[amsat-bb] Re: How to make phasing cable for 440 MHz Arrow II cross Ya gi?
w4mvb at juno.com
Sun Dec 2 04:55:46 PST 2007
My experiments over the last several years indicates that the value of circular polarization for LEO satellites is somewhat over rated. The key to success is switchable polarization. My current antennas consists of a often modified M2 2M 14 element (7+7) crossed yagi and a pair of K1FO style yagi's for 435 - one mounted vertically and one horizontally. Both are circularly polarized (or nearly so) and I can switch the polarization on both. I built a number of antennas before I arrived at this configuration and during the process I used linear fixed polarization, circular fixed polarization and switchable polarization both fixed and circular. No matter what polarization I used the key was always the ability to switch. When FO-20 and 29 were active you could get "locked out" for a considerable part of an orbit if you could not switch polarization. Switching between vertical and horizontal polarization worked as well as switching between RHCP and LHCP. I once decided that if you mount one antenna at 45 degrees the problem would be solved. As those of you who use handheld arrow style antennas know even that had to be reversed at some point to get a reasonable signal.
Circular polarization can provide a 3 db advantage over linear polarization but that is only true if both transmit and receive antenna are the same polarization and both are aimed directly at each other. Although this works well on the HEO stabilized birds it almost never happens on the lower orbit satellites. It's doubtful if the 3 db advantage could be detected on the low orbit birds even if it did happen given the other variables in the transmit/receive path.
Although I'm sure there are those who would disagree I think it is just as effective (and much less complex) to simply switch between vertical and horizontal elements where both are available.
My 2 cents worth.
73 de Jess - W4MVB
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