12.12. Propagation of SW Signals
One of the questions raised frequently is to determine which
transmissions can be received at what distance from the transmitter.
There are a number of factors that affect short-wave reception and
distance vary greatly depending on time of day and year, sun activity
and amount of traffic in a particular band. Each band has its own
characteristics and as a thumb-rule says, one could say ‘the higher the
frequency, the greater the distance’, at least during day time. At night
there is also a considerable increase in the distance for low frequency
bands. For instance, a typical transmitter in the 4 MHz band would cover
a distance of 250 nm around noon time which would increase to as
much as 2,500 nm at night. The following comparison looks at the various
bands and their respective propagation characteristics.
2 MHz Band: Maximum distance during day time is about 60 nm which
increases at night to 200 nm under good conditions. Thunder storms and
lightning severely affect reception quality.
4 MHz Band: Reception in this band works best in the early morning hours
with a gradual decline towards noon time. Towards the evening
propagation may exceed 2000 nm. Reception of transmitters in closer
proximity may not be possible.
8 MHz Band: Best propagation during the early morning hours. Distances
up to 800 nm should be possible during the entire day. Reception of
transmitters in closer proximity may not be possible.
12 MHz Band: Until late afternoon no long distance coverage. Thereafter
propagation increases gradually but transmitters located within 600 nm
may not be received.
16 MHz Band: This band is similar to the 12 MHz band. Reception of
stations within 800 nm is not normally possible. In the late evening hours
propagation increases to 6000 nm.
22 MHz Band: This band is not normally usable for stations located within
1000 nm. under good atmospheric conditions distances of up to
8000 nm may be covered. Best connections are obtained in North /
South direction. Right after sunset this band is not usable.
