There are very significant practical implications to this phenomenon. First, for single/fix polarity antenna arrays (H or V only), the proper selection between Horizontal or Vertical polarity can make a huge difference in terms of RX performance. A good survey of the noise level at different AZ/EL should be done before making the final selection between H or V pol. Of course, there may be other factors why an EME practitioner may favor one polarity over the other, but certainly, the noise factor should be a key one to consider.

 

Second, Rotatable Polarity capability (RPOL) can enable "Signal-to-Noise" optimization which can make all the difference in the world in the context of EME/weak communications. For instance, using the current example as an hypothetical case, let's imagine that the polarization of a rare and very "weak" DX station is at 45-deg. Using 0-deg (H-POL) or 90-deg (V-POL) would result in a theoritical polarization mismatch loss of 3.0 dB. In addition, at 90-deg (V-POL), there is a noise increase of 4.0 dB compared to 0-deg (H-POL), so at V-POL, the total penalty would be up to 7.0 dB compared to ideal polarization (45-deg) and using H-POL noise level as basis (0.0 dB). That would for sure obliterate any chance of making the QSO with that weak station...!

 

How would one perform the "Signal-to-Noise" optimization in the example above? Knowing that the noise level increases exponentially from 0 to 90 deg and that the polarization of the DX is 45-deg, setting the antenna polarization to about 25-deg would result in a relatively small noise increase and would greatly minimize the polarization mistmach penalty as well. For the sake of this example, let's assume that the relative noise increase at 25-deg pol is ~1.0 dB, and that the polarization mismatch penalty is 0.5 dB (45deg - 25deg = 20 deg ==> 0.5 dB), the total penalty would be around 1.5 dB. Therefore, considering that at pure H-POL or V-POL the theoritical polarization mismatch loss is 3.0 dB, the overall "net gain" would be around 1.5 dB (3.0dB-1.5dB) compared to pure H-POL, and up to 5.5 dB compare to pure V-POL (using H-POL noise level as basis). That can make all the difference in the world between realizing the QSO with that rare weak DX station or not.

 

Now, let's assume that the least noisy polarization is 45-deg (that is between H and V-POL), and that both H and V-POL are 2.0 dB noisier than at 45-deg. In such case, using the case example above, setting the polarization to 45-deg would not only provide a 3dB advantage over H and V-POL by eliminating the polarization mismatch with the rare DX station, but would also provide an additional 2dB advantage due to the noise reduction, totalling a 5 dB advantage!

 

An alternate strategy would be to wait for Faraday to "align" the rare DX with the most advantageous polarization which would be H-POL in this example (lowest noise level). "Waiting" for Faraday to do the optimization work however may be a very "risky gamble" where Faraday may be rotating so slow that one may lose its moon window before the actual aligment occurs, or at the time where the polarization alignment is favorable, other much stronger stations may consume the window of opportunity...