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(i.e. frequency modulation), which is less
subject to interference. Many
customers, such as forestry, also use
it for communications. Anyhow, a
modulator's job is to combine signals
from the radio and audio amplifiers.
The simplest method of transmitting
information is to turn a signal on
and off in a recognisable code, as
used by older NDBs which break the
signal in a pattern matching the
Electricity & Radio 121
Morse Code ID of the station, called
wireless telegraphy, or continuous wave
(CW). This is known as an A1
transmission, whereas a carrier wave
by itself would be known as A0.
Note: Although Marconi
transmitted the first CW signal, a
Canadian, Reginald Fessenden,
transmitted the first voice signal from
Massachusetts to ships along the
Eastern Seaboard. Mind you, Nikola
Tesla was way ahead of them both.
Otherwise, you can adjust the
amplitude (or volume, if you like), the
frequency, or the phase, if the frequency
of both waves is the same. Simple
AM is known as A2, and complex
AM (i.e. voice) is called A3. Jumping
a bit, the VOR, mentioned below, is
A9W, because its carrier wave varies
with amplitude and the frequency at
the same time.
Wavebands
The range of possible
electromagnetic waves is quite large,
but radio waves only occupy a small
part of it, actually between about 3
KHz to 3,000 GHz. This area is split
up by International agreement
between the people who wish to use
it, and consists of frequency ranges
that share similar characteristics:
· 3-30 KHz. VLF – Very Low
Frequencies, with very long waves.
Used by Omega.
· 30-300 KHz. LF – Low
Frequencies, with long waves.
Used by Decca, NDBs, Loran.
· 300-3,000 KHz. MF – Medium
Frequencies, with medium waves,
used by most AM stations, and
NDBs, with static problems.
· 3-30 MHz. HF – High
Frequencies, with short waves,
used for long-range SSB
communications between
aircraft and ground stations,
with static problems.
· 30-300 MHz. VHF – Very
High Frequencies, used with
amplitude modulation for voice
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
122 Canadian Private Pilot Studies
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 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 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
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