Jim, I’m going to give you the same answer you gave me. "I don't buy any of
this.”
In a perfect world, where connectors perfectly match the characteristics and
construction of coax, I would agree. We don’t live in that world.
The proof? The impedance bump reported by a TDR when it “sees” a connector. Our
sensors are telling us that something is up there that doesn’t agree with your
stance. In fact we have a lot of evidence that something else is going on from
that and other sensors.
Like Wes, I would love to see a good mathematical analysis that shows what
happens at a connector, and what happens at the very end of the coax shield.
That should be quite revealing. I don’t remember such an analysis from my
Fields and Waves class from oh so long ago. I do remember the treatment of
theoretical coax, and being asked about it on tests. Alas, there was never any
discussion of coax activity at terminations or on connectors, theoretical or
not.
73,
Jack, W6FB
> On Oct 1, 2025, at 3:34 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> 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@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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