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Re: Polarity questions



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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