[TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Thu Jun 28 01:14:09 PDT 2012

For those who still have an open mind on whether or not antenna system 
balance guarantees zero voltage across the CM choke, I've sketched an 
equivalent circuit showing various currents, voltages and impedances at 
the feedpoint of a coax-fed dipole:



* Z1 and Z2 together represent the dipole's impedance; if the dipole is 
perfectly symmetrical wrt ground Z1 = Z2
* Vd and Id are the differential mode signals delivered by the coax 
feedline at the feedpoint.
* Vcm is the voltage wrt ground of the coax braid at the feedpoint
* Zcm represents the impedance of the CM choke mounted at the feedpoint
* Zx represents the CM path impedance along the coax braid outer surface 
back to ground


* If there were no CM path, the system would be balanced and the current 
into side 1 of the dipole, I1, would equal the current in the other side Id

* The existence of the CM path potentially allows a common-mode current 
Icm to flow, leading to a disparity between the dipole leg currents Id 
and I1

* We attempt to make Zchoke as high as possible in order to minimise 
Icm, and equalise I1 and Id

* In the situation where we have achieved balance, Icm = 0 and Vcm = 
Vd/2; note that Vcm is NOT zero under balanced conditions!

* The proportion of Vcm appearing across the choke will be Zchoke / 
(Zchoke + Zx)

* Under balanced conditions, the only way the choke voltage can be zero 
is the trivial case where Zchoke is zero, or the special case where Zx 
is infinite; if that special case pertains, the choke is redundant 
because Icm would be zero anyway.

All of the above can be demonstrated using simple NEC modelling.

Hence my statement that balance does _not_ lead to zero CM voltage 
across the choke.

Steve G3TXQ

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