[TowerTalk] Stub question

[EC1CT Fernando]

Hi Chris,
Wow, thank you very much for the reply. I think I will test the RG-8 coax in 
shortcut as the guy at the radioclub told me. The idea of the resistor 
sounds quite attractive too. Forgot to mention that I´m gonna use it with 
1-1.2 Kw with this antenna. Thanks all for your inputs. 73s!
EC1CT Fernando
E-mail: ec1ct at ure.es

----- Original Message ----- 
From: "Christopher J Galbraith" <cgalbrai at umich.edu>
To: "'EC1CT Fernando'" <ec1cwg at dxhunters.com>; "'towertalk'" 
<towertalk at contesting.com>
Sent: Thursday, January 25, 2007 5:54 PM
Subject: Re: [TowerTalk] Stub question


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