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Re: [TowerTalk] 40M rotary dipole

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 40M rotary dipole
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 20 Sep 2017 21:33:51 -0700
List-post: <mailto:towertalk@contesting.com>
Very good post, Grant.  I want to emphasize/clarify a few points. It's clear to me that YOU get them, but others might not. :)
The post to which I was replying was about bonding coax to a tower.  It 
was not about a dipole strung between trees. And you're entirely correct 
that if the dipole center was at a tower and the coax was properly 
bonded to the tower, a choke at the feedpoint would isolate the dipole 
from the tower. Ditto for a beam on the tower.  The purpose of the bonds 
is to prevent arcing between the tower and the coax in a lightning event 
by keeping every point on the coax as close as practical to the same 
potential as the point on the tower that is physically next to it. It is 
standard practice at commercial VHF/UHF radio sites.
Further, the tower is NOT ground, it's a vertical antenna with its base 
(usually) grounded!  It's only a tower at DC. Lightning is NOT a DC 
event, it is an RF event.  The word "bond" in the electrical contest 
means an very low impedance connection between grounded points that is 
electrically and mechanically robust and can carry all possible load 
current. The purpose of bonding, is, in general, to keep the bonded 
elements at the same potential. While the purpose of this bonding (coax 
to tower) is lightning protection, proper bonding within a premises 
(home, shack, audio/video system, building, etc.) also minimizes issues 
with hum, buzz, and RF noise.
BTW -- all of this stuff is in Ward Silver's new ARRL book on Power, 
Grounding, Bonding, etc. and much of it is in 
http://k9yc.com/GroundingAndAudio.pdf
73, Jim K9YC

On 9/20/2017 8:59 PM, Grant Saviers wrote:
It is a bit confusing since "bonding" usually refers to providing a ground path for lightning protection as in the case you mention as a means to keep the coax shield at the same potential as the tower along its length if there is a strike.  For tall towers multiple bonding points are recommended.   For hardline it is a bit easier to understand since the jacket is stripped for an inch or so and a copper strap wrapped around the solid shield and a heavy gauge lead then connected to a bonding plate on the tower or the grounding point at the base.  There is no penetration or interruption of the shield at a bonding point.  The hardline probably continues to an antenna or to a  jumper coax where the end of the shield may or may not be connected to the tower (ground) at the antenna, not for a dipole.
As you  conclude, if the shield was grounded at a dipole feedpoint the 
pattern would change.  A choke between the bonding point and the 
antenna feedpoint effectively disconnects the outside of the shield 
from those two points as well as preventing currents from flowing on 
the outside of the shield if the antenna is not balanced.  Even though 
a dipole is a "balanced" antenna I think they are rarely perfectly 
balanced due to all sorts of things nearby - houses, powerlines, 
trees, etc.   So to keep the feedline from becoming part of the 
radiating (and listening) antenna system a choke is a very good idea.  
Note that the coax may still become part of the system, particularly 
when elevated and it acts as an antenna.  Another good reason to bury 
feedlines.
OTOH, if you don't care about the pattern of your dipole, don't have 
feedline induced receive noise, or don't have RF in the shack, one 
might not bother with a choke.  Generally, not too bad a bet with 
dipoles since they really want to work.  For OCF, end feds, G5RV's, 
verticals with limited radials, and other wildly unbalanced antennas, 
probably a bad bet.
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