Tom Donaly, KA6RUH wrote:
"---forget what Richard just posted."

Suits me. You recall the old story about leading a horse to water.

Tom rejects measurements of r-f currents at both ends of a loading coil
at work. The currents were clearly differing.

Tom must be stuck with battereis and the rise and fall these produce in
the current of an inductor.

A loading coil is usually in the antenna field in our examples. The
loading coil is subject to an incident wave and to a reflected wave.
These waves combine in a continuously varying phase relation along the
coil to make current, impedance, and voltage all functions of their
positions along the coil. Every spot along the coil is different when
both waves are sensed together. That`s the way SWR works, and we`re
discussing standing-wave antennas.

I have a 1982 ARRL Antenna Book. On page 13-3 there is a Fig. 6,
"Improved Current Distribution Resulting From Center Loading". The
loading coil clearly shows less current at its top than at its bottom.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
I have a 1982 ARRL Antenna Book. On page 13-3 there is a Fig. 6,
"Improved Current Distribution Resulting From Center Loading". The
loading coil clearly shows less current at its top than at its bottom.

My 1988 version shows the same thing on page 16-4, Fig. 7. Unfortunately,
two pages later, in Fig. 10 it shows how the current at the top of a
loading coil can be four times higher than would exist in an equal
top length of a 90 degree antenna. Since V*I*cos(theta) is the power, does
that mean that the voltage at the top of the coil is 1/4 the value of
an equal top length of a 90 degree antenna? So the impedance looking into
that 15 degrees of whip above the coil is 1/16 of the impedance looking
into that same 15 degrees of that same whip mounted above 75 degrees of
wire??? Doesn't sound reasonable, does it? Exactly how does that coil
manage to change the impedance looking into 15 degrees of whip by a
factor of 16 AT THE TOP OF THE COIL?????

That's exactly what happens when one uses circuit analysis on a distributed
network problem. If circuit analysis worked on such a problem, we wouldn't
need distributed network analysis.
--
73, Cecil http://www.qsl.net/w5dxp


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