> I'd appreciate your recommendations for Slant Feeding a 88'
> self-supporting
> tower for 160-m. The only antenna on the tower is a Cushcraft XM-240
> (Shorty-Forty) on top. The XM-240's elements are not grounded therefore
> only the 22'
> boom is electrically attached. (the tower is 36" wide at the base and 18"
> at
> top)
By slant feed, I am assuming you mean a wire from the center of your radials
(30 ft away from the tower) to somewhere on the tower?
I've run a couple dozen NEC4 models on this scheme with various taps up and
down the tower over a medium ground. All of them show huge losses,
basically due to not centering the radials on the tower and connecting them
to it.
The problem seems that you are establishing an "inducing" loop, where the
three loop components are coax center conductor to tower tap, tower tap down
to tower base, tower base to coax feed shield by some means. The last is the
killer. It is worst with only a grounded tower, and only somewhat better
with a connecting wire.
No matter what you do with a tap, you will be forcing current from tower
base to feed coax shield. That path along or through ground causes the
combination to be extremely lossy, generating pattern gains of -7 dBi and
worse. The reference antenna is a commercial broadcast quarter wave which
is modeled at +1.2 dBi using the same environment assumptions.
Speculation on a 50 ohm tower tap seems pointless with a performance that
bad.
73, Guy.
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