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@contesting.com wrote:
> Send TowerTalk mailing list submissions to
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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@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <34dbf78a-4b01-4d0e-b0dc-558fde9c4c17@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@mebtel.net>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <5d823b09-50f0-4814-80ca-b628449ca305@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@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <116e91bf-d3eb-498e-a0bf-ec5218875e1c@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@yahoo.com>
> To: "towertalk@contesting.com" <towertalk@contesting.com>, Jim Brown
> <jim@audiosystemsgroup.com>
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <105714405.1566405.1759344697922@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@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@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> ------------------------------
>
> Message: 5
> Date: Wed, 1 Oct 2025 15:06:26 -0400
> From: Jim Miller <jim@jtmiller.com>
> To: Wes Stewart <n7ws@yahoo.com>
> Cc: "towertalk@contesting.com" <towertalk@contesting.com>, Jim Brown
> <jim@audiosystemsgroup.com>
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <CACYeN9z8+TYs-NZUhdchpbC20igWTN1frHb2Pr9uKh+cOVHDZQ@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@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@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@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@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@subich.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <518b9e20-6ec8-4a14-a680-bd699cf4ea18@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@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@audiosystemsgroup.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID:
> <9cc77d06-523d-437a-899e-e415362b3cd8@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@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@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
> ------------------------------
>
> Message: 8
> Date: Wed, 1 Oct 2025 17:09:30 -0500
> From: Jack Brindle <jackbrindle@me.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <B174EF96-FFFB-40B9-BEE7-D4655419CDC6@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@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
>>> _______________________________________________
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
> ------------------------------
>
> Message: 9
> Date: Wed, 1 Oct 2025 17:21:28 -0500
> From: Jack Brindle <jackbrindle@me.com>
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
> Message-ID: <621D87FD-3FC2-42C2-9B95-5D5F8D31D8FB@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@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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