That is a great question, and one I have been contemplating for quite a while.
My conclusion is “it depends”. If any of the signals are strong enough to cause
problems, then the answer is yes. I believe that this is actually the basis for
K9YC’s work on having chokes at the antenna. It eliminates the common mode
signal from the antenna end of the coax. The question is where else a choke may
be needed and the cause.
I learned 20 years ago that a roof-mounted vertical for 40 meters (16 radials)
absolutely needed a choke at the feedpoint. But I also saw pickup on the line
that caused sensor issues in the shack where the next connector plugged into my
test amplifier. They were solved by wrapping the coax through another toroid
several times at the end of the coax. My conclusion was that the coax was
picking up RF along the way. I assumed it was from the antenna radiation, but
there were other radiators in the area, including a strong 1500 KHz AM station.
73,
Jack, W6FB
> On Sep 30, 2025, at 9:44 PM, Wes Stewart via TowerTalk
> <towertalk@contesting.com> wrote:
>
> Well, an open ended cable is the limiting case, a connector just extends it a
> bit. Along the length of the cable, if the frequency is high enough, skin
> effect separates the shield into two conductors. At an open end they are
> "tied together" and as you put it, the two signals "mix".
> This is all very interesting and is something that I haven't thought about
> before. But it does beg the question, should we have a CM choke before and
> after every connector pair?
> Wes N7WS
> On Tuesday, September 30, 2025 at 05:41:37 PM MST, Jack Brindle via
> TowerTalk <towertalk@contesting.com> wrote:
>
> I don’t believe it has to be open-ended. The theory states that the inside of
> the shield and the center conductor carry the transmission signal, usually
> between equipment or from antenna to whatever it connects to. The outer
> portion of the shield is open to pick up whatever RF signals that come its
> way, whether these are from the local antenna on transmission or someplace
> else (maybe the AM station down the road?). Each end of the coax may have a
> connector -or- perhaps the antenna end is directly connected to a wire. In
> both cases the outside and inside of the shield are tied together at the
> connection. It is at these points that the inner and outer signals may mix,
> unless the connector somehow is able to keep the two signals (carried by
> skin-effect) separate. I have never seen a connector that is able to do that.
>
> Dual-shield coax (like bury-flex) may keep the signals even more apart - the
> inner shield (aluminum foil) would carry the inner one (again, the depth is
> determined by frequency and skin effect), while the braid would carry the
> induced, or received signal. In this case the signals would again mix at the
> connector where the two conductors come back together. Note that there may be
> many more than two signals of concern - there may be multiple
> received/induced signals on the outer portion of the braid.
>
> In this situation it would make sense that the only way to keep this mixing
> from occurring would be to wind the coax through a toroid next to a connector.
>
> I would also note that a coax cable will carry far more than one signal on
> its conductors. We like to think there is only the one we are interested in,
> but it is the radio (transmitter or receiver) that determines the signal of
> interest and filters out everything else (usually).
>
> I don’t remember seeing n a detailed discussion of this situation, but have
> observed it many times. I would love to see a really good analysis.
>
> 73,
> Jack, W6FB
>
>> On Sep 30, 2025, at 5:45 PM, Wes Stewart <n7ws@yahoo.com> wrote:
>>
>> Ok. So this happens anytime you have an open ended coax.
>>
>> Wes N7WS
>>
>> On Tuesday, September 30, 2025 at 02:29:58 PM MST, Jack Brindle
>> <jackbrindle@me.com> wrote:
>>
>>
>> No. All connectors that connect the inner and outer edges of the shield.
>> Purely from skin effect.
>>
>> 73,
>> Jack, W6FB
>>
>>> On Sep 30, 2025, at 4:22 PM, Wes Stewart <n7ws@yahoo.com> wrote:
>>>
>>> I'm trying to understand the mechanism for this. Are you suggesting that
>>> this is unique to UHF connectors due to the serrations?
>>>
>>> Wes N7WS
>>>
>>>
>>> On Tuesday, September 30, 2025 at 01:52:55 PM MST, Jack Brindle via
>>> TowerTalk <towertalk@contesting.com> wrote:
>>>
>>>
>>> The problem is in the way you connect it. If you use a PL-259, then you mix
>>> the outer and inner currents at the connector, thus “polluting” the signal
>>> going into the radio. But then the same thing will happen if you add a
>>> choke with connectors.
>>>
>>
>
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