The polarization sense of an antenna can be determined from the phase slope of a rotating antenna. Before starting the phase measurement, determine that the setup is proper. Some older phase–amplitude receivers are ambiguous, depending on whether the local oscillator frequency was above or below the signal frequency. We use the convention that increased distance between antennas gives decreased phase. Move the antenna away from the source and observe decreasing phase or correct the setup. A rotating linearly polarized source field is given by

where α is clockwise rotation viewed from the direction of propagation (forward). A horizontally polarized linear antenna has the response E_a = E₁ˆx. It responds to the rotating linear source field,

E₁E₂cos α. The phase is constant under rotation until the null is passed and it flips 180◦ through the null.

An RHC polarized antenna has the response E₁(ˆx − j ˆy). It responds to the rotating linear source field,

The magnitude remains constant, but the phase decreases with rotation. Phase increases when the antenna is LHC. By observing the phase slope, the sense of the predominant polarization can be determined: RHC = negative phase slope; LHC = positive phase slope. It is easily remembered by considering the basis vectors of circular polarization:

In rotation from the x-axis to the y-axis, the phase decreases 90◦.