Dave wrote:
I reported that the addition of 3 or 4 inches above the coil produces a much
larger shift in frequency than adding the exact same length below the coil [by
almost a factor of 100:1]. That is empirical data. It is real and measurable.

My question is one of Physics. I think everyone who reads this list knows it
happens. The real question is: Why?? Why the difference in antenna resonant
frequency?

Dave,

Any antenna is a large network of distributed impedances. You have
series inductance in every conductor and capacitance to the outside
world. There is also some series resistance and even shunting losses in
dielectrics, but looking at the reactances is good enough to answer
your question.

Look at what we have in the short mobile antenna. Below the loading
coil we have a conductor that carries an almost uniform current,
voltage and current are nearly in phase throughout the entire length.
We have the loading coil that adds a high value of series reactance all
in one spot. Above the loading coil we have an area with very high
power factor. The current and voltage are nearly 90 degrees out of
phase and voltage is very high.

When we perturb the system below the coil, it isn't terribly sensitive
to changes in capacitance because voltage is so low. The top of the
mast below the coil is terminated in a fairly low impedance so it isn't
sensitive to shunt capacitance and the additional series inductance
added by the small additional length is very small compared to the
system's overall reactance and electrical length at that point.

Above the coil it is another story.

Now we have very high voltage (it can still have almost the same
current above as below the coil if the antenna is a good design). The
coil is terminated in a very high value of reactance that is mostly
comprised of the distributed reactance of the small whip, and that is
mostly capacitive reactance of a very high impedance.

Altering the whip length or diameter above the coil greatly changes the
system resonance because the small value (very high reactance) of the
whip is what actually resonates or terminates the coil.

The loading coil and the capacitance above the coil form a series
resonant circuit. The area above the coil has very high impedance, very
high power factor, and as such is very sensitive to any additional
shunting impedance caused by additional length or diameter.

The area below the loading coil has almost unity power factor, has low
impedance, and has much lower levels of electric field so it isn't
nearly as sensitve to shunt impedance (distributed capacitance)
changes.

73 Tom