I'm not the one designing the antenna. I'd build a collinear. I'm
just interested in the properties of the annular slot.

What I mean by "I guess the annular slot is the complement to a halo?"
is this:

A halo as I know it is a half wave or so loop broken opposite the
feedpoint and in a plane parallel to the ground. It has an
omnidirectional azimuth pattern but radiates horizontally polarized
radiation.

It seems that the annular slot is a slot in a plane parallel to the
ground, with an omnidirectional azimuth pattern, and vertically
polarized radiation.

So, is an annular slot antenna a halo cut out of a sheet?

73,
Dan
N3OX