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Re: [TowerTalk] Common Mode Choke Locations

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Common Mode Choke Locations
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 26 Jan 2023 19:28:37 -0800
List-post: <mailto:towertalk@contesting.com>
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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