor to support 1xV 1xU each side, dish at center.
Power is 10W (Microwave modules xverter).
My sat are SSB ones, maybe FM too.
Here comes the question.
Circular polarization is hard to achieve; you have to put a delay line exactly (exactly???) 0.25*Vf, that is 13" on 2M and 4.5" on 435Mhz, in-line with one coax line of one of the antennas (after the splitter); if you use a couple of coax connectors 1" error seems easy to achieve. Circular polarization switching is worst, as 1. if you have an asymmetrical - as depicted above - inline delay line (on one side only), how long is to be computed the switching system? and 2. if you choose a symmetrical - either included in the divider or inline on both antenna cables - solution, how can you compute correctly the lenghts / difference of lenghts of the needed (coax, connector, T, relay) items?
So the question is: how have you done your polarization switch (I have seen SM and PA hams solutions, but no one consider the problem I oulined here)? And the second part of the question is: do we really need a switching or do we have a recognized standard - LHCP I think - for our sats?
Thank you for suggestions, If not clear enough I'll clarify on demand, 73s from
Marco, I4MFA - W4MFA
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