H. Adam Stevens, NQ5H wrote:
Any difference in current between the ends of a compact loading coil in the
center of a short whip is due to radiation from the coil.

That's an old wives' tale. But don't feel bad. Kurt N. Sterba made essentially
that same mistake in his Nov. 2004 article in "Worldradio". Anyone who believes
that the current is zero at the tip end of a standing-wave antenna because it
has been radiated needs a refresher course in standing waves.

************************************************** *******************
* Any difference in forward current between the ends of a loading
* coil is due to radiation from the coil.
*
* Any difference in reflected current between the ends of a loading
* coil is due to radiation from the coil.
************************************************** *******************

Please read those two statements until they soak in because they
agree with you about component currents but disagree with you about
superposed currents.

The main difference between the net superposed current at each end of the
coil is caused by the phase shift through the coil acting on each of those
current components in opposite phase-rotation since they are traveling in
opposite directions.

Is any difference in current between the ends of a section of transmission
line due to radiation from the transmission line? Yes, but most of the
difference is due to phase shifts between the forward and reflected waves.
Exactly the same concepts apply both to a transmission line with standing
waves and a standing-wave antenna. In fact, a horizontal wire antenna is
nothing more than a "lossy" transmission line losing energy to radiation.

A mobile antenna is a standing-wave antenna possessing a Z0, just like a
transmission line. There's a forward current and a reflected current. The
superposition of those two component currents causes the main difference
in the magnitude of the net current at each end of the coil.

When Roy, W7EL, measured the phase of the current at each end of the coil,
he measured the phase of the NET superposed current, not the phase of the
forward current or reflected current. The phase shift of the NET superposed
current is only a couple of degrees max from feedpoint to end as asserted by
Kraus in "Antennas for all Applications", page 464.

In the ARRL Antenna Book is an equation for the characteristic impedance of
a single wire over ground. It is 138*log(4h/d) where 'h' is the height of
the antenna and 'd' is the diameter of the wire. The Z0 for a piece of #16
wire positioned 24 feet above ground is about 600 ohms. A quarter wavelength
of that wire used as an antenna responds essentially the same as a quarter
wavelength of lossy transmission line. There is a large reflected current
component at the tip of a mobile (standing-wave) antenna.

In fact, my earlier quarter-wavelength matching section plus its 100w load
acts similar to a quarter-wavelength antenna radiating 100w.
--
73, Cecil http://www.qsl.net/w5dxp


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