Propagation - Short Wave Bands - AOR AR7030 Operating Manual

system. If in doubt consult an experienced electrician.
Connecting an external earth wire may greatly reduce the
local noise encountered when listening on the short wave
bands. It is very important to provide a good earth should
you use an aerial tuning unit.
A short length of thick gauge earth wire may be connected
to a nearby central heating radiator or water pipe but never
use a gas pipe for earthing. Ideally a separate earth rod
should be used but not if the receiver must be distant from
the rod - if too long the earth system may well pick up
noise rather than remove it. If a long run of earth wire is
necessary, it may be worth considering a screened earth
system. This simply comprises a 50 ohm coaxial cable
with the centre and screen shorted at the earth rod end
and only the centre connected to the GND connection of
the AR7030, the outer screen braid being cut back and
insulated.

12 Propagation - short wave bands

Unlike VHF and UHF transmissions which generally
propagate only short distances (to the horizon plus a small
amount), short wave transmissions may travel for many
thousands of kilometres. Depending upon the frequency
in use, time of day, season of the year and sun spot activity
transmissions may propagate completely around the
world.
Radio signals are electromagnetic waves very similar to
light beams. As such they do not readily follow the
curvature of the earth but attempt to travel out into space.
Luckily the frequency spectrum of short wave is often
reflected back down to the earth by the upper layer of the
earth's atmosphere called the ionosphere.
When the reflected signals reach the earth again they may
either be received or reflected back up into space. Often
they will be reflected back by the ionosphere yet again
providing reception into another and possibly more distant
location. The ionosphere is constructed of many layers of
ionised gas. Of particular interest to short wave listeners'
are the lower E and upper F1 and F2 layers although a
lower D layer exists during day time.
D layer: During day time the lower D layer forms around
60 to 80 kilometres above the earth's surface. This layer
tends to absorb low frequencies reducing the distance
covered by medium wave transmissions. At night time
when the D layer dissipates, medium and low frequency
transmissions may propagate over much greater distances
because they can reflect from the higher layers.
If the transmitted frequency is too high to be reflected by
an ionospheric layer or the wave meets the layer at too
steep an angle, transmissions will pass straight though
without being reflected and will travel upward to the next
ionospheric layer.
E layer: Above the D layer is the E layer - located at a
height of about 100 kilometres. The E layer tends not to
absorb signals as much as the D layer but reflects some
signal back to earth where it may be received some
distance from the original point of transmission.
Usually in autumn and spring sporadic E propagation
PAGE 32 AR7030 OPERATING MANUAL
results from dense pockets of E layer ionosphere,
reflecting even the higher VHF and UHF transmissions
causing patterning on television sets. This is to the delight
of radio amateurs who are then able to communicate for
many hundreds and even thousands of kilometres on
frequency bands usually capable of only local reception.
F1 and F2 layers: During the day time there are two upper
layers of the ionosphere, these being the F1 layer at about
200 kilometres and the F2 layer at about 400 kilometres.
As evening falls, these layers combine to form a single F
layer. It is the F layer that is largely responsible for short
wave propagation over great distances. The density of
the ionospheric layers vary depending upon season, time
of day and sunspot activity (which is believed to follow an
eleven year cycle of good and bad propagation conditions).
Often large areas of the earth's surface lie between the
point of transmission and the region were the transmission
is reflected down to - in this area there will be little or no
reception. For this reason F layer propagation is often
referred to as skip propagation and the reflected signal
as sky wave.
Generally speaking only frequencies below 30MHz are
reflected by the ionosphere. Higher frequencies pass
straight through even the F layers and continue outwards
into space for ever.
Choice of frequency: Depending upon the time of day
and desired skip distance, different frequencies will be
selected by radio amateurs and commercial users such
as short wave broadcasters and oceanic air traffic control.
For instance the MUF (maximum usable frequency) is
often stated for a path between two locations at a particular
time. Choosing a frequency above the MUF will not
produce results as transmissions will pass straight into
space.
Many propagation predictions and statistics are published
and usually available from most country's national amateur
radio and short wave listeners' representatives. Various
publications are produced giving transmission and contact
details for world-wide reception. These titles include:-
Passport To World Band Radio, IBS North America,
Box 300, Penn's Park PA 18943, USA.
World Radio TV Handbook (WRTH), PO Box 9027,
1006 AA Amsterdam, The Netherlands
or BPI Communications, 1515 Broadway, New York
10036, NY USA.