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Re: [TowerTalk] Stub question

To: "'EC1CT Fernando'" <>,"'towertalk'" <>
Subject: Re: [TowerTalk] Stub question
From: "Christopher J Galbraith" <>
Date: Thu, 25 Jan 2007 08:54:50 -0800
List-post: <>
Hi Fernando,

A 46 ft length of RG-8 (or other coax with 0.80 velocity factor) is a
quarter wavelength at 3.75 MHz.  If you connect one end to the antenna
feedpoint (center conductor to vertical element, braid to radials) and short
the other end, any DC (static charge) sees a short circuit (everywhere), but
RF at 3.75 MHz (plus or minus about 10% or less) sees an open circuit at the
antenna feedpoint.  It's the magic of a quarter wave line (or the math of
forward and reflected waves, whichever you prefer!).

Other ways to bleed off static charge I can think of are: 

(1) use an RF choke across the feedpoint with a minimum value of about 22 uH
(for a reactance of 10x 50 Ohms = 500 Ohms at 3.5 MHz) that has a self
resonant frequency (SRF) higher than about 10 MHz.  Note that this will only
work on 80m; if you use the vertical on higher frequencies, the choke must
have a SRF of about 3x the highest frequency used.

(2) Put a 300 kOhm resistor (0.5 W rating) across the feedpoint.  This will
discharge static and your TX/RX won't know the difference.  With 1500 W at
the feedpoint, the resistor will dissipate 0.5 W (you can use a lower R for
lower power levels).  I use this method and it works FB.

73, Chris KA8WFC

-----Original Message-----
[] On Behalf Of EC1CT Fernando
Sent: Thursday, January 25, 2007 8:01 AM
To: towertalk
Subject: [TowerTalk] Stub question

Hello all,
I recently aquired a quarter wave vertical for 80 meters. It´s a 33 ft
aluminium mast with a capacity hat designed by EA5JK. It has such a kind of
loading coil. Yesterday at the radio club someone told me to put a stub
formed by 40 ft or so of coax line at the feedpoint. He said that would help
me to prevent the static but this stub would put the antenna in shortcut as
it would connect in any way the radial system with the copper core of the
coax line to the radio. It doesn´t convince me at all. Just wondering if any
of the low banders here have added any stub to their verticals and how you
did it. Thanks a lot. 73s
BTW: lot of snow in norther spain today. Prop is as bad as the weather....
EC1CT Fernando

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