I think I can answer my own question now, and the answer is faulty measurement
technique...
I am only using an MFJ259 to measure both impedance and resonant frequency, but
my instrument cannot differentiate between real and imaginary parts of an
impedance. When I measure the antenna resonances without the added series
resistor, they are all very close together, but the series resistor changes the
apparent resonance by different amounts on each one. But of course, the
frequency at which the MFJ shows minimum SWR is _not_ the resonant frequency -
it is just the frequency at which the impedance is closest to 50 Ohms.
I am pretty sure that the antenna with its feet in the swamp does have a better
ground than the other three and although the feed impedance is very similar in
each case, that only has general implications for the resonant frequency.
As I don't have any better instrumentation easily available, I will adjust the
verticals to have the same 'resonant frequency' without any added resistance
and then put that in later.
73 Roger
VE3ZI
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