TopBand: Re: [TowerTalk] Chokes on Dipoles

[Tom Rauch]

> Date:          Wed, 29 Apr 1998 08:38:18 -0500
> From:          Lee Buller <k0wa@southwind.net>

Hi Lee,

> Someone mention chokes on dipoles.  I have used current chokes on beams, but
> never on 40, 80 and 160 dipoles.   Can someone illuminate me on the whys and
> wherefores concerning current chokes on feedline to the dipole?  I hae read
> the articles in QST, but what practical experiences do we have out there in
> the collective wisdom of the reflector.

I use chokes because they prevent the feedline from arcing over to 
the tower legs or other cables, as well as the more common reasons.

I had two experiences where this occurred.

One was with a 160 droopy dipole. I had an insulated feedline running 
down a tower leg, and about 90 feet down from the dipole it arced 
through the insulation and paint to the tower leg.

Another was with my 20 meter yagi, a bit less than 1/4 wl from the 
feedpoint the coax arced from shield to mast causing TVI. 

A coil of coax, much like Roy Lewallen describes in his articles, 
cured it. 

By the way, when I measured FS from elevated radials, there was 
almost a one dB decrease in FS when I omitted a choke balun and 
allowed the elevated radial common point to terminate to earth via 
the feedline, plus I had feedline radiation. With a bunch of radials, 
the choke made no detectable change.

Choke baluns can help with verticals using poor grounds, and I also 
use them on my receiving antennas to prevent noise ingress from 
common mode currents.

73, Tom W8JI
w8ji.tom@MCIONE.com

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