Topband: Automatic Bandswitching 80/160m Vert

[Michael Sapp]

Hello Bob & Group: Take a look at March 1998 QST article "Building
Efficient
Short Vertical Antennas" (AC7A) There are some antenna efficency
comparisons
for wire capacity hats with 0 and -25 degree angles and inductive loading
at
different points along the vertical. (Keep the voltage and current
distribution
across the vertical antenna in mind as you read the text.)  If you could
keep the capacity hat wires
horizontal and fold them back like a folded dipole that might be a
reasonable
comprimise. If you top load the antenna with capacitance, inductance, or
some
combination of the two, the 160m loading device should look like an RF
choke at
80m, so no switching would be needed. In practice, you may want the 80m
part
of the antenna slightly shorter than 1/4 wave and use a small variable or
tapped inductance at the
base to achieve resonance at your preferred operating frequencies. That
should give
some roaming capabilities on both bands. There is an interesting vertical
antenna
modeling program at www.smeter.net/software/loadcoil.exe that would allow
you
to compare vertical antenna efficiencies with a small coil at the base for
tuning purposes---or larger
coils on different points along the vertical for loading purposes. At the
end of the day we are
all limited by what is practical to put up in the back yard (and bury in
the ground).

                                Mike, WA3TTS