Which of the following results from the fact that signals propagated by the ionosphere are elliptically polarized?
Why When a signal passes through the ionosphere, the magnetized plasma rotates and reshapes the wave's polarization (Faraday rotation), so what arrives is elliptically polarized and its orientation drifts constantly. Because the arriving wave has components in both planes, a vertical or a horizontal antenna will pick up usable signal. That is why HF skywave operators mix dipoles, verticals and beams freely and still work each other.
Watch out The idea that both stations must match polarization is true for line-of-sight VHF/UHF work, where cross-polarization can cost 20 dB or more, but it does not apply to ionospheric paths. Polarization has nothing to do with whether a mode like FM or a digital mode works.
Skywave scrambles polarization: on HF, vertical or horizontal both work. Matching polarization matters only line-of-sight.