Increasing interest in W-band communication systems and sub-millimeter wave imaging systems has stimulated research in sub-millimeter wave antennas. Circularly polarized (CP) antennas are important for point-to-point communication for airborne transceivers and for satellite communication. Conventional methods for CP gain characterizations require measurement of the radiated field (magnitude and phase) using an LP antenna in the two orthogonal planes. In this method, the antenna under test (AUT) or LP antenna must be rotated by 90°. Such rotations normally cause phase errors due to cable bending and possible position inaccuracies. For sub-millimeter waves, these errors can be significant in the phase of the vertical field with respect to horizontal. Furthermore, for WR-8 band (90-140 GHz), coaxial cables are not available. Frequency multiplier boxes with WR-8 output waveguides needs to be also used for antenna measurements in this band. Rotation and alignment of these boxes adds more phase errors, since rotation of the box can change the distance between the AUT and LP antenna. For example, at 100 GHz, a variation of distance as small as 0.25 mm can cause a phase error of 30°. These complications make antenna CP measurements a daunting task for frequencies beyond 100GHz.