On Thu, 25 Mar 2010 07:39:31 -0700, Dan wrote:
>I should have mentioned that the 40/80 antenna for my son will be almost
>vertical. He has a single pulley in a tall tree to suspend the antenna.
>I agree that the fan dipole or bent full size dipoles would be good
>solutions but a single 2 band dipole is better suited to his needs.
>Thanks for all the comments. I was hoping to get an inductance value and
>location in a 90 ft dipole. Antennas of this type are widely available
>from suppliers but it is more fun to build it and he will be visiting in
>a week or so. It makes a nice educational project. I do have an
>instrument to measure inductance at those frequencies.
Several thoughts. First, a vertical dipole is not a very good antenna. I
say that from direct experience -- I have a 40M vertical dipole with the
top suspended at about 100 ft from a tall redwood. It would work better
as a sloping dipole, coming away from the tree as far as possible. That
would also allow it to be longer.
Second, you might consider a vertical long wires, loaded against radials.
Third, if you still want to wind a coil, look up an article in QST from
about four years ago that shows how to design shortened dipoles using
loading coils. Or, even better, use the $80 or so you would have spent on
a pre-built antenna on a copy of EZNEC from W7EL's website. You'll still
have to design it, but you'll be able to show him how various design
changes affect antenna performance.
In my professional life, I design sound systems for large public spaces,
and I use acoustic modeling software extensively. It is a WONDERFUL
LEARNING and TEACHING tool, because it graphically shows results as you
make changes to the design. So is EZNEC, and for the same reasons.
EZNEC is also a very good predictor of actual antenna behavior (provided
that you actually build what you model (or accurately model what you have
built). The mediocre performance of that vertical dipole that I clearly
observed on the air (by switching it between my horizontal dipoles) was
clearly predicted by EZNEC.
Jim Brown K9YC
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