conditions, because the lower layers have gone and those frequencies
penetrate further before being refracted.
6 Signals at 16MHz and above will not normally return to earth during
night-time conditions, going right through the top layer of the
ionosphere - just like VHF signals.
The following table is provided as a starting point when trying to
work out what frequency to choose, day or night, for a given distance.
Ranges given are very approximate and change from season to season,
and through the 11 year sunspot cycle (which affects the level of energy
in the ionosphere and, therefore, the refraction properties at each
frequency band). hn
~d/22 MHz band
16 MHz band
12 MHz band
8 MHz band4 MHz band
Exceptionally:A =-....
;P/T~~1600-3600 miles
1200 -3600 miles
800-2400 miles
600-1600 miles
(signals absorbed)(gone to the moon)
(gone to the moon)
2400 -3600 miles
1200-3600 miles
800-2400 milesa Signals in the 2MHz band will exhibit sky-wave properties during
the hours of darkness. This is generally considered a nuisance, because
of the interference caused to ground wave signals at great distances
(over 1000 miles).
b Where multi-hop propagation takes place, distances of 7000 miles
and more can be achieved, at the right time of day/night, when the
sunspot cycle is high, and using the right frequency.
The best way to understand sky-wave propagation is to listen in to
signals from various coast stations around the world (radio telex call-
bands are good for this - see Chapter 4). Listening to broadcast stations
may mislead you, as those stations (BBC World Service, etc.) tend to
share transmitters at different locations around the world and so, even
although you may be listening to the BBC in the Far East, the transmitter
may be somewhere other than in the UK.
MARINE COMMUNICATIONS 153