...since the antenna is isolated from ground the counterpoise seems to have a much greater effect on the resonant frequency than if they were grounded.
They certainly do affect the resonant frequency. You basically have
a vertical dipole with the bottom half widened out. You can tune it by
adjusting either end, which is handy because the bottoms of the radials
are often easier to reach. Adjusting the relative lengths of the top and
bottom sections changes the feedpoint impedance, and is one tool for
And, yes, it's just a vertical ground plane with sloping radials. Just
because it can have a variety of names doesn't change how it works.
...the un grounded radials provide a "sheild" from ground at the resonant freq...
The ground is still significant. Though the immediate ground losses will be somewhat
lower (primarily due to more distance) the low angle radiation still depends on the
local ground conditions out to 50 - 100 wavelengths.