Instead of this, why not use identical feedline lengths into a Tee and
position one cross polarised antenna 0.25WL in front of the other, to obtain
the 90-deg phase shift?

David, VK3BDK

Doing as you propose is possible, and would produce the same circular
polarization on the antenna boresight.

But moving away from the boresight, the circularity of polarization would
degrade (into elliptical polarization) faster than if instead using a
quarter-wavelength delay on one of the two feedlines.

73

Tony I0JX