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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