In reply to GW0DSP:
That’s a strange one Steve. I would have thought that an antenna once
tuned to a set qrg, stays on that qrg and the effect you were seeing
was a difference in VSWR due to antenna orientation, which in turn
fools the analyser into seeing a different resonant frequency for the
antenna.
Back to basics: an antenna presents, at a particular frequency, a feedpoint impedance, which in the general case is a complex value. Changing almost anything will change that impedance in some way. There are two degrees of freedom: resistance and reactance. Or, equivalently, magnitude and phase angle. “Resonant” is simply a shortcut for saying that the reactive part of the impedance is zero, i.e. purely resistive. It says nothing about the magnitude of the impedance.
Except in one special case, the feeder will transform the impedance to a different value. That special case is where the load impedance is the same as the characteristic impedance of the feeder. So if the antenna impedance is 50+j0, and you use 50 Ohm cable, you will “see” 50+j0 (which happens to look “resonant”).
If the antenna is resonant but at the wrong magnitude of impedance, then unless you are extremely lucky, there will be a reactive component at the far end of the feeder, and it will appear not to be resonant. Conversely you may see a resonance when in fact the antenna is not. Resonant does not necessarily mean that the load will be a good match for the transmitter.
So, keeping on topic, what you put on the Santa list is the type of antenna analyser that can compensate for the feeder. Typically this is all smoke and mirrors in software: the analyser can only measure one thing - complex impedance, but by calibrating with known loads at the far end of the feeder, you can calculate the effect of the cable and know exactly what is happening at the feed point as you change things.
The AIM4170 is an example of such a device.