Tom,
Let's revisit Jaan's original question:
Background:
I have a tower that is 137ft. or 42m tall. It is triangular 1ft 10inches
or
40cm wide. The tower is guy wired at three levels with non-isolated wire.
The foundation is a concrete slab on on rocky ground.
The surroundings is quite flat and conductivity is perhaps not the best.
About 1/2 mile away I have the Baltic Ocean in almost 360 degrees.
My question is what options do I have to build a good TX antenna out of
this? What can I do? On RX side I will use separate antennas.
I could think of isolating the guy wires or replace them for
non-conductive
type. I'm not sure if it possible to isolate the base today. Shunt fed
the
tower, use it as a folded monopole? Or should I just use the tower as a
support for a L-antenna?
73 de Jaan, SM0OEK
What would you recommend?
I wouldn't attempt to suggest someone else do work on a system with a large
tower with multiple uninsulated guylines and other antenna by guessing.
Guessing is easy, but there are two major problems with design by guessing:
1.) There often are so many things in the near field of any 160 meter
antenna, that actual patterns and real results are difficult to predict with
guesses.
2.) If we work what we want to work with satisfaction to ourselves, we
automatically think we have the best solution possible. This is true even if
our signals are actually 5-10 dB down from what they could be.
With so many modeling tools available, it would be worth the time to model
the entire installation. The only difficulty is knowing how to use the
model.
What I would do is different, and might not be for someone else. If it were
my installation, the installation would have had insulated or sectionalized
guylines to start with. I haven't had a tower or mast since 1963 with
conductive guylines, because I don't want nine or more random things thrown
into the equation of every low band antenna I plan on trying.
If I was serious about low bands, I would model the tower and see how the
guylines and other antenna impact all of the bands before doing anything.
If I just wanted to get something on the air without any real effort, I'd
put up an Inverted Vee dipole and/or an Inverted L and live with it. The
only real choices are vertical or horizontal. All the differences and any
possible gain from things like loops and slopers and magical ground and feed
systems is just a few dB at the most, and no one would notice. The real
influence is what the tower system looks like electrically, and no one knows
that..
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|