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Re: RG-6 quad shield for 72 ohm cable and other antenna questions



Michael,
If you offset the two antennas as you mentioned, that will give you the 90 
degree phasing for circular polorization. You only need to keep the phasing 
( length of coax) equal between the two antennas for 0 phase shift.

The second part of the problem is getting half the power to each antenna, 
this is done with a power divider.
Lines in 1/4, 3/4, 5/4 etc wavelengths will transform impeadence. You must 
connect two 50 ohm antennas to a 50 Ohm coax, so one solution is to use two 
1/4 or 3/4 wavelength coax of 72 ohms to make each antenna 100 ohms and both 
in paraell will equal 50 Ohms. Using 72 Ohm comes out to be 103 OHms ( 
square of the coax matching section divided by the impeadence of the 
antenna, equals the impedence at the other end of the coax.

CATV cable is usally a foam type and the Vp varies dependent on the foam, 
and any deformities in the cable.  The solid polyethlene (Vp .66) dielectric 
is durable, and stands up better to weather, and is harder to pinch with 
cable ties etc.
Caculation is a good  method with polyethlene cables, foam types are better 
measured and extreem care must be used in handling foam type cables. They 
are easly deformed.

Good Luck,
Art,
KC6UQH


----- Original Message ----- 
From: "Michael A. Tondee" <mat_62@netcommander.com>
To: <amsat-bb@amsat.org>
Sent: Wednesday, December 28, 2005 7:35 PM
Subject: [amsat-bb] RG-6 quad shield for 72 ohm cable and other antenna 
questions


> I'm trying to homebrew  a crossed yagi array for 2 meters and since I have 
> a
> bunch of RG-6 quad shield cable lying around here from when I had 
> sattellite
> TV, I'm wondering if I could possibly use it for the matching/phasing 
> harness
> or do I need some other type of 72 ohm coax? I plan on offsetting the 
> elements
> a quarter wavelength on the same boom. I've been reading the Satellite
> Handbook and I'm still a little confused about measuring the electrical 
> length
> of the matching/phasing lines. I looked up the velocity factor of the 
> cable
> but I have no access to a dip meter. Is this super critical or can I wing 
> it?
> I've been homebrewing antennas all my life but the phasing  and circular
> polarization stuff is new to me. Any help would be appreciated.
> Tnx and 73,
> Michael, W4HIJ
> AMSAT # 36017
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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----
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