[TowerTalk] Shunt Feeding, Can this be?
w8ji at contesting.com
Tue Nov 15 19:04:49 EST 2005
> The 1:1 reading is absolutely flat for about 60-70Khz. It
is the same on both bands in as far as being flat and wide.
I thought I might have a poor ground, but I have double
checked all connections and resoldered everything that looks
even remotely bad. I also have 15 ground rods connected to
the tower on three legs besides the radials. This is my
first attempt at shunt feeding a tower and my question is if
it is possible to have such a flat SWR on a shunt fed tower?
I don't know why Hams persist in repeating the false idea
that bandwidth tells us something about efficiency.
Bandwidth is related to efficiency ONLY when everything in
the system is a constant and we add loss resistance. In
other words if we compared two exactly identical systems
with no changes other than loss resistance, the wider
antenna would be the lower efficiency antenna. That is where
it starts and stops.
My widest BW mobile antennas, even though they are the same
overall height as other narrower band antennas, also are the
most efficient by far. It can also be the other way around.
For example, a one foot diameter 130 foot tall vertical with
ONE thin elevated radial can be much less efficient and also
have much less bandwidth than the very same vertical with
100 radials! Why? Because we changed the structure. On the
other hand if we had that same vertical and added an
additional series or shunt lumped resistance, the vertical
with the additional resistor would always have wider
bandwidth. Clearly we can only apply that rule when nothing
in the antenna changes, not even the ground system. So what
good is it?
Unless you are looking at one structurally unchanged system
(you can't even change the ground) where only *losses* are
changed, BW is virtually useless to estimate or determine
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