The currents in the wires of a folded dipole or monopole are neither in
phase nor 180 degrees out of phase, as you can easily see from EZNEC.
Because they obey superposition, you can, for convenience, consider them
as two separate sets of currents, common mode (or "antenna" current) and
one differential mode (or "transmission line" current). Neither one is
zero. The phase of the antenna current is a function of the velocity
factor of the two wires excited in parallel. For TV twinlead, for
example, this would be something like 3 percent slower than for bare
wire -- about the same as any other typical insulated wire. In contrast,
the phase of the transmission line current is dictated by the velocity
factor of the two wires excited out of phase as a transmission line. In
this mode, there's an intense field between the conductors, so the
dielectric between the conductors has much more impact. The velocity
factor for this mode is more like 0.8, so the transmission line will be
electrically considerably longer than a quarter wavelength.

The TV twinlead "antenna" will be resonant, then, at a length about 3%
shorter than if it were made from two parallel bare wires -- not
because the currents in the two wires are in phase, but because the
common mode part of the currents are in phase -- by definition, in fact.
But the effect of the transmission line stub also affects the feedpoint
impedance, and its velocity factor has to be accounted for in the
calculation of its contribution. I've seen a recommendation that the
conductors of a twinlead folded dipole be shorted about 80% of the way
from the center. What this does is to make the transmission line nearly
a quarter wavelength long, so its contribution to the feedpoint
impedance is negligible. Then you don't need to make any adjustment of
the length to compensate for the transmission line. Alternatively, you
can short circuit the wires at the ends in the normal fashion, and
slightly adjust the length to compensate for the impedance change caused
by the transmission line.

Roy Lewallen, W7EL

Cecil Moore wrote:
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