Jerry Martes wrote:
I have a severly limited capacity for understanding alot of theoritical
presentations. I did read some of Cebik's information, and I still wonder
if the velocity of propagation of the "twin lead" used for the folded dipole
can be ignored. I wonder if the VP of the twin lead is an important
consideration when designing a folded dipole.

The VF of twin-lead used for a folded dipole is approximately the same
as the VF of insulated wire used for a dipole. I'm just not sure how
much "end effect" actually exists in a folded dipole since there is,
technically, no end.

If the resonant frequency of a folded dipole is identified as that
frequency where the input (feed point) impedance is R+/-j0, it seems that
the 1/4 wave stubs that shunt the feed point might strongly effect the input
impedance.

There seems to be some confusion about exactly how the feedline connects
to the folded dipole. Here is the correct way:

+------------------------------------------------------------------+
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+-------------------------------+ +-------------------------------+
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Here is the incorrect way:

|
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+-------------------------------+--------------------------------+
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+-------------------------------+--------------------------------+
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|

For the incorrect way, the feedline is indeed seeing two shorted 1/4WL
stubs in parallel. The currents for the incorrect way would be 180 deg
out of phase and defeat the purpose of the antenna.

However, for the correct way, the currents in the adjacent wires are
in phase and there is a current phase reversal (current minimum point) at
each end of the antenna. After all, a folded dipole is just a one-wavelength
loop with the conductors brought close together. When Mr. Moore invented
the Quad beam, he envisioned a folded dipole with its conductors being
separated incrementally by a distance until it came out to be a square.

The key to understanding the folded dipole is that there is an electrical
phase reversal at the same point as a physical 180 degree reversal.
180 + 180 = 360 degrees, i.e. in phase.

I recognize that there is a good chance that my reasoning is wrong. It
may be that the "stubs" that I consider to be shunting the fed point, are
not acting the same as a 'non disipative' stub. But, this is where my mind
could benefit from having some "lab data" which is what I refer to as "real
life" data.

A classic stub is a current-balanced device with the currents 180 degrees out
of phase. That is not true for a folded dipole antenna. Therefore, a folded
dipole antenna is not composed of true stubs. Semantics strikes again. A series
"stub" is different enough from a parallel "stub" that we probably should not
use the same word for the two of them.
--
73, Cecil http://www.qsl.net/w5dxp



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