At 08:33 AM 11/23/98 -0500, w8ji.tom wrote:
...
>Can you imagine "typing in" a wire table that represents all the elements
>and the boom of a TH7-DXX?? Or a stack of antennas of various types? It's
>all part of the tower, and it all affects performance and optimum height.
>
>One way to improve luck would be to install relays at the feedpoint and
>ground all unused slopers to the shield and tower. That almost guarantees a
>low impedance connection for the other half of the feedline (the shield)
>and a decent SWR.
>
>As an alternative you could put the relays 1/4 wl away on a transmission
>line and open the unused antennas, but that results in high open circuit
>voltages (into the kilovolt range) across the relay contacts in the open
>position that might ruin the relays with high power.
>
>Myself, I'd just use sloped dipoles (or ground-end fed slopers if the tower
>was too short) if I was firmly convinced I wanted a vertical. Otherwise,
>I'd use a regular dipole or Vee. W4EX had a system of inverted Vees using
>his guy lines on 160 and 80, and he had a killer 160 and 80 signal and a
>totally uncluttered yard. It allowed him to work DX like he was shooting
>sitting ducks.
just a couple of comments. I have a complete model of my one-tower
installation and all the antennas and guy wires. It isn't totally accurate
(for example, the lower halves of the dipoles in my lazy-vee array describe
more of a quarter circle than a straight line coming back to the tower
base), but the resulting currents on various conductors are fascinating.
The array works better than modeled, for example, which I attribute to the
tower acting as another reflector, judging by the large amount of current
it shows in the model.
I've used high inverted vees, but never had anything like the consistent
performance that the lazy vee array gives me -- of course, "high" for me on
80 means the feedpoint at only about 3/8 WL (90 feet). To me that's the
big drawback of low-band horizontal antennas -- it's hard to get them, high
enough unless, like some, you have a really TALL tower.
73, Pete N4ZR
Loud is good
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