Tom Donaly wrote:
So, since the phase shift has to be 90 degrees, the antenna
should always resonate at the same frequencies a quarter wave
stub of the same electrical length would resonate at, right?

Not sure what you mean by this statement. 90 degrees
is 90 degrees. A mobile antenna physically shorter
than 1/4WL is still close to 90 degrees long at
resonance. (It is not exactly 90 degrees because of
the well-known end effects.)

In order for the reflected wave to be in phase with
the forward wave at the feedpoint (purely resistive
feedpoint impedance), the reflected wave must traverse
180 *electrical degrees* during its round trip. That
fact inticates that the antenna is electrically 90
degrees long.
--
73, Cecil http://www.w5dxp.com