曝光台 注意防骗
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comms, etc. Relatively staticfree,
needs line of sight. Used
also for VOR, ILS Localiser.
· 300-3,000 MHz. UHF – Ultra
High Frequencies. DME, SSR,
ILS Glidepath, GPS.
· 3-30 GHz. SHF – Super High
Frequencies, with centimetre
waves, as used in radar. Also
known as microwave frequencies, so
good for MLS, Radar, Doppler
and radio altimeters.
Propagation
An invisible connection between two
points is called a field – since radio
depends on the interplay of
electricity and magnetism, there is an
electromagnetic field joining your
radio with whatever is transmitting.
A change of one type of field causes
a change in another, so if you vary
an electric field, it will induce
changes in a magnetic field and vice
versa, which is how an aerial is used
to transmit – flip-flop movement of
electricity up and down creates a
magnetic field around it, and the
movement of the magnetic field
creates an electric field, which
creates another magnetic field, and
so it goes on until the power fades in
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
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).
The trick is to flip-flop the electricity
so fast along the antenna that it
effectively falls off the end and keeps
on going, which doesn’t happen
122 JAR Private Pilot Studies
below a certain frequency (the
frequency of the field is the same as
the AC along the antenna).
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.
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, or dead space:
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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