> * ON4UN in his book recommends detuning the tower by
running a wire
> down the side in series with a variable capacitor
and tuning for
> minimum current with an RF current probe. However,
it seems that
> this would only work for one of the bands, and you'd
have to
> accept some interaction with the tower on at least
one of the bands.
I always wonder where some of this stuff comes from.
That's generally the opposite of how you should tune if you
are measuring current. You would generally tune for MAXIMUM
current in the sectionalizing drop wire, NOT minimum. That
insures the sectionalizing area is resonant. Since it is a
"loop", current flowing up equals current flowing down, and
that canels radiation. The end-to-end impedance is highest
at resonance, so the section you tune acts like a
high-impedance parallel tuned network isolating the tower
above and below that area.
The exception to tuning this way is if the area below or
above the tuned area is self-resonant. It also is not the
preferred way to detune a structure, but the preferred way
requires a FS meter.
> * Does anyone have any experience with this detuning
trick? Did you
> find that it was really necessary?
Well, I've done hundereds of them commercially. I'm not sure
how necessary it is in a Ham installation, especially since
you never said if the antenna was vertically polarized or
what band it was on.
> * Do you have any suggestions for how to accomplish
detuning for two
> bands? Two wires and two capacitors?
That works, but it interacts if the wires overlap the same
area of the structure.
> * Does a crankup tower alter this detuning requirement
in any way?
Only if the connections are intermittent.
I have a question, why are you using a delta loop or a loop?
There are good mechanical reasons for using a loop, but
generally electrical reasons are not good. Will this be
vertically polarized?
73 Tom
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