Steve,
I wrote a two part article on the Isotron in AntenneX magazine. If you have a
subscription you can take a look. Basically, I examined the real theory of
the Isotron and performed a controlled experiment. The Isotron is a top loading
structure for the "ground wire" and the shield of the coaxial cable. It forms
a top loaded vertical and nothing more.
With this in mind, it can perform well if the ground wire and/or coaxial
feedline is routed as you would route a vertical. A good ground system will
improve the performance.
You can acheive the same effect using a loading coil and a top hat or wire.
For example, a Hustler 80 meter 'resonator' and a DX Engineering top hat will
get you on top band. The longer the vertical wire and the better the ground the
better things will work. Another way to go that is simple and relatively
inexpensive is a Hustler 5BTV vertical with the DX Engineering top hat. The
first
top hat for this vertical was the Nantenna top hat that I manufactured and
sold a few years ago. With this and a few 25' radials one can acheive a
radiation
efficiency of 8-12%. Not high but OK for a 22' vertical.
Your screwdriver antenna should resonate on 160 meters with a top wire that
is about 20'. I think you were just a bit short on your experiment with the 15'
wire. The radiation efficiency will be 2-3%. With 100 watts that will get you
plenty of Q's in the ARRL 'test. With 5 watts it will be a struggle. I
normally operate the 'test with a radiated power of 1.25 watts.
This year I'll be running 5 watts to a 40' base loaded vertical. The mast is
the fiberglass one available from The Wireman. It is not listed on the order
form yet but it is $99. It uses 12, 1-meter sections that telescope. They also
have a 60' version. I won't burn up the band with this but it is the best I
can do in my CC&R neighborhood this year. Maybe I'll go with the 60' next year
or push it up only 40' and put a top hat on it for that big top band signal.
If you need any simulations or ideas just email me. 73 OM.
Dave WX7G
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