You seem to be equating feed point VSWR and that seen by the built in
tuner. That might be the case if the transmission line was lossless.
It is unlikely to be, and we don't know again due to lack of
information.

Owen,

I appreciate your responses, but I am at a loss as to what additional
information one would need to model a center fed 75 foot long half wave
antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal
roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of
three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz.

What parameter is missing to provide a good model of this?


Ed