The 1986 handbook has the details for this type of antenna on page 33-14,
under the title of "Simple Antennas for HF Portable Operation." For 7.15
MHz, the value of the capacitor is 152 pF, and the length of the matching
stub is 6'11-1/2". An open-end stub, made from twin lead, of length 20'-1/2"
can be substituted for the capacitor.

"Roy Lewallen" wrote in message
...
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