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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