[TowerTalk] 40M rotary dipole

[Grant Saviers]

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

On 9/20/2017 19:50 PM, Dave Sublette wrote:
> 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
>> On Sep 20, 2017, at 8:04 PM, Wes Stewart <wes_n7ws at triconet.org> wrote:
>>
>> It's not even pretty easy to measure these values.
>>
>> On 9/20/2017 3:29 PM, Jim Thomson wrote:
>>> ##  Pretty easy to get 7500-11,000 ohms of RS these days.... using just 4 x cores +  teflon  RG-142U..and thats
>>> from   160-10m. http://myantennas.com/wp/product/cmc-230-5k/     He can put a balanced output on the box...
>>> with whatever coax connector you want on the input side.   Put 2 of these in series for an eye opener.
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