H. Adam Stevens, NQ5H wrote:
The current below the loading coil is as if the antenna were full length,
max power radiated; the voltage above the loading coil is as if the antenna
were full length.

That's semi-close but not entirely true. As Tom and Roy say, the coil
indeed does distort the current away from the cosine pattern common in
1/4WL wire antennas. It just doesn't distort the current as much as they
say. But the current at the top of the coil is a greater magnitude than
it would be if your above statement were true. In one case, it is 66%
higher than in a wire antenna.

The current at the top of the coil is greater than it would be for the
same stinger mounted on a physical 1/4WL antenna but it is not equal to
the current at the bottom of the coil. The coil probably causes a larger
phase angle between the voltage and current than exists in a wire antenna.
If theta is small, V*I*cos(theta) can be fixed while V and I become larger
than they are in a wire antenna. There's a lot happening around that coil.

Remember the boundary conditions; the current is max at the feedpoint and
zero at the end (can't go anywhere).

Sorry, the current is not max at the feedpoint. There is a current maximum
point located inside the coil that is a greater magnitude than the feedpoint
current. If we say there is 90 degrees from current max inside the coil to
the tip of the antenna, a center-loaded mobile antenna is longer than 90
degrees. One in particular, calculates out to be 110 degrees long.

The coil causes an impedance discontinuity at each end in the standing-
wave antenna. It is somewhat like the following where the 10k ohm feedline
represents the coil with a Z0=SQRT(L/C):

---600 ohm feedline---+---10k ohm feedline---+---600 ohm feedline---open

One can see that there would be reflections in both directions at the
'+' points. That's why there will probably never be an equation to
represent a mobile antenna.

Here's the key. (Vfor+Vref) must be in phase with (Ifor+Iref) at the
feedpoint for the feedpoint impedance to be purely resistive. But
Vfor doesn't necessarily have to be in phase with Ifor for that to
happen. Neither does Vref necessarily have to be in phase with Iref
for that to happen. The coil has a different phase effect on voltage
than it does on current and the natural Z0 of a vertical antenna is
not fixed since every point on the antenna is a different distance
from ground. It's probably impossible to quantify using equations.
--
73, Cecil http://www.qsl.net/w5dxp


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