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an inverse square relationship,
meaning that a signal 2 nm from its
source will have a quarter of the
strength of one only 1 nm away.
The result is an electromagnetic
wave with of one of each type at
right angles to each other. A wave’s
Electricity & Radio 279
polarisation is noted with reference to
the electrical field, so a vertically
polarised wave has a vertical electric
field, which will come from a vertical
aerial. For efficiency, the receiver
must have the same orientation.
NDBs are vertically polarised, whilst
VORs are horizontally polarised.
Attenuation, or weakening, of a signal
occurs for three reasons. Firstly, the
circumference of the wave front
increases as the wave spreads,
reducing the amount of energy per
given unit of length. Secondly, some
energy is soaked up by the Earth's
surface, the rate increasing with
frequency (the land absorbs 3 times
as much as than sea). Finally, the
ionosphere will take up some, but
the rate decreases with frequency.
The trick in the first place is to flipflop
the electricity so fast along the
antenna that it effectively falls off
the end and keeps on going, which
doesn’t happen below a certain
frequency (the frequency of the field
is the same as the AC).
A space wave may leave the antenna at
an upward angle, or be bounced off
the ground. If contained within the
troposphere, it will also be known as
a tropospheric wave, or Direct Wave.
Otherwise it will be a sky wave when
headed for the ionosphere, where it
might be bounced downwards again,
if the angle is right, and reach further
distances (on HF). The ionosphere is
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.
There are four layers to the
ionosphere, called the D, E, F1 and
F2 layers. The D & E and F layers
merge during the night, so the D
layer is only there during the day,
between 50-100 km high. It weakens
LF and MF waves, and refracts VLF.
The E layer is higher, between 100-
150 km, and defracts sky waves at
night because the D layer isn't there.
By day, the F layer (actually F1 and
F2) ranges between 150-1000 km
high, but both merge into one higher
and thinner layer at night.
In Summer, the F2 layer is higher by
day than it is in Winter.
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 does the turning.
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 at which 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:
280 Canadian Professional Pilot Studies
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
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
　
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