Elliptical Polarization as a Result of Propagation
Elliptical or circular polarization can be found in ionograms. Figure 7 shows an ionogram of the Belgian Dourbes. The signals reflected by the ionosphere are shown in two colors.
Figure 7 — Dourbes’ ionogram with ordinary (red) and extraordinary (green) reflections.
We can imagine a linear polarized field, such as that of a dipole for example, as consisting of the sum of a right-hand and a left-hand circularly polarized component. Together, they form the linearly polarized field. In the ionogram, we see both circularly polarized components as the ordinary and extraordinary wave. If now, due to propagation by the ionosphere, for example, the left-hand circular component is attenuated, a right-hand circular field remains.
On the top bands, the extraordinary wave (green) normally seems to have a higher attenuation. In these cases, the ordinary wave (red) is dominant. We will then be left with a circular (normally elliptical) polarized field. This effect in propagation is of course not constant and depends on propagation, on the time of day, the time of year and solar activity.
NM7M has written about this topic in his article "Power Coupling on 160 Meters" in Communications Quarterly, Summer1999, page 95-101. It is a very good and understandable article.
There is, however, not much information about this in normal handbooks or on the Internet.
On the 80-meter band, the ordinary wave appears to dominate during the day (phase difference around –90°) and the signals are normally circularly polarized. As it gets dark, this moves to linear polarization with a once dominating extraordinary wave (phase difference around +90°). On the 160-meter band, we at least see circular polarization in the evening. Ground wave propagation is mainly vertically polarized, not circular. I have found no information as to how this is for DX-signals. This will undoubtedly depend on whether or not propagation takes place by daylight. The Prolab program uses this division into ordinary and extraordinary wave and should be able to state this for DX.
Last update: April 15, 2007