>The currents will flow on the outside of the coax shield. Now, why is that a
>problem?"
A few potential problems come to mind, all of which have been documented in the
literature for decades:
1) Unintended line radiation;
2) Directional antenna pattern distortion;
3) Inaccurate SWR measurements due to common-mode current.
Paul, W9AC
Where do these currents go to? According to your statements, they will simply
stay on the outside of the shield, never combine with the currents on the
inside, and somehow not go into the transcribing equipment. Or perhaps they
will re-radiate from the coax, but if so what will receive them?
If the outer currents never combine with the inner ones, then there should be
no problem.
We both know this is incorrect. We seem to disagree on exactly how these
currents cause problems. What is it that you see as that mechanism?
73,
Jack, W6FB
> On Oct 1, 2025, at 5:09 PM, Jack Brindle via TowerTalk
> <towertalk@contesting.com> wrote:
>
> 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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