[TowerTalk] Coax, conduit and toroids

Jim Brown 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 
matching network.

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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