Non-reciprocal paths via the ionosphere at HF

During a discussion on Pedersen rays at a recent CAKE meeting it was claimed these were “asymmetrical” paths and an explanation was needed.

We all know that much of our hobby depends on the ionosphere to return our transmitted signals back to earth and we may tend to think in terms of mirror-like reflections. Nothing could be further from the truth as the ionosphere is an unstable mix of charged particles and gases.It has both regular and irregular behavior.

Although the ionosphere was key to commercial world-wide radio communication from around the 1930s the great deal of knowledge accumulated was practical rather than scientific. The science of the Ionospheric began sometime around the 1950s with the invention of atmospheric sounders. Scientific studies were intensified in the 1960s for security reasons when understanding propagation in high northern lattitudes became especially important. Hang on I will answer the question soon HI

In 1967, NATO held a conference on Ionospheric communications with a focus of the Arctic and the conference proceedings has a section on HF entitled The difference in levels of signals travelling in opposite directions. The following three statements were selected from that section:-
1.   Paths between two antennas can be non-reciprocal but this is NOT due to differences in ray path attenuation or phase. On the basis of ray theory these are the same for signals travelling in either direction.
2.   The reciprocity or non reciprocity arrises only because of interaction of the upward or downward waves with the antennas at the ends and the earths magnetic field.
3.   A reciprocal path is formed between linearly polarized antennas (most amateur antennas) if they are both polarized in, or perpendicular to, the earths magnetic field otherwise the path can be non-reciprocal.
The section reached the conclusion that “most ionospheric paths are non-reciprocal with the result that there can be large instantaneous differences in the levels of signals travelling in opposite directions.”

To go beyond that statement and understand the mode and practical implications one needs to look into the phemomena of the ionosphere converting an incoming plane polarized wave into two outgoing circularly polarized waves known as O ordinary and X extraordinary.

That’s all for now
Ron W6WO