On 1/26/2023 6:28 PM, David Gilbert wrote:
If so, depending upon the circumstances, it seems you might want a CMC
at the shack end of the coax as well.
If grounding and bonding is properly done, cable shields should be
bonded to ground before they hit the operating desk. If adding a choke
at the shack end of the feedline reduces common mode current in the
shack there's probably something wrong with grounding and bonding. The
only important practice I can think of not covered in this tutorial is
what to do about two-wire feedlines. The CAN (and usually do) carry
common mode current if there is any imbalance in the system.
http://k9yc.com/GroundingAndAudio.pdf
There is a mechanism related to the quality and construction of cable
shields quantified as a cable's Transfer Impedance. It is the ratio of
the differential voltage induced inside the cable by current on the
shield. This could be a good reason for an additional choke. BUT -- it's
important to realize that in the common mode circuit the feedline is an
antenna and exhibits wavelength just like any other conductor; in the
case of coax, its VF related to the o.d. of the shield and the
dielectric properties of the outer jacket. So think on the order of 0.97.
The lower limit of the transfer impedance is the resistance of the
shield at the frequency(ies) of interest. Transfer impedance is reduced
by making them more uniform and continuous. One of the virtues of hard
line is that its transfer is quite low. This mechanism is most likely to
matter in multi-transmitter setups, like DXpeditions, or
multi-transmitter and SO2R contesting. It's a VERY big deal in 2-way and
broadcast sites where multiple transmitters are present. If you want to
put a ham repeater on one of these sites, prepare for extremely
stringent requirements every element of the transmitter and its antenna
system.
In large cities like Chicago where I spent most of my life, lots of TV
and FM transmitters are diplexed in a handful antennas that are split
between the Sears and Hancock buildings (or whatever they're called now)
that are about 1.5 miles apart.
There is another good reason for using a choke at a second point on the
line -- I break up the feedlines to my high dipoles so that they will
not act as parasitic elements to my 160M vertical. The choke acts like
an egg insulator.
73, Jim K9YC
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