[TowerTalk] Shunt Feed Woes

[Jim Brown]

The N9NB/W5JAW installation seems not bond coax shields to the tower at 
top and bottom, which is in conflict with long standing good engineering 
practice for lightning protection. This bonding, combined with running 
the coax in close proximity to a tower leg, puts the coax at the same RF 
potential as the tower, point-by-point along the height of the tower, 
and because the tower has a far greater cross section, it carries nearly 
all of the current. So if these coax runs were properly bonded, the only 
need I see for common mode chokes is at each Yagi feedpoint to prevent 
noise coupling and null filling due to common mode current at the 
operating frequencies of those antennas, and these chokes would be wound 
for those operating frequencies, not 160/80M. Perhaps I'm missing 
something about physical construction of the antenna(s) in question.

I have a very low level of confidence with respect to Balun Designs 
products. That's based on one of their units (brand new, no possibility 
of damage) that an NCCC member who's building a station in the Caribbean 
asked me to measure. It looked NOTHING like its data sheet that was on 
their website at the time. I circulated that data to several trusted 
engineers, and within a few months, the data sheet plot had disappeared 
(I suspect that one of those engineers asked them about it). My 
assessment is also based on things I've read on their website about 
chokes that indicate that they don't understand how chokes work.

73, Jim K9YC

On 12/7/2019 11:30 PM, K9MA wrote:
> The shunt chokes may not be as critical with the slopers, as the voltage 
> near the top of the tower is probably lower than with a shunt fed tower.