I have absolutely no experience with analyzing a folded dipole. I would
like to take advantage of this thread to ask for some information.
I had thought of a folded dipole as an antenna with a pair of 1/4 wave
shorted stubs across its feed point. I thought the antenna length for a 1/2
wave dipole would be the same if it was fed as a 72 ohm dipole or fed as a
300 ohm folded dipole. But, if there was some dielectric loading in the
twin lead it would make it necessary to account for the VP of the twin lead
used for the *folded dipole" so the impedance of the stubs would be
accounted for.

Specifically -- If a 1/2 wave folded dipole an antenna is constructed
with a twin lead with a VP of 0.9, and the total antenna length is close to
1/2 wave, the shorted stubs will impose a capacitive reactance shunting the
feed point impedance.
But, when I take my thinking to extreme configurations, my ideas seem to
fail.

I'd sure like to get some "real life" data. I'm sure some of you guys
know if the real life folded dipole gets measurably effected by VPs of 0.8
or 0.9.

Jerry



"Cecil Moore" wrote in message
...
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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