Richard Harrison wrote:
Tom Donaly, KA6RUH wrote:
"Richard, there is no such thing as a "current drop."

This is where i came in almost a year ago. Yuri may havr posted Fig 9-22
from page 9-15 of the 1994 edition of ON4UN`s "Low-Band DXing". There
are base loading, center loading, and continuous loading examples of
short vertical antennas, and their current distributions. In every case,
the currents at the two ends of the coil are different.

Unfortunately those diagrams are misleading. They draw a current profile
against a scale of electrical height, which makes it look as if there is
a significant change in current along the length of the coil.

The current profile can only be correctly drawn against a scale of
*physical* height. Then the error goes away, and the current is seen to
be uniform through the coil. What does change is the shape of the
current distribution above and below the coil.

All of ON4UN's math is good, and if you follow that instead of the
pictures, you will find that the current through the coil is always
assumed to be constant. In other words, he assumes an ideal inductor.
The text and math shown that ON4UN actually understands the situation
perfectly. I don't believe for a moment that he expected those diagrams
to be so fundamentally misinterpreted.


The impedance of an antenna is a function of position along the antenna.
There is radiation from an antenna so not only is the impedance along an
antenna a variable, but the the transmit power level power level along
an antenna is a variable, too. These variables ensure a difference
between the current into and out of a loading coil.

No: there is no change in current through a coil that is physically
tiny, and doesn't have any capacitive coupling with the rest of the
antenna. All the changes occur above and below the coil.

Any deviation from equal currents in and out has to be caused by the
physical size of any real coil being non-zero. Then, even a physically
small coil will behave like a very short section of helical monopole
antenna. It is radiating an EM field, so there must also be some
variation in current along its length. (The extreme case of a long coil
is the fully helical whip, where the antenna and the coil are one and
the same.)


There is also a lot of loose talk about "phase variation" due to an
ideal inductor. This is NOT a phase variation in the current from end to
end! What changes is the phase of the voltage between the input and the
output. At each end you are measuring the phase of the voltage,
*relative* to the phase of the current which does *not* change.



Now someone is going to come right back at me, talking about "real life"
this and "practical" that. But if someone does not understand how an
inductor is even *meant* to behave, all their practical knowledge is
built on sand - they may know lots of stuff, but they don't truly
understand it.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek