Hi Ed,
actually, I can already tell you that it is very different on this vertical / up-and-outer, because I tested in with the previous versions, like this one
Electrically, it is equivalent to the new design; the difference is that the loading coil is an air coil in the first version and wound on a toroid in the new design.
When tuning the first version, even 10 cm +/- make a difference. So 1/8 will not work.
What still surprises me, though, is that the resonant length for the counterpoise is typically significantly less than 1/4 lambda.
This is also equivalent to the counterpoise lengths recommended for the ATX-1080 antenna - they indicate a length of
counterpoise_length = 54 / frequency_in_MHz
But I do not understand where this formula comes from.