  Linear polarization angle, $\theta$, dependent measurements of the microwave
radiation-induced oscillatory magnetoresistance, $R_{xx}$, in high mobility
GaAs/AlGaAs 2D electron devices have shown a $\theta$ dependence in the
oscillatory amplitude along with magnetic field, frequency, and
extrema-dependent phase shifts, $\theta_{0}$. Here, we suggest a microwave
frequency dependence of $\theta_{0} (f)$ using an analysis that averages over
other smaller contributions, when those contributions are smaller than
estimates of the experimental uncertainty.
