[TowerTalk] 40M rotary dipole
John Simmons
jasimmons at pinewooddata.com
Thu Sep 21 08:28:59 EDT 2017
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 at 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>
>
> ______________________
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
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> Jim Brown <mailto:jim at 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 at 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 at 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 at triconet.org>
> Wednesday, September 20, 2017 7:04 PM
> It's not even pretty easy to measure these values.
>
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