How critical is the phi alignment of the antenna w.r.t. the probe?
The phi angle is measured around the z-axis of the range measured from +X to +Y. A small phi error (dphi) has a minor effect on the principal-pol of the antenna, but a more serious impact on the cross-pol of a linear antenna. Principal-pol amplitude will decrease as the cosine of the angle whereas, cross-pol increases as the sin of the angle. The following table was generated using 20*log10(Cos(dphi)) for principal-pol and 20* log10(Sin(dphi)) for cross-pol. Note that the cross-pol level increases dramatically with dPhi (the error). This cross-pol is actually a “false” cross-pol and interferes with the ability to measure the true cross-pol of the antenna. It can be considered as a noise effect in the measurement. It will affect both cross-pol and principal measurements at low levels.
