At 07:09 16/06/1998 -0700, Mike Michon AB5XP wrote:
>
>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?? 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.
>
>Would this be any better antenna for contesting than a quarter wave
>vertical? We would have a dipole at 100 feet for higher angle stuff.
My money is on the vertical dipole, because the high current area will be
well above ground. This will reduce the look angle and minimise the ground
losses. I wouldn't worry about the tower being grounded, and in fact think
it might help antenna performance rather than hinder it, providing you
stretch out a few radials.
The recent articles by K5IU and VE2CV in Communications Quarterly show that
we still have much to learn about vertical antennas, especially ground
absorption from radials. Clearly the further you keep the antenna return
currents away from ground, the better.
If you have a problem, I think it is more likely to be a breeze at the top
of the tower, blowing the balloon sideways and turning the antenna into an
inverted L. This could mess up the pattern somewhat.
Peter Nesbit VK3APN
pnesbit@melbpc.org.au
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