When we speak about radio communication using the ionosphere we use the term reflection, as if the signal is reflected from a single point in the ionosphere. What actually happens is that the signal is continuously bent or refracted as it travels throughout the ionised plasma. The two situations are shown in the diagram below.

Virtual reflection height

This shows ray paths for a vertical radio wave as transmitted by an ionosonde and an oblique radio wave as used for communication. The actual ray paths are shown as solid lines, with the virtual paths shown dotted. The two circuits are shown 'reflecting' from the same height in the ionosphere. This is only true when the two frequencies are related by f = fv sec(θ), showing that an oblique signal path will allow the transmission of higher frequency signals than a vertical path, as the incidence angle θ is increased.

The concept of virtual paths and virtual heights is not only a convenience. There are some solid relationships between the actual and virtual paths. When the signal travels in the ionosphere it is slowed down, and we find that the actual travel time between the transmitter and receiver (along path TAR or TiAiRi) is identical to a signal travelling the virtual path (TVR or TiViRi) which is not slowed down, but which continues to move at the speed of light (c = 3 x 108 m/s). This result is called the theorem of Breit and Tuve, who constructed the first ionosonde.

Ionosondes produce an image which shows the virtual height of the ionospheric reflection point as a funstion of frequency, and thus are instruments where reality is virtual! Another theorem relating the virtual and actual is due to the Australian physicist D F Martyn. It states that if a vertical signal is refracted from the same actual height as an oblique signal, then the two signals will have identical virtual heights of reflection (ie if A = Ai then V = Vi ). This result allows ionograms to be used to predict communication circuit behaviour.

ASAAustralian Space Academy