Panasonic RFB55 - Manual (Page 73)

Guide to Short Wave (SW)
13 m Band (21.450-21.750 MHz)
This band is used for iong-distance interna-
tionai broadcasts, it is used regardiess of the
season when the soiar spots are active. Con­
dition of this band changes from time to time.
11 m Band (25.600-26.100 MHz)
This band is used for iong-distance interna-
tionai broadcasts during heavy activity in the
soiar spots. Like the 13 m band, its condition
changes from time to time. When the condi­
tions are good, reception is easy as there is
neither noise nor interfering signais.
■ Propagation of electromagnetic
waves
Eiectromagnetic waves of a broadcast are
separated into surface waves which are prop­
agated aiong the surface of the earth and
space waves which are propagated upwards.
Low frequency waves (LW & MW) are surface
waves, whiie high frequency waves (SW) are
bounced off the ionosphere which is iocated
100-300 km above the earth's surface. This is
charged and acts like a mirror toward electro­
magnetic waves. That is why high frequency
waves can be received over great distances.
On the other hand, the electromagnetic waves
of higher frequencies (VHF) extend beyond
the ionosphere and can only be used for short
distances.
The ionosphere is considered a product of the
sun’s ultra voilet rays, and consequently, the
conditions of the ionosphere change from time
to time due to the position of the sun. Seasonal
variations also occur. Since it is multilayered,
the path of propagation of electromagnetic
waves is extremely complex.
In particular, the number of solar spots chang­
es every 11 years, and this change affects the
shape of the propagation, in receiving SW
broadcast, all the foregoing must be kept in
mind. Well known SW staions take all these
factors into consideration and change the fre­
quency from season to season or broadcast
with two or more different frequencies at the
same time for optimum reception.
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