The 1:1 choke at the ladderline/coax junction will impede conducted CM
current from flowing back along the outside of the coax sheath. That
will drive equal and opposite leg currents in the ladderline *at that
As you rightly suppose, other factors which unbalance the antenna or
ladderline could still cause CM current at other points along the
ladderline, despite the choke at the junction.
On 17/01/2011 03:48, Grant Saviers wrote:
> I'm using a ladder line fed 80m dipole as an all band antenna with a
> tuned length of line, high Z 1:1 choke at the LL to coax junction. One
> assumption in these antennas kinda baffles me, so here is a question for
> tower-talkians with expertise in transmission lines and baluns:
> Why is the current into the antenna balanced? If I adequately choke the
> coax at the ladder line junction the current should be balanced there.
> Theory says ladder line doesn't radiate IF the current is equal (must be
> equal in phase and amplitude I assume) in both wires, but it will get
> unbalanced if one side is near metal, etc. So what happens on the
> ladder line if the antenna is not balanced at the feedpoint?
> Is there such a device as a ladder line balun ala the baluns used to
> drive ethernet twisted pairs? (seems like a similar problem but at 100
> ohms pair Z, a ferrite 1:1 current balun is not difficult to build).
> Grant KZ1W
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