On 1/16/2019 9:43 AM, Rob Atkinson wrote:
Where does the idea come from that a grounded tower holding up an
inverted L is a problem. Is this just some notion that took hold?
The tower is some how an RF sponge? Hams have been doing this for
decades. Of course the tower and ground system are bonded to each
other so the tower provides a return current path as with a ground
system.
To help others think about this, remember, we're talking about RF, not
DC. The BASE of the towers are at ground potential, but the top of a 106
ft tower is about 0.2 wavelength above ground on 160M, and the aluminum
that we add to the top of these towers can easily make them a resonant
passive reflector.
When I first moved in to my W6 QTH in 2006, I had an 80 ft tee vertical
for 160M and fan dipoles for 80/40 and 20/15/10, all suspended from
redwoods. I quickly learned that the coax for the dipoles acted as
passive reflectors for the 160 vertical, so I added ferrite chokes to
take them out of the equation.
Several years later, I found a spot for a 120 ft tower to hold a
SteppIR, and used it to support two 160M wires that sloped away from the
tower to the east and to the west. Each are insulated from the tower and
are fed from their base against four elevated radials. The tower acts as
a passive reflector, giving me about 2 dB gain in the direction of the
slope. I've added about ten radials to the tower to make it a more
efficient reflector.
When I described this setup to friends at our contest club, NI6T and
N6BV both collared me and urged me to look at interaction between those
antennas and the tower. I spent much of the summer doing that, building
an NEC model that included the tower, the Tee vertical, and the two
sloping wires. I learned that the tower, combined with the two sloping
wires, if shorted, which are about 200 ft N of the Tee vertical, act as
a passive reflector for the Tee, giving me about 2 dB of gain to VK/ZL,
with less signal to the north. I also learned that the Tee, if shorted,
acts as a passive reflector for the two sloping wires, moves the main
lobe of the two sloping wires northward, so that the one to the west
peaks at JA and the one to the east peaks to EU. I accomplished those
shorts with feedlines that are close to 180 degrees long and a selector
switch that shorts the unused antenna.
Another point. Rudy Severns, N6LF, has done a lot of definitive work on
radial systems and vertical antennas, all of which is on his website,
and some of which is in the ARRL Antenna Book. One of his papers shows
that even a tower that is relatively short electrically can cause
significant pattern distortion. google to find his website.
73, Jim K9YC
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