Mike;
Thanks. This is very interesting information, and I agree with you. The
discussion at hand is not about connector leakage, in fact one of the
assumptions is that the signals are being fully contained within the connector.
Rather the question is that in the presence of common mode (or other) signal on
the outside of the coax shield, what happens with that and the desired signal
carried on the inside of the shield inside the connector.
To be certain, there will be “leakage”, both into and from the outside of the
coax shield due to it being in an RF-rich environment (probably far more in
than out). This is the exact same thing that makes antennas work. We really
want the antenna picking up all sorts of RF, including the signal we are
interested in. We would rather that the coax shield not pick up these signals,
but because of the nature of things, it will. The good news is that for the
most part the signals will be weak enough as to not cause any problems. Of
course, we do not want any of those signals breaking through the coax (or
connector) to get in (or out) and interfere with our desired signal. Those of
us who have dealt with inter-station-interference know the lengths we have to
go to eliminate those issues. Usually the problem is more of one antenna
getting into another, but there are multiple modes for the problem to occur.
One of the problems we see in SO2R and Multi-transmitter stations is that over
time connectors may loosen and cause leakage. I periodically check all my
connections and make sure they are tight. Even when PL-259s are tightened with
a wrench, they can loosen over time. It’s not a good experience to discover
this has happened in the midst of a contest!
73,
Jack, W6FB
> On Oct 1, 2025, at 8:47 PM, Michael Tope <W4EF@dellroy.com> wrote:
>
> Hi Jack,
>
> Coaxial connectors leak, but generally not very much. The amount depends on
> the quality of construction, the applied torque, and the frequency. I worked
> in the CATV industry for a few years in the early 90's and I remember
> cumulative leakage index (CLI) being a big deal. The operating frequencies of
> Cable TV systems overlap the VHF/UHF ham-bands, public service communication
> frequencies, over-the-air television frequencies, and aeronautical
> frequencies. To successfully coexist with these services, CATV plants have to
> be well-shielded.
>
> Customer premises equipment (CPE) such as F-connectors were a source of
> trouble as were passive devices (e.g. RF splitters and couplers). The really
> low-end passives were housed in zinc castings with back-plates that were
> glued on with an inexpensive non-conductive adhesive. I think the castings
> had a lip that surrounded the outer edge of the back-plates that was
> flared-in a bit with some sort of mechanical press to improve electrical
> contact before the glue was applied. The higher quality passives were
> tin-plated so that back-plates could be solder-sealed. These solder-sealed
> passives radiated significantly less than the ones that had the glued-on
> backs. Likewise system operators were always looking for connectors that were
> easy too install and provided consistently good connections with low
> "ingress/egress". Nevertheless, the radiation from a run-of-the-mile
> F-connector might be 90 dB down from an isotropic radiator. A better
> connector might be down 115dB (I don't remember the exact numbers, but that
> was the order of the magnitudes involved)
>
> So yeah, connectors can radiate, but not very much. If you don't believe me,
> hook a good shielded 50 ohm load to the RX input of your HF receiver and
> measure the noise floor. Then hook up a long run of coax comprised of
> multiple sections of coax spliced together with multiple barrel junctions to
> your HF receiver input and put the 50 ohm load at the far-end of that run. In
> most cases if you are using properly-installed decent-quality connectors and
> adapters, you won't hear much, if any, change in the noise floor. And of
> course, the minute you loosen the connector on the back of the receiver and
> pull it out a little so only the center conductor is making contact, the
> noise floor will (depending on where you live) go up something like 20 or
> 30dB as that long run of coax is converted from a shielded enclosure into an
> antenna.
>
> 73, Mike W4EF......................
>
>
> On 10/1/2025 3:09 PM, Jack Brindle via TowerTalk 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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