Richard Harrison wrote:
My ARRL Antenna Book (19th edition, page 8) says:

"Since the antenna section (of 300-ohm twin-lead) does not operate as a
transmission line, but simply as two wires in parallel, the velocity
factor of twin-lead can be ignored in computing the antenna length."

I wish the author had said:

"---the transmission line velocity factor of twin-lead can be
ignored---."

The phase of the currents in the adjacent sections of twinlead is what
is important. If the phase of the adjacent currents is 180 degrees, the
twinlead is acting like a transmission line and T-line VF must be taken
into account. If the phase of the adjacent currents is zero degrees, the
twinlead is acting like an antenna and the VF is considerably higher,
essentially equal to insulated wire. If the phase of the adjacent currents
is zero degrees, all the current is "common-mode current", something not
desirable for transmission lines but something most desirable for antennas
since common-mode currents do not inhibit radiation.

Bottom line: The currents flowing in a folded dipole antenna are common-
mode currents which radiate, not transmission line currents which do not
radiate (much), and that's a very good thing for an antenna.
--
73, Cecil http://www.qsl.net/w5dxp



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