Once and for all, let’s understand extraordinary wave propagation on HF. Why does it happen and what does it mean to us?
Back around 1930, scientists like Edward Appleton began to study the dispersion of electromagnetic waves in the atmosphere. We know that these experiments led to creation of ionograms. We see frequencies and virtual heights of radio signals reflecting from the ionosphere at vertical incidence.
Unexpectedly, ionograms showed us two primary wave echoes coming back from each pulse sent upward. Ordinary wave propagation (in red, above) is what we would expect without a magnetic field. Extraordinary wave propagation (in green, above) is what we see when Earth’s geomagnetic field is added into the mix.
Simply put, our HF radio signals experience two different indices of refraction, and two different critical frequencies which are separated horizontally and vertically. FCX is typically half the gyrofrequency higher than FCO. Geomagnetic gyrofrequency varies across the globe, 1.2 ± 0.5 MHz. Extraordinary waves use a slightly different path, has different (usually higher) absorption and different times of arrival. Both E and F layers have extraordinary wave propagation, but it is quite weak from the E layer.
After experiencing our magnetic fields, radio waves are elliptically polarized, especially if traveling perpendicular to magnetic field lines.. Waves running parallel to the magnetic field are typically circularly polarized, combining to form Faraday Rotation due to different phase speeds.
Back around 1930, our friend Appleton developed a formula for ionospheric refraction in a magnetic field. His complex formula had two roots (love math) which is where ordinary and extraordinary wave propagation arise.
What Does Extraordinary Wave Propagation Mean to Us?
Most of the time, not much. We lose 3 dB of signal power from polarization mismatch. We get a bit more fading from multiple signal paths. We get a slightly higher MUF from extraordinary wave propagation.
But as we move lower in frequency, especially for Near Vertical Incidence Signals, we can experience different forms of propagation. And at northern latitudes, the two characteristic waves can take weird paths which are not great circle. And, for long paths, which reflect multiple times through different magnetic characteristics, all bets are off.