Polarization - As Jerry K4SAV stated, the electron gyro-frequency plays an important role on 160m since our ionosphere is immersed in a magnetic field - it also affects ionospheric absorption and refraction. For those of us at mid to high latitudes, vertical polarization on 160m is *theoretically* optimum since it couples the most energy to the limiting polarization at the entrance to the ionosphere. I don't understand Mark K3MSB's comment about the polarization terms disappearing unless it has to do with that fact that the polarization going up to the ionosphere has nothing to do with the polarization going thru the ionosphere (which is dictated by the ionosphere).
Elevation angles - Ray tracing shows that elevation angles up to about 10 degrees are E hops since there is still enough E region ionization at night to refract 160m. I don't know how important these E hops are - probably okay for short distance, but the losses (absorption and ground reflection) add up quickly for the longer distances. Above 15 degrees or so we get F hops. From 10-15 degrees is where ducting occurs in the electron density valley above the nighttime E region. Ducting in the valley likely requires shallow angles. But when a signal gets dumped out of the duct, that suggests a higher down-coming angle. K4SAV said it best: ". . . and the real world on 160 is very complicated." Carl K9LA _________________ Topband Reflector Archives - http://www.contesting.com/_topband