# [TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Wed Jun 27 13:34:05 PDT 2012

```For anyone without access to a modelling tool, let me try to give you a
simple qualitative analysis.

Say we have a perfectly symmetrical dipole fed with coax that drops away
exactly at right angles from the dipole. An extremely high-impedance
common-mode choke has been fitted to the coax at the feedpoint, which
reduces the CM current to a negligible amount. Let's say the
differential-mode voltage across the dipole feedpoint is 100v.

The presence of the choke ensures that the balance of the system is
preserved; so, measured with respect to ground, one side of the
feedpoint will be at 50v and the other side will also be at 50v but
opposite phase. So we know that there is a voltage of 50v wrt ground
trying to "drive" CM current through the choke and back along the coax
braid.

Now, for Jim's claim to be true that there is zero CM voltage across the
choke, the CM voltage at the radio side of the choke must also be at 50v
wrt ground.

Ask yourself how that can be, when there is negligible current flowing
along the braid; where does that 50v "come from"?

By the way, this wouldn't be the first time that someone tried to
discredit NEC because it produced answers that disproved their claim!

Steve G3TXQ

On 27/06/2012 21:05, Steve Hunt wrote:
> Jim,
>
> I'm not going to waste my time in long debate with you again!
>
> I'll just leave other readers to check what I have said using NEC. You
> might believe that NEC can't model the common-mode currents, but that's
> the first time I've heard that claimed, and there are plenty of other
> well-respected folk - Roy Lewallen, VK1OD and W8JI for example - who I
> expect would disagree with you. Look at Tom's website and you'll see how
> he uses EZNEC to model common-mode situations.
>
> I encourage other readers not to believe me or Jim but to check it for
> themselves; model a simple coax-fed symmetrical dipole and see if Jims
> claim that "a perfectly balanced antenna system must have zero voltage
> across the choke and zero current through it" holds up.
>
> And remember, if the choke has zero voltage across it and zero current
> through it, you can safely remove it and nothing will change :)
>
> Steve G3TXQ
>
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```