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AM (i.e. voice) is called A3. Jumping
a bit, the VOR, mentioned below, is
A9W, because its carrier wave varies
with amplitude and frequency.
The emission character codes are:
· N – unmodulated
· A – AM double sideband
· J – AM single sideband
· F – Frequency modulated
· P – Pulse modulated (radar)
· G – Differential phase shift
keying (for MLS)
The numbers include:
· 0 – unmodulated
· 1 – keyed carrier wave
· 2 – Simple AM
· 3 – Complex/voice modulated
· 4 – Pulse modulated
· 9 – Miscellaneous (VOR)
Classes of emission are:
Class Aid
N0MA1A NDB (BFO on)
N0NA2A NDB (BFO on for tone,
off for ident)
A2A NDB
J3E HF
A3E VHF/VDF
A8W ILS
A9W VOR
P0N DME, SSR
N0X/G1D MLS (DPSK)
Sidebands
When you combine two signals,
another two are created
automatically, above and below the
resulting combination, being the
equivalent of the sum and difference
frequencies of the original two, so
you get four in total. The extras are
called upper and lower sidebands, which
are exact mirrors of each other, in
terms of power and information
carried (the complete range is called
the bandwidth). You can remove three
waves, and transmit one sideband
only, adding the ones that were
taken away at the receiver, which
means you don’t need so much
power to transmit the same distance.
Also, the signal doesn’t take up so
much bandwidth, and you can get
more channels in.
This is called Single Sideband
Transmission, or SSB. FM has
sidebands as well, but more are
278 Canadian Professional Pilot Studies
produced, and make things more
complex anyway. In effect, with SSB,
you are transmitting with a narrower
bandwidth. Clarity can be improved
by monitoring the carrier trace.
The whole range of frequencies
transmitted at a time is known as the
bandwidth, in the above case including
the upper and lower sidebands. The
narrower the bandwidth of a signal,
the longer the range, and the more
signals you can cram into a given
space, not to mention less noise.
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.
It is important to remember which
equipment belongs in each range.
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
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