15 m band (18.900-19.020 mhz), 13 m band (21.450-21.750 mhz), 11 m band (25.600-26.100 mhz) – Panasonic RFB33 User Manual
Page 69: Propagation of electromagnetic waves
Attention! The text in this document has been recognized automatically. To view the original document, you can use the "Original mode".
15 m Band (18.900-19.020 MHz)
When the solar spots decrease, DX station
and traffic communication signals are difficult
to receive. When the spots are active, it is pos
sible to receive low-power DX stations as sur
prisingly strong signals.
13 m Band (21.450-21.750 MHz)
This band is used for long-distance interna
tional broadcasts. It is used regardless of the
season when the solar 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 long-distance interna
tional broadcasts during heavy activity in the
solar 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 signals.
■ Propagation of electromagnetic
waves
Electromagnetic
waves
of
a
broadcast
are
separated into surface waves which are prop
agated along the surface of the earth and
space waves which are propagated upwards.
Low frequency waves (LW & MW) are surface
waves, while high frequency waves (SW) are
bounced off the ionosphere which is located
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.
69