Solid State Shortwave Receivers For Beginners - The Listeners Guide

High frequency signals pass straight through the E layer usually,
and go on to the F2 layer where they are reflected back to earth. The
F2 layer is at a height of about 200 miles as compared to about 70
miles for the E layer. This greater height of the F2 layer causes the
reflected high frequency radio waves to be reflected back to the
earth’s surface at a great distance from the transmitter, and it is
partially this factor that makes the H.F. bands (10, 15 and 20 Metres)
so good for DX reception. This type of propagation is represented by
line A - E - F in the diagram.
Another factor for the good DX reception on the H.F. bands is
the absorbtion of the ground wave which prevents local transmitters
from blotting out DX signsis. When listening on the 20 Metres amateur
band it is quite common to hear a relatively local station in contact with
one several thousands of miles away. Often the distant station provides
a really strong signal whereas the local one is barely perceptible. In fact,
when listening on the 20 Metre band only very rarely will a British
station be heard at all.
In order for a signal to pass from one side of the earth to the
other it has to bounce from the atmosphere to the earth, back up to
the atmosphere and then down to earth again for several cycles in order
for it to negotiate the curvature of the earth’s surface.
How well, or otherwise, the signals are reflected by the F2 layer
is largely dependent upon unpredictable events, and the sun is a major
factor here as it is it’s radiation that ionises the relevant layer of the
atmosphere. It is quite possible for an H.F. band to suddenly have a
proliferation of DX signals which disappear a few hours later just as fast
as they arrived. For quite long periods the H.F. bands can seem to be
completely dead, particularly towards the upper end of the S.W.
frequency spectrum. Therefore one should not be too perturbed if
a newly constructed receiver does not seem to operate as well on the
H.F. bands as it does on the L.F., ones. This is probably due to a lull
in the propagation conditions on the H.F. bands rather than because
of some fault in the receiver.
Broadcast Bands
There are twelve broadcast bands and their frequency limits are
shown in the table below. Note however, that broadcast stations tend
to sprawl outside these limits, and they are not adhered to as strictly as
in the case of the amateur bands.
120 Metres 2.3 to 2.495 MHZ
90 Metres 3.2 to 3.4 MHZ
75 Metres 3.9 to 4.0 MHZ
60 Metres 4.75 to 5.06 MHZ
49 Metres 5.95 to 6.2 MHZ
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