When radials are in the same plane, they don't radiate a significant
amount. But when slanted downward, they do, pretty much the same as a
conical element of the same outside dimensions. You can call a quarter
wave vertical with sloping radials a "quarter wave ground plane" if you
like, but unlike one with horizontal radials, the radiating portion is
longer than a quarter wavelength.

In the case of the antenna being described, the upper radiating part of
the dipole is a quarter wavelength and the lower part is about 0.7 times
a quarter wavelength but effectively large diameter.

We have to get over the notion that calling some part of the antenna
structure "ground" gives it some kind of special properties. It doesn't.

Roy Lewallen, W7EL

wrote:
The bottom half of each
antenna consists of 9 quarter-wavelength wires that connect to the
center of
the tower and slope downwards to the ground at a 45 degree angle.

Sure, thats a "1/2 wave" antenna, but I would always consider that
a ground plane, rather than 1/2 wave vertical. It's got sloping
radials.
Same thing I used to run on 40m. I always called it a 1/4 wave ground
plane... MK