[TowerTalk] Feedline (choke) question

[Jim Brown]

Very well put, Joe. Exactly right.

There's another issue at play too-- shielding effectiveness based on the 
quality of the shield. It's quantified as the Transfer Impedance of the 
shield, defined as the ratio of the differential voltage induced by 
shield current divided by that current. The lower that number, the 
better the shield. The lower limit is the resistance of the shield at 
the frequency of interest. Factors that affect it are the shield 
construction, like the weave of braid, the combination of foil and 
braid. One of the major virtues of hard line is that the shield is 
solid. That's also why cables are made with dense double braid shields 
silver coated copper.

Years ago, shielding effectiveness came up in work we were doing in the 
EMC WG of the AES Standards Committee, and I found a book by Anatoly 
Tsaliovich on the topic, who was at AT&T Bell Labs when he wrote it.

73, Jim K9YC

  10/1/2025 12:43 PM, Joe Subich, W4TV wrote:
> 
> 
> On 2025-10-01 2:51 PM, Wes Stewart via TowerTalk wrote:
> 
>> At the very end of the cable (or connector) there is no inside and 
>> outside of the outer conductor, there is just the conductor, hence
>> there is no skin effect at that point. 
> This is only true if the shield is simply "cut" as in the case of
> the coaxial vertical.  If the cable is terminated in a connector
> - either soldered or crimped - the finite thickness of both the
> shield and the connector will maintain the two wire behavior of the
> shield through the "splice" so long as the shield and connector
> are more than 'n' skin depth thick at the operating frequency.
> 
> Even in the case of a braided shield, RF flows *on the surface* -
> it does not "weave back and forth" with the braid.  This is one
> reason that "hardline" and cables with a second foil shield have
> lower losses than equivalent size size "double braided" cables.
> 
> Common mode currents - unbalanced currents on the exterior of
> the shield - are an electromagnetic phenomena and only possible
> because RF fields force the current to the *surface* of the
> shield - either the outer surface for externally applied (common
> mode) fields or the inner surface for differential (transmission
> line mode) fields.
> 
> The only time those currents are combined is when the transmission
> line is interrupted - e.g. the shield is formed into a pigtail -
> at an antenna or when brought into equipment without proper
> concern (design) for "pin 1" issues.
> 
> In any case, common mode currents can be present in non-coaxial
> lines.  Even simple "zip" cord or other parallel lines can be
> treated by applying an impedance to the unbalanced circuit (as
> is quite common in noise suppression applications).
> 
> 73,
> 
>     ... Joe, W4TV
> 
> On 2025-10-01 2:51 PM, Wes Stewart via TowerTalk wrote:
>>   Jim,
>> I think you're missing Jack's very interesting point.  I've used an 
>> open ended cable as an example, but a mated pair of your favorite 
>> connectors is no different.
>> At the very end of the cable (or connector) there is no inside and 
>> outside of the outer conductor, there is just the conductor, hence 
>> there is no skin effect at that point.  I'm not smart enough to figure 
>> out how far down the cable the skin effect develops.  But this raises 
>> a question in my mind. We've all seen a thousand times the drawing of 
>> a coax-fed dipole, where current is "spilling over" the open end and 
>> becoming a common-mode current on the outside of the cable.  A smarter 
>> mind than mine needs to 'splain this to me.
>> Wes  N7WS
>>
>>
>>
>>      On Wednesday, October 1, 2025 at 10:28:53 AM MST, Jim Brown 
>> <jim at audiosystemsgroup.com> wrote:
>>   On 10/1/2025 7:46 AM, Jack Brindle via TowerTalk wrote:
>>> Connectors are very important in this system. They must be added to 
>>> the analysis. Without them, we have to question the validity of the 
>>> tests.
>>
>> No. Common mode and differential mode currents are a characteristic of
>> transmission lines, and common mode can be present on 2-wire lines if
>> the system that includes the antenna, the transmission line, and
>> termination in the shack has imbalance. The mechanism by which common
>> mode in coaxial line is on the outside of the shield is skin effect, and
>> it's present in those connectors.
>>
>> Soldered or crimped, the connector(s) is/are simply part of the
>> transmission line, carrying the differential and common mode current
>> that is in that system (antenna, line, shack). Depending on their
>> construction, they can introduce some discontinuity in the differential
>> circuit.
>>
>> 73, Jim K9YC
>>
> 
> 
> 
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