[amsat-bb] Re: Polarity questions

Edward Cole kl7uw at acsalaska.net
Fri Sep 19 22:59:57 PDT 2008


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.
>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.


73, Ed - KL7UW              BP40iq, 6m - 3cm
144-EME: FT-847, mgf-1801, 4x-xp20, 185w
http://www.kl7uw.com     AK VHF-Up Group
NA Rep. for DUBUS: dubususa at hotmail.com

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