Cecil,

1) Only the total current matters. I have never found a detailed
treatment of antennas that was based on anything other than the total
current (or total current density) at each point on the antenna. Have you?

Current components may be useful for discovering the total current or
for handwaving explanations, but they have no further role in antenna
analysis.

2) Where did this 90 degree phase shift requirement come from? There is
virtually no phase shift in the current of a half-wave dipole (or
quarter-wave monopole) from feedpoint to tip. I am looking at figure 9.6
on page 370 in Kraus "Antennas" (2nd Ed.), and it shows perhaps a few
degrees phase variation over the entire length of the dipole antenna.
This figure is located in the chapter on the Moment Method for
calculating cylindrical antennas, in case you do not have the second
edition.

I suspect you may be confusing the argument (AKA, the phase) of the
cosine function presumed to describe the behavior of the current
amplitude. However, current amplitude and current phase are not at all
the same thing.

Have you been seduced by your math models?

73,
Gene
W4SZ


Cecil Moore wrote:

[snip]


Because nobody has made that assertion since the original eHam article, it
appears to be a straw man. The coil occupies whatever number of degrees
that it occupies and it does NOT occupy zero degrees.

For instance, using a particular EZNEC segment model of a coil, the
current at the bottom is 1.0 amp and the current at the top is 0.5
amp. Assuming the cosine distribution of standing-wave current is
accurate, the coil occupies about 60 degrees. The whip would occupy
about 30 degrees, the rest of the 1/4WL.

Nobody has attempted to explain how one can obtain 90 degrees of a
1/4WL antenna on 4 MHz using a ten foot (15 degree) whip. That is
one hell of a velocity factor. If the bottom-loading coil really
occupies zero degrees, then the ten foot whip would be forced to
occupy 90 degrees. That is so impossible as to be laughable.

The notion that a coil replaces some sizable portion of the total
phase shift in an antenna has been shown to be incorrect. Experiments
reported by Roy and Tom R. convincingly demonstrate the phase shift
behavior of coils.


The total current undergoes virtually no phase shift since it is a
standing wave. That's in the textbooks and nobody is arguing that
point so it's just another straw man.

It's the forward current and reflected current that is undergoing a
phase shift through the coil just like they do on a wire standing-
wave antenna. Nobody has measured those two current components so
the jury is still out on that subject. There is no argument about
the phase of the total current that Roy and Tom measured. Please,
there are enough arguments already without having to introduce
straw men.

If you will look at my phasor diagrams of forward and reflected
currents at:

http://www.qsl.net/w5dxp/current.htm

you will see that the phase of the total current is exactly the same
in both cases. That's the phase that Roy measured. Since it is a
standing wave current, the phase of the standing-wave current is almost
constant. It is the magnitude of the standing-wave current that
changes and it changes as a cosine function of electrical length
in degrees. The coil has an electrical length in degrees. That's
what causes the current to be different at the bottom and at the
top in a 1/4WL antenna. Assuming the phase shift from the feedpoint
current to the tip of the antenna is 90 degrees, if an accurate
measurement of the current at the top and bottom of a bottom-loaded
antenna coil is made, the number of degrees occupied by the coil can
be calculated from

arccos(Itop/Ibottom)

just as it can be calculated between two points on a wire. This
assumes that Ibottom is an Imax point on the standing wave.