Topband: Feedline Grounding and Feedline Chokes

[Jim Brown]

Hi Bob,

Lots of well established science on most of this.

Bonding to the tower top and bottom is for lightning protection. 
Combined with proximity to the tower, the feedline and the tower are 
essentially one conductor for common mode, so whatever is induced in the 
tower will couple to the feedline, except that the tower, by virtue of 
its cross-section will carry much more of the common mode current.

Chokes are always a good thing at antenna feedpoints to prevent noise 
(or other signals) picked up on the length of coax near the antenna from 
coupling common mode to the antenna, thus filling in the nulls.

The single most important place for a choke is right at the antenna 
feedpoint. In general, it should be in addition to whatever "balun" or 
matching network is supplied as part manufactured antennas.

Lots of detailed info at k9yc.com/2018Cookbook.pdf

73, Jim K9YC

On 3/25/2020 3:24 PM, Bob K6ZZ wrote:
> Folks,
> 
> I see a lot of references to grounding coaxial feedlines at the top of
> towers, bottom of towers, and at house entry points.  Is this purely for
> static and lightening protection purposes or does it also help mitigate
> Common Mode problems as well?
> 
> If feedlines are well grounded, are chokes still useful for controlling
> Common Modes problems?  I suspect that both can be used, and should be
> used, in a well designed station.
> 
> Are there specific recommendations on the use of chokes on grounded
> feedlines?  Does placement matter?
> 
> Thanks, Bob K6ZZ
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