[TowerTalk] TowerTalk Digest, Vol 274, Issue 2
Edward McCann
ag6cx1 at gmail.com
Wed Oct 1 21:31:54 EDT 2025
Those interested may find solace and dine answers in the following:
Understanding Common-Mode Signals | Analog Devices
https://share.google/MxoIBb4TYICZuraNR
And the Ott Note mentioned earlier
Common-Mode Current Measurements
https://share.google/f7YlrJIilovlVnin5
Ed McCann
AG6CX
> On Oct 1, 2025, at 3:26 PM, towertalk-request at contesting.com wrote:
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> Today's Topics:
>
> 1. Re: Feedline (choke) question (Jim Brown)
> 2. Re: Feedline (choke) question (Howard Hoyt)
> 3. Re: Feedline (choke) question (Jim Brown)
> 4. Re: Feedline (choke) question (Wes Stewart)
> 5. Re: Feedline (choke) question (Jim Miller)
> 6. Re: Feedline (choke) question (Joe Subich, W4TV)
> 7. Re: Feedline (choke) question (Jim Brown)
> 8. Re: Feedline (choke) question (Jack Brindle)
> 9. Re: Feedline (choke) question (Jack Brindle)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 1 Oct 2025 10:27:36 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <34dbf78a-4b01-4d0e-b0dc-558fde9c4c17 at audiosystemsgroup.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> 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
>
>
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Wed, 1 Oct 2025 13:46:47 -0400
> From: Howard Hoyt <hhoyt at mebtel.net>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <5d823b09-50f0-4814-80ca-b628449ca305 at mebtel.net>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
>>> Date: Tue, 30 Sep 2025 21:22:23 -0700
>>> From: Jim Brown via TowerTalk wrote:
>
>>> The shells and center conductors of connectors splicing two sections of line
>>> exhibit skin effect just like any other conductor. The only difference are
>>> the dimension of the shield as it goes through the connector(s).
>>> 73, Jim K9YC
>
> Exactly, following EM theory the energy is contained in the interstitial space between the inner and outer conductor. The CM currents are due to potential difference between the outer surface of the shield and the capacitance with the environment. This is why EM fields in the environment cause CM currents to flow. CM Chokes therefore reduce external pickup from the coax in receive.
> The reciprocal is true in transmit.
>
> If you look at the magnitude of CM currents using an inductive pickup on feedlines as I have you will see both CM chokes and grounding can reduce CM currents at the shack, but the grounding is perhaps more problematic given it's variable impedance as part of the changing environment. A well designed CM choke as Jim well describes in his documentation can provide reliable CM attenuation regardless of ground impedance I have such chokes on my single feedline at the feedpoint, where it hits ground level and in the shack.
>
> Cheers & 73,
> Howie / WA4PSC
>
>
>
>
> ------------------------------
>
> Message: 3
> Date: Wed, 1 Oct 2025 11:38:12 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <116e91bf-d3eb-498e-a0bf-ec5218875e1c at audiosystemsgroup.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> On 10/1/2025 10:46 AM, Howard Hoyt via TowerTalk wrote:
>> If you look at the magnitude of CM currents using an inductive pickup on
>> feedlines as I have you will see both CM chokes and grounding can reduce
>> CM currents at the shack, but the grounding is perhaps more problematic
>
> Mostly good observations, Howie. but problems in the shack have NOTHING
> to do with grounding, and everything to do with BONDING! Earth
> connections are about lightning safety. If chokes solve problems in the
> shack, some element of bonding is not done properly.
>
> And when trying to measure CM on the line, we get into the obvious
> problem of doing so without disturbing the system we're trying to
> measure. Any conductors that couple to the line can do that if they are
> large enough (as a fraction of wavelength, and depending on the
> impedance of the common mode circuit at the point of measurement).
>
> 73, Jim K9YC
>
>
>
>
>
> ------------------------------
>
> Message: 4
> Date: Wed, 1 Oct 2025 18:51:37 +0000 (UTC)
> From: Wes Stewart <n7ws at yahoo.com>
> To: "towertalk at contesting.com" <towertalk at contesting.com>, Jim Brown
> <jim at audiosystemsgroup.com>
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <105714405.1566405.1759344697922 at mail.yahoo.com>
> Content-Type: text/plain; charset=UTF-8
>
> 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
>
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> ------------------------------
>
> Message: 5
> Date: Wed, 1 Oct 2025 15:06:26 -0400
> From: Jim Miller <jim at jtmiller.com>
> To: Wes Stewart <n7ws at yahoo.com>
> Cc: "towertalk at contesting.com" <towertalk at contesting.com>, Jim Brown
> <jim at audiosystemsgroup.com>
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <CACYeN9z8+TYs-NZUhdchpbC20igWTN1frHb2Pr9uKh+cOVHDZQ at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> And yet with coax there is no "inside or outside". The shield wires are
> woven and pass over and under each other. No different than at a connected
> junction with a connector.
>
> The fields are still maintained separately.
>
> jim ab3cv
>
> On Wed, Oct 1, 2025 at 2:52?PM Wes Stewart via TowerTalk <
> towertalk at contesting.com> 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
>>
>>
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> ------------------------------
>
> Message: 6
> Date: Wed, 1 Oct 2025 15:43:26 -0400
> From: "Joe Subich, W4TV" <lists at subich.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <518b9e20-6ec8-4a14-a680-bd699cf4ea18 at subich.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
>
>
> 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
>
>
>
>
>
> ------------------------------
>
> Message: 7
> Date: Wed, 1 Oct 2025 13:34:47 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <9cc77d06-523d-437a-899e-e415362b3cd8 at audiosystemsgroup.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> 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
>>
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
> ------------------------------
>
> Message: 8
> Date: Wed, 1 Oct 2025 17:09:30 -0500
> From: Jack Brindle <jackbrindle at me.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <B174EF96-FFFB-40B9-BEE7-D4655419CDC6 at me.com>
> Content-Type: text/plain; charset=utf-8
>
> 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 at 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 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
>>> _______________________________________________
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
> ------------------------------
>
> Message: 9
> Date: Wed, 1 Oct 2025 17:21:28 -0500
> From: Jack Brindle <jackbrindle at me.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <621D87FD-3FC2-42C2-9B95-5D5F8D31D8FB at me.com>
> Content-Type: text/plain; charset=utf-8
>
> Let?s try this another way. What is the problem that a choke is supposed to solve? I believe you have told us that a good, high-impedance Choke will block, by absorption, common-mode currents that flow on the outside of coax shields. So, what is the problem that occurs if the choke is not employed? The currents will flow on the outside of the coax shield. Now, why is that a problem? 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 at 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 at 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 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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