NEC-LIST: radiating transmission lines

Here is a simple, but fundamental question relating open-wire
transmission line to antenna behavior. That is, when does an
open-wire, free-space length of twin-lead become an antenna? I guess
the general answer is, "when the spacing between wires is electrically
large, but this also depends on the terminating impedance."

How can you use regular transmission line formulas to quantify
radiation from "twin-lead?" Assume a 90 degree length of twin-lead
with a velocity factor of 1.0 (i.e. air dielectric) and perfect
conductors. Also assume the spacing between parallel wires is 90
degrees. If this transmission line were non-radiating, I would expect
you could terminate it in a pure resistance equal to is surge
impedance, and measure that same pure resistance at its input.
Terminating such a line with a third wire loaded at its center might
not be a good approach, since the introduction of a third wire
complicates the model. How could one avoid adding a third wire, but
still connect a load between the ends of the twin-lead? Maybe I
should read the NEC manual !!

The same question remains when doing the open-circuit, short-circuit
test to estimate surge impedance.

But let us assume that a valid resistive load has been connected at
the end of the twin-lead, and for all possible values of load
resistance there is no purely resistive input impedance to the
twin-lead. So we have a reactive term at the input which must relate
to radiation is some explicit, quantifiable way. And perhaps the
surge impedance of the line is not a pure resistance, either. In
other words, the line looks lossy, but the loss R is actually
radiation resistance, not conductor I2R loss or dielectric loss.

Any ideas or examples?

Grant Bingeman
Received on Tue Jun 27 2000 - 14:03:54 EDT