Michael:
At 05:53 PM 9/19/2008, Michael Tondee wrote:
I may have misunderstood but the way I got it when I was researching homebrewing yagi's for my sat station is that if you had to fix the antenna to either RHCP or LHCP and could not switch between the two because of cost or complexity or whatever other reason that your best compromise would be to go with linear polarization.
It might be easier to wire for just one sense of CP (maybe a little cheaper), but the complexity is not that big a deal. You can read how to do it in the Satellite Experimenter's Handbook by Davidoff (available from Amsat or ARRL).
Feed harnesses for homebrew CP antennas are a stumbling block for me as I don't have a grid dip meter or SWR anylyzer to properly figure coax velocity and electrical length. If you don't get the phasing harnesses the right length then you won't get CP anyway.
You are making too much of a big deal out of this. Velocity factor is published by coax manufacturers so you do not need instruments to get close. 1/4 WL = Vf * 492/Fmhz in feet 1/4 WL (RG-213) = 0.66 * 492/144 = 2.25 feet or 27-inches
If you are off 5% it will not destroy the circularity. Most hams get it "close enough". Of course if you are off by a quarter wavelength or more it will matter (20-inches at 144-MHz). I know you can get it within an inch and it will work fine. More accuracy produces a better SWR.
If you plan to build much VHF and above stuff a SWR meter is a good basic piece of equipment to have.
I use two of the "cheap yagi" designs by Kent Britain in vertical polarization and rotate them with a homebrew "SAEBRTrack" Az/El rotor box and old Gemini OR-360 TV rotators. Works well enough for LEO's anyway. 73, Michael, W4HIJ
Sure, linear antennas only sacrifice 3-dB of gain in crosspolarization with a CP signal. Often squint angle make such signals elliptical so the loss can be less.
GL
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