[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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