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[TowerTalk] coax as capacitor

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Subject: [TowerTalk] coax as capacitor
From: (W0UN--Signal Hill Ranch)
Date: Wed, 25 Jul 2001 16:38:26 -0600
At 03:55 PM 2001-07-25, you wrote:

>Is it feasible to use coaxial cable as the series capacitor in a shunt feed
>system, simply pruning the length of cable as appropriate?  Or is there
>some reason why not?
>73, Pete N4ZR


Yes, it can.  Coax has been used by many for gamma match
caps--including Cushcraft.  I even published an article about it
around 35 years ago.

Only problem is that it is not a pure capacitance so if the
length of cable needed starts to approach a significant
fraction of a 1/4 wave then the inductive component
reduces the capacitive reactance--meaning it takes more
cable than what is calculated by just using the cap/ft
figure for the cable.  Most low impedance matches (Yagis,
etc) don't require much C so this is pretty easy way to go.

I like to do the initial match with a receive variable capacitor
and get the match set by tweaking the gamma rod length
and cap value.

Then fix the rod and replace the RX variable with the coax and
trim to length.  Best (cheapest) tool to trim RG-213 is the
Radio Shack cable cutter.  Less than $10 and cuts coax cable
easily and pretty cleanly.

Good luck--John  W0UN

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