[TowerTalk] Feedline (choke) question

[Jim Brown]

On 9/25/2025 8:27 PM, David Gilbert wrote:

> 2.  A common mode choke at the output of the rig to keep RF that has 
> been picked up by the coax from getting into the rig.  Choking common 
> mode current at the antenna doesn't necessarily keep the coax run to the 
> rig from picking up unwanted RF.  The coax shield is an antenna and 
> making it look like an open at one end doesn't keep it from being an 
> antenna.

Hi Dave,

You're right about pickup on the feedline, BUT: that should NOT matter 
if 1) Proper bonding and grounding is done for electrical safety and 
lightning protection per NEC, which includes, but is not limited to, 
everything bonded to everything and feedlines grounded and lightning 
protected at a bonded entry panel; and 2) the Station, especially the 
operating desk, is properly bonded and grounded.  If it helps, there's 
something seriously wrong with Bonding Grounding.

I think I read into your observation that you're talking about a 
feedline that goes straight to the amp or the radio without that proper 
bonding and grounding. >
> 3.  A common mode choke, like any other inductor, isn't going to have 
> much effect at a current minimum, so placing common mode chokes at 
> current maximum(s) along the coax seems like it would be good additional 
> protection ... and they aren't necessarily located at either end of the 
> coax.  Note that this would be the current maximums for the common mode 
> current, not necessarily for the differential current.  It would 
> probably require a field probe to find those points if they exist.

Yes. EZNEC will show that if you model enough of the antenna system to 
include it.  That observation is the basis for my tutorial on the 
placement along the feedline of stubs to reduce amplifier harmonics, and 
why what W2VJN published on that topic missed the mark, and why I 
published a more correct method in NCJ a year later (and confirmed by 
measurements in my station).

It has been my experience that if you give NEC ALL of the information, 
it will provide very accurate results. The challenge is to SEE all of 
the information, which requires both visual powers of observation and a 
thorough understanding what things can contribute to the system. N6LF 
published a great piece maybe 15 years ago showing that even a tower 
that is relatively short as a fraction of a wavelength can become 
parasitic to a nearby vertical antenna. Rudy's work is some of the most 
important stuff published in the last 30 years on antennas and radial 
systems.

73, Jim K9YC


> 
> Thoughts?