> 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!
Hi Larry and all,
No matter hat the shape of the coil or what the mutual coupling from
end-to-end, the only way current can taper is if current has a second path
to ground other than the outer end of the antenna. Such current change can
happen two ways:
1.) Voltage at some point within the coil relative to space surrounding the
antenna can be many times higher than the end-voltage of the whip or
whatever is on the end of the antenna.
2.) The physical area of the coil must be large compared to the area of the
rest of the antenna beyond the coil.
Another way to say this this is this displacement currents from the center
region of the coil to the outside world must be a significant percentage of
current into the coil's terminal.
Consider the case of a small loop antenna. Current throughout the small loop
antenna is essentially equal, even through the loop tuning capacitor. The
only way this changes is if the small loop antenna becomes large in terms of
the wavelength. It has nothing to do with loss, radiation, or mutual
coupling.
In a small inductor we can have a condition where current is not essentially
uniform throughout the inductor when the inductor is operated near the
series-resonant frequency of the inductor, or when the inductor is very near
other objects that increase the capacitance from some area inside the coil a
large amount as compared to the termination capacitance of the inductor as
presented by the antenna. Such a condition would have nothing directly to do
with mutual coupling between turns or ends of the coil.
Either the claim is based on a measurement error, or the coil area is
significant compared to the physical area of the antenna above the coil.
73 Tom
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