On 10/31/2020 6:07 PM, David Olean wrote:
I would put the ferrite at the end of the coax before it goes into the
rig if there were rf coming down the coax. Common mode energy can and
will get on the outside of the coax. I am not an expert on this,
I AM an expert on this. :) The most important place for a choke in any
antenna system is at the feedpoint. It's function is to prevent the
feedline from becoming part of the antenna by adding a very high common
mode impedance to the common mode circuit. The only good reasons for a
choke elsewhere along the line are
1) to prevent the feedline from becoming a parasitic element of another
antenna; for example, I choke the coax feedlines to my high (125 ft)
dipoles for 80 and 40 so that they do not become parasitic to my three
160M verticals. e
2) to prevent coupling shield current by a mechanism quantified as the
"transfer impedance" of the coax shield. It is the ratio of the
differential voltage generated inside the coax as a result of shield
current. Its unit is Ohms, lower is better, and the lower limit is the
RF resistance of the shield at the frequency of interest. But the
quality, density, and uniformity of the shield makes an additive
contribution -- a poor quality shield increases the transfer impedance.
Both of these measures minimize inter-station interference, both in-band
and cross-band. W6GJB's contesting trailer has antennas for 15, 20, 40,
and 80 supported on a single pneumatic mast, with their feedpoints
within about 2 ft of each other. Because they are very effectively
choked, and because we have dual stubs on the outputs of the power amps
for the two stations, we can operate on adjacent bands at 500W
(K3/KPA500) with minimal interference from the lower frequency station's
harmonics to the higher frequency station. One day, Glen rebuilt the
connection at the antenna and accidentally left out the choke, and the
interstation interference was terrible.
A choke at the rig is only a band-aid for failure to do proper grounding
and bonding in the shack and throughout the house. See N0AX's ARRL book
on the topic, on which I collaborated, or the slides for my tutorial
talk at Pacificon, Visalia, and to various ham clubs.
http://k9yc.com/GroundingAndAudio.pdf
The half-wave section of line you described in an earlier post may take
care of the feedpoint for one band (and it may not be as effective as a
well-designed choke), but it's not a solution for other bands. Properly
designed chokes can cover multiple adjacent bands. See
http://k9yc.com/2018Cookbook.pdf
73, Jim K9YC
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