In response to Mike, Bruce, and others:
There seems to be a trend towards forgetting the physical dimensions of the
loading scheme in question. I'd like to caution those trying to wrap their
mind around this problem that the "coil" in question is not what you would
normally see in a mobile (or fixed) antenna system. It is definitely not
your usual high Q inductor of 2:1 or 3:1 L/D.
The loading "coil" that Barry, Greg, and myself have used consists of about
a pound of #18 enameled wire close wound on a piece of 1/2" schedule 40 PVC
pipe. This works out to about 3/4" in diameter and 20 to 24 inches long.
This is not a real "coil". The inductance can't be calculated with the
normal equation used for solenoid wound inductors. The top and the bottom
ends of the "coil" are so loosely coupled that if you remove 1/2 of the
turns, the inductance will not be reduced by a factor of four, but closer
to TWO!
In a practical top band mobile implementation, there would be about three
feet of base section, five feet of top section, with around two feet of
"coil" in the middle. This means that the "coil" occupies 20% of the total
antenna length! It is NOT a mathematical construct consisting of electrical
properties with NO physical dimensions.
Mention has been made of temperature as a current taper indicator. You have
to be careful doing this because the short thermal time constant of a
conventional loading coil is such that a long transmission at normal power
levels will heat the coil fairly evenly. I solved this problem by using
very short transmissions of very high power. (coax from bumper to shack,
big amp, assistant on key, watch for corona, grab coil - hopefully after
power is shut down!) Results of my tests showed almost no heat taper in a
coil of "conventional" dimensions (4 or 5:1 L/D) and very much heat taper
in the "Minooka" design. Another interesting observation made during these
tests was that the "Minooka" design would handle far more power than I
expected. A curious person might wonder where the power is going if not
into heat to melt the plastic!
Similar results were obtained when the "coil" was used for base loading or
near top loading. I have also used similar L/D ratio loading coils near the
top of some very successful fixed station verticals. Didn't run up the
tower and grab those coils!
Make of all this what you will.
73,
Larry - W7IUV
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