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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