Vertical Section = 52 feet using two #14 wires connected at the base and diverging upwards to attach with a spread of 6 feet. Horiz section = Tee arrangements with equal wire (#14) length on either side with flat top for 16 feet (using 1.5" aluminum tubing; 8' per side) then sloping downward at 45 degs
for the remaining length using #14 wire. Desired F
Not a good approach, actually, because the end wires reach nearly to ground level
by the time the resonant frequency reaches 1.8 MHz, which lowers efficiency (besides
being a safety hazard.) Radiation resistance of the antenna is about 5 ohms.
A general rule is that sloping wires used for top loading shouldn't extend much more
than half way down the antenna to maintain efficiency. (Actually I think the number
from the study I read was 40% of the antenna height, but half way is easier to
So instead I fixed the end height of the wires at 25' and varied the length, letting the
angle be whatever it is. The required that the wires extend out about 70' from the
center on each side (68' of wire on each end of the center crossbar.) Radiation
resistance is 7.5 ohms: when matched to that value, the SWR bandwidth is about
40 kHz at 2 : 1 (though part of that is below the bottom of the band.)
Adding 15 ohms of ground loss resistance and matching the antenna to 22 ohms
gives a 2 : 1 SWR bandwidth from 1.75 to 1.87 MHz, or 120kHz.
This assumes that the vertical wires maintain equal spacing most of the way down
to the feedpoint. If I bring the bottom ends closer together (there are some
modeling issues with bringing wires together at small angles) the passband lowers
by about 20 kHz. Shortening the loading wires to 64' on either end of the
aluminum tubing puts the 2 : 1 passband from 1.79 MHz to 1.9 MHz, with an
SWR of 1.5 : 1 at 1.8 MHz.
One observation is that there is no current flowing in the center of the aluminum
tubing between the attachment points of the vertical wires. That means that using
an insulator that point would allow such an antenna to serve as a doublet for 80m
and higher bands simply by changing the configuration at the feedpoint.