I am curious. The coax/tower (we'll call it Antenna 1) is isolated from
Antenna 2 (the real antenna) with a choke.
Antenna 1 is still radiating the noise without, let's say, direct connection to
Antenna 2.
So there is no propagation such that Antenna 2 picks up the noise radiated from
Antenna 1, end of which is at most a few inches away?
Stan, K5GO
> On Sep 20, 2017, at 11:33 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> 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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