Two minor points to add by an amateur in this area (who has been
traveling recently and has not kept up with this thread)...
To begin, THHN wire actually has a nylon jacket over [normally] vinyl wire
insulation. The nylon jacket provides abrasion resistance and a higher
voltage and temperature rating. While I have used THHN wire for amateur
radio field-day antennas, I have never successfully used it for long
outdoors. The nylon has very poor ultraviolet resistance. It also has
considerably different dielectric properties than the plasticized
polyvinyl chloride insulation. It is much lighter than the common THW
thermoplastic vinyl insulated wire. Water can be carried by capillary
action between the nylon jacket and the underlying vinyl too.
The second point involves the traditional folded dipole made from
twin-lead or ladder line. Normally the top and bottom conductors are
shorted to each other at an electrical quarter wavelength from the
feedpoint. This distance takes in the velocity factor of the cable which
is proportional to the reciprocal of the square root of the effective
dielectric constant of the cable (since the fields around a section of
typical ladder line include air as well the cable dielectric). From this
short, the antenna is normally extended with a single (or paralleled
conductor in the case of ladder line) until the folded dipole is resonant.
Having never used a folded dipole except as the feed to a VHF yagi where
other considerations determined the feed impedance, I cannot personally
attest to this practice. I can, however, say that every time I have used
insulated wire with "thick" insulation, the resonant frequency was
slightly lower than the same configuration using bare or enamelled wire.
73, Barry L. Ornitz WA4VZQ ornitz_at_eastman.com
Received on Mon Feb 05 1996 - 22:24:00 EST
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