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@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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