Communications in the VHF through microwave regions normally takes place on a ‘line-of-sight’ basis where the radio horizon defines the limit of sight. In practice, however, the situation is not so neat and
simple. There is a transition region between the HF and VHF where long distance ionospheric ‘skip’ occurs only occasionally.

This effect is seen above 25 MHz, and is quite pronounced in the 50MHz region. Sometimes the region behaves like line-of-sight VHF, and at others like HF shortwave.

Auroral propagation
The auroral effect produces a luminescence in the upper atmosphere resulting from bursts of particles released from the sun 18 to 48 hours earlier. The light emitted is called the northern lights and the southern lights.

The ionized regions of the atmosphere that create the lights form a radio reflection shield, especially at VHF and above, although 15 to 20MHz effects are known. Auroral propagation effects are normally seen in the higher latitudes, although listeners in the southern tier of states in the USA and Europe are often treated to the reception of signals from the north being reflected from auroral clouds. Similar effects exist in the southern hemisphere.

Non-reciprocal direction
If you listen to the 40 metre (7–7.3 MHz) amateur radio band receiver on the East Coast of the United States, you will sometimes hear European stations – especially in the late afternoon. But when the US amateur tries to work those European stations there is no reply whatsoever.

The Europeans are unable to hear the US stations. This propagation anomaly causes the radio wave to travel different paths dependent on which direction it travels, i.e. an east–west signal is not necessarily the reciprocal of a west–east signal.

This anomaly can occur when a radio signal travels through a heavily ionized medium in the presence of a magnetic field, which is exactly the situation when the signal travels through the ionosphere in the presence of the Earth’s magnetic field.

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