On Tue, 16 Jun 1998 07:09:49 -0700 "Michael Michon" <michon@eatel.net>
writes:
>A friend and I are thinking of a vertical dipole for 160 meters. We
have a fire tower 100 feet tall. 22 feet sqare at the base tapering to
6 foot square at the top. It is grounded. We would extend a balloon
from the top for the other half of the antenna.
Is the bottom being grounded bad, good??
==============
Answers:
Bad if you intend to feed it as a normal dipole, i.e., center-fed. You'd
find the feedpoint will be about 10k ohms. This is because the bottom
half of the "dipole" is grounded at its lower end.
Possibly good if you intend to do one of the following:
1) Insulate the tower from ground/elevate it.
2) Shunt feed (at the base) the 1/2-wave combination of the tower and
wire. (but only good if the soil is "very good" or better.)
==============
>We could drop a wire in between the tower or extend a mast from the cab
and hang a hundred foot wire down if this is better.
==============
I feel this would be more practical, but if the tower is near 1/4-wave
resonance on 160m (it probably is), it will effect the pattern and
feedpoint Z. (I assume you're talking about base-feeding a 100' wire
spaced away from and insulated from the tower.)
==============
>Would this be any better antenna for contesting than a quarter wave
vertical?
==============
Over your "poor" Mississippi soil the answer is no, unless the tower is
insulated from ground.
==============
>Any thoughts as to the difference between a vertical dipole and a
quarter-wave vertical??
==============
A shunt-fed half-wave grounded vertical will only perform better than a
quarter-wave
vertical if the soil conductivity is "very good" or better. Also no
matter what the size of the grounded vertical, it requires a good radial
system.
An elevated vertical dipole should perform better for DX than a grounded
quarter-wave vertical because of its lower take-off angle. Also, it
doesn't need radials under it. And the more elevated it is, the better.
========
My above answers are based on modeling results of the following:
1) Elevated half-wave center-fed vertical dipole
2) Grounded half-wave center-fed vertical dipole
3) Shunt-fed grounded half-wave vertical dipole
4) Base-fed (either series or shunt-fed) grounded 1/4-wave vertical
73, de Earl, K6SE
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