[TowerTalk] Coax, conduit and toroids
jim at audiosystemsgroup.com
Thu Jun 28 12:36:31 PDT 2012
On 6/27/2012 1:34 PM, Steve Hunt wrote:
> By the way, this wouldn't be the first time that someone tried to
> discredit NEC because it produced answers that disproved their claim!
Nor would it be the first time that someone didn't understand the
circuit or the model.
Finally had time to look up what I had remembered reading. Quoting from
W7EL's manual for his EZNEC software (pdf download from his site), Roy says:
"I don't know of any way to accurately model common-mode effects on a
two-wire transmission line (that is, how to model a radiating two-wire
line). If it is necessary to do this, the line will have to be modeled
as two parallel wires. See Closely Spaced Wires for precautions."
= = = = = = = = = =
Roy's instructions for closely spaced wires note that segments must be
precisely the same length and precisely aligned with each other. They
must also be several wire diameters apart. It has been observed that NEC
has difficulty with very closely spaced wires.
That aside, two question remains, where in the model of parallel
conductor line might one insert a common mode choke? And even if you
COULD insert it, what would be the impedance at the transmitter between
the line and the earth? To answer that you would have to know about the
Let me repeat that the fundamental definition of "balance" is defined by
balances of the IMPEDANCES of the entire SYSTEM, and that SYSTEM
includes the antenna, it's relationship to the earth and surrounding
conductors, the feedline, the transmitter/matching network, and
everything between that and the earth. Few transmitters are connected to
the soil by wires of zero length, and most earth connections are pretty
lossy at RF.
73, Jim K9YC
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