Dave wrote:

Circular polarization means you have equal H-Pol and V-Pol radiation,
with one polarization 90 degrees ahead (or behind) of the other.

That's not a very good description, although it's correct. You can
separate a circularly polarized field into vertically and horizontally
polarized components, the sum of which is the circularly polarized
field. And if you do that, you'll find that the two components are 90
degrees out of phase with each other.

But here's a little more complete description: The electric (E) field of
a horizontally polarized wave is horizontal, and the E field of a
vertically polarized wave is vertical. But the E field of a circularly
polarized wave rotates at the transmission frequency, one revolution per
cycle. The instantaneous amplitude of a vertically or horizontally
polarized field is sinusoidal, varying at the transmission frequency.
The amplitude of a circularly polarized wave is constant.

A quad antenna is a folded dipole.

No, it's not.

Roy Lewallen, W7EL