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From: TexasRF@aol.com
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To: no6x@inreach.com
Cc: .towertalk@contesting.com
Subject: Re: [TowerTalk] Gamma matching assistance needed
To: <towertalk@contesting.com>
Date: Wed, 29 Jul 1998 08:27:44 EDT
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Mike, NO6X asked about using a coax center conductor and dielectric as gamma
capacitor,
The answer is is yes, and it was done 30 years or so ago by Mosley in their
Classic series of antennas except they placed the coax center conductor
directly inside the driven element and on both sides of the driven element.
>From a practical viewpoint though, most gamma matches I have worked with are
too short to create enough capacitance using this scheme. For example, a 40m
beam might have a 6-ft long gamma arm and need about 280 pf of capacitance to
resonate the system. RG213 has a capacitance of only about 25pf per foot with
the shield in place and a lot less if the shield is replaced with a 1 inch
tube. I suppose one could place 2 or 3 center conductors in parallel inside
the gamma tube but I have not tried that approach.
The correct way to 1>analyze this is to determine the needed capacitive
reactance 2> determine the characteristic impedance of the center
conductor/gamma tube as a coax line 3>calculate the capacitive reactance of
the transmission line of length = gamma length and z=answer in step 2. If the
reactance is too low, then great! Just shorten the center conductor, if too
high, the length is too short; punt!
73 de Gerald, K5GW
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