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Old February 15th 06, 07:38 PM posted to rec.radio.amateur.antenna
Roy Lewallen
 
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Default 300 ohm folded dipole from ARRL Handbook, early 1990's

Geoffrey S. Mendelson wrote:
. . .
The shorted length makes perfect sense, the ratio of total to shorted is
.8425 which is a reasonable velocity factor for 300 ohm twinlead.
. . .


Actually, shorting the antenna at an intermediate point between the
center and ends rather than just at the ends doesn't make much sense.

The feedpoint impedance of a folded dipole consists of four times (or
other ratio if the conductors are different in size or there are more
than two folded conductors) the impedance of a standard dipole, in
parallel with two series connected shorted stubs. The dipole consists of
the two conductors in parallel. This behaves as a single fat wire which
has the effective velocity factor of ordinary insulated wire, around
0.97 or 0.98 that of bare wire. That's why a folded dipole is about the
same overall length as a standard dipole. The stubs have the physical
length of half the dipole, or a bit shy of a quarter free space
wavelength. Unlike the dipole part, they operate as transmission lines,
so their velocity factor is around 0.8 -- a value which varies somewhat
with cable construction. Folded dipoles are sometimes shorted about 0.8
of the way from the center to the ends in an apparent attempt to make
the stubs an electrical quarter wavelength, resulting in their impedance
being very high as seen at the feedpoint.

But if the intermediate short circuit isn't done, the effect of the
somewhat longer stubs is only to add a bit of capacitive reactance
across the feedpoint. This lowers the antenna resonant frequency roughly
50 kHz at 14 MHz, and has very little effect on the feedpoint
resistance. Antenna resonance can be restored by simply shortening the
antenna a little. So why bother with the intermediate short?

Roy Lewallen, W7EL