... a 3/8-wavelength vertical (ALL vertical) with
>two conductors in a folded fashion is said to present a resistive 200-Ohm
>impedance.
My quarter wave base insulated vertical supports an 80 m. dipole @ the 119
ft. level. Even though the dipole and feeder are not electrically connected
to the tower at any point, the close proximity of the open wire feeders
along the length of the tower (they run concentrically up the inside of the
tower) provides coupling. When used as a vertical, the feeders are
disconnected at the base of the tower and left floating.
Depending on frequency, the base impedance measures between 150 ohms and 350
ohms, with considerable +j (which also varies). RF ammeter readings at
known power output verify the measured resistance. The antenna is very
effective on transmit, but I never have tried it as a simple quarter wave
without the dipole, for comparison. Each dipole leg would be 1/8
wavelength, so that should raise the maximum current point similarly to a
3/8 wave vertical. An unexpected benefit of this arrangement is that the
antenna is broadbanded, staying less than 3:1 swr across the band, using a
simple L-network to match the base of the tower to 50-ohm feedline. The
ground system consists of 120 quarter-wave radials buried approximately 2"
below the surface.
Don K4KYV
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