Motorola R56 manual. It is the bible for fixed sites, including bolting
the racks down to the floor in case of earthquake.
73,
-John NI0K
Clay Autery <mailto:KY5G@montac.com>
Thursday, September 21, 2017 6:34 AM
Are you endorsing the purchase of this book, Jim? I believe I saw that
you had some input into it, but I've been hesitant to purchase it.
Have to say, I found a Motorola tech manual that has an entire chapter
and appendix on bonding/grounding communication facilities that seems
like pure gold.
Thanks for all your efforts to stamp out misinformation in this realm of
the hobby. <smile>
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Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
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Jim Brown <mailto:jim@audiosystemsgroup.com>
Wednesday, September 20, 2017 11:33 PM
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
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Grant Saviers <mailto:grants2@pacbell.net>
Wednesday, September 20, 2017 10:59 PM
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.
Grant KZ1W
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Dave Sublette <mailto:k4to@arrl.net>
Wednesday, September 20, 2017 9:50 PM
Well regarding the bonding of the coax shield at the top and bottom of
the tower… I’m having a hard time understanding this. If the shield of
the coax is connected to the top of the tower(or at the point on the
tower where the antenna is mounted), one side of the dipole then is
connected to the tower at that point. I would think that would disturb
the radiation pattern, the match, and anything else that can be
disturbed (including me) !
Dave, K4TO
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Wes Stewart <mailto:wes_n7ws@triconet.org>
Wednesday, September 20, 2017 7:04 PM
It's not even pretty easy to measure these values.
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