Hi Ed, I have The Satellite Experimenters Handbook, it was one of my very first purchases when I decided to get into this. Actually that is where I got the impression that velocity factor and electrical length of the phasing harnesses was a big deal. Davidoff states that there can be a 10% variation off published values from cable to cable. That is what I. thought prevented me from building a phasing harness without proper measuring equipment. If I can get "close enough" without a grid dip meter ot SWR anylyzer, that would be great. I actually already have a second set of elements cut for my 70cm antenna and I purposely left the boom long enough to accomodate them. At any rate I wish I did have an SWR anylyzer that would cover 2 meters and 70cm but my hobby budget won't allow it at this time. I have a garden variety 2 meter SWR bridge but nothing that works on 70 cm which is a problem for me because I'm having trouble with the TX on the 70cm uplinks of VO-29 and AO-7 and I suspect it's SWR related. 73, Michael, W4HIJ ----- Original Message ----- From: "Edward Cole" kl7uw@acsalaska.net To: "Michael Tondee" mat_62@netcommander.com Cc: amsat-bb@amsat.org Sent: Saturday, September 20, 2008 1:59 AM Subject: Re: [amsat-bb] Re: Polarity questions
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
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@hotmail.com
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