[TowerTalk] Common Mode Choke Locations

[Jim Brown]

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