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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