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a region where the Sun’s UV rays
dislodge electrons from the gas
molecules, making them positively
ionised. This happens mostly during
the daytime and is at its minimum
just before sunrise.
Anyhow, any wave that hits the
ionosphere is bent, as the side of the
wave that hits a layer first starts to
speed up, which has the bending
effect. Eventually, if the angle is
increased, the bending will be
enough to bounce the wave back to
Earth (we won’t get into Moon
bouncing here!). The angle when this
first happens is called the critical angle.
A ground or surface wave, in contrast,
may go directly, or curve to follow
the Earth’s surface, depending on
the frequency. Friction with the
ground and the widening
circumference of the wave will
eventually weaken its power, though.
When a wave leaves an antenna, the
ground wave will be detected until it
fades, or attenuates. Between that
point, and where the first sky wave
comes from the ionosphere, is an
area where nothing is heard, called a
skip zone:
Surface and atmospheric attenuation
increase with frequency, while
ionospheric attenuation decreases.
Ground range increases if critical
angle, frequency, dead space and
skip distance decrease, and vice versa.
The skip distance is the Earth distance
taken by a signal after each refraction,
or the distance covered by the first
sky wave. The maximum useable
frequency exists where skip distance is
the same as that between the
transmitter and receiver. The optimum
Electricity & Radio 123
useable frequency is about 85% of that,
to allow for variations in the height
and thickness of the ionosphere (see
VOR for the formula).
The ionosphere moves all the time,
affecting it considerably, which is
why the ADF suffers from what is
called night effect just after sunset and
before sunrise when the needle
swings erratically (on the other hand,
during night is when you will receive
distant stations best). Refraction can
occur in many ways, from the
ionosphere (see above), the coast (a
wave crossing at anything other than
a right angle will be bent) through to
the atmosphere – under certain
inversion conditions, a phenomenon
called ducting occurs, which enables
waves to travel unusually large
distances (the author has certainly
seen British TV programs in
Germany with this effect).
Fading is due to multi-path propagation,
amongst other things, where signals
may be received from many sources
and be out of phase with each other
at the aerial. Sometimes, under such
circumstances, waves will cancel
each other out.
Ground waves are associated with
LF/MF waves, sky waves with HF
and direct waves with VHF frequencies
and above. In fact, the latter are also
called line-of-sight, meaning that
anything in the way, like hills or
buildings, will have a detrimental
effect (they will not bounce like HF).
You will get best reception if the
transmitter and receiver are in sight
of each other, but, in practice, you
can expect a little more than that,
actually to just beyond the horizon,
due to effects like refraction within
the troposphere (see VOR, below).
The actual figure is greater by a
factor of around 4/3.
HF frequencies need to be higher
during the day or when you are at
greater range from the station. At
night, you can use lower frequencies,
generally about half (that is, use
Double During Day).
VOR
This stands for Very High Frequency
Omnidirectional Range, so is based on
VHF, using the phase difference
between two signals to signify your
direction from the transmitting
station. The frequency range is
between 108-112 MHz on even
decimals, plus 50 KHz (to prevent
confusion with the ILS), and 112-
118 on odd and even, plus 50 Khz.
VORs are identified on maps with a
compass rose around the station
aligned with Magnetic North. It is not
sensitive to heading, as is the ADF
(below) - it shows track.
The Station Identifier is transmitted in
Morse Code every 15 seconds, and
you must confirm the frequency and
ID before using a VOR for
navigation. If there is no ID, but
behaviour is otherwise normal, the
system is on maintenance.
The transmitter sends out a
reference signal in all directions,
frequency modulated at 30 Hz,
which is received by all stations at
the same phase, if they are the same
　
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