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Re: [TowerTalk] Testing a vacuum variable Cap for an Inv-L

To: "" <>
Subject: Re: [TowerTalk] Testing a vacuum variable Cap for an Inv-L
From: "Jim Brown" <>
Date: Sun, 29 Aug 2010 16:24:14 -0700
List-post: <">>
A question to ask is whether you really need a vacuum variable at 
all. Unless the antenna is close to a half wave and fed from the end, 
the voltage at the feedpoint is pretty low, even running very high 
power. Rather, you need a capacitor that can carry relatively high 
current, because the antenna is probably being fed at a low impedance 
point. If that's true, all you really need is a capacitor of stable 
value and with a low equivalent series resistance (so that its 
dissipation is low when carrying high current). A length of good 
quality RG8 would do that job! I've done it that way, but now I use a 
parallel combination of fixed disc capacitors. I tweaked the value  
by adding and subtracting smaller caps in parallel until I got it 
where I wanted it. Several years ago, I stocked up on a selection of 
HV caps at our local electronic surplus house, HSC, also known as 
Halted. They have an internet presence. 

The other question is why it needs to be variable. I tune my vertical 
with a fixed capacitor whose value I tweaked to get a match. As 
initially installed, the SWR was under 2:1 over half the band. That's 
good enough for most uses, but I wanted the antenna to be a bit more 
broadband to make it easier to QSY during contests. My antenna is a 
Tee vertical, which is a close relative of yours. So I simply added a 
second conductor in parallel with the vertical section, using 8-inch 
pieces of 1/2-in PVC conduit as spacers. That made the SWR under 2:1 
over the entire band! 

73, Jim Brown K9YC

On Sun, 29 Aug 2010 09:37:58 -0400, krishna kanakasapapathi wrote:

>  Hi all,
>Greetings. I am in the process of building a 160m Inv-L and have
>offered to purchase a 5Kv vacuum variable cap from another ham.
>He has offered to make it good if the cap has lost the seal.

>How do i test this one?
>I have an antenna analyzer, but no Hi-pot test gear.
>The Inv-L + radials are not ready as yet.

>I am in the  Raleigh NC area. Any help would be appreciated.

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