The International Telecommunication Union (ITU) classifies emissions by short codes, and four matter for aviation:
N0N — a carrier without modulation, as used by non-directional radio beacons (NDBs);
A1A — a carrier with keyed Morse code modulation, as used by NDBs;
A2A — a carrier with amplitude-modulated Morse code, as used by NDBs;
A3E — a carrier with amplitude-modulated speech, used for communication (VHF-COM).
Read the codes as a story of increasing content: N0N is a bare carrier, A1A switches that carrier on and off in Morse rhythm, A2A modulates a Morse tone onto it, and A3E modulates speech. The NDB lesson will lean on the first three.
Four kinds of modulation carry this course's aids, each varying a different property of the carrier:
amplitude modulation — the information is impressed onto the carrier by altering its amplitude;
frequency modulation — the information is impressed by altering the carrier's frequency;
pulse modulation — used in radar: short pulses transmitted, followed by longer interruptions;
phase modulation — used in GPS (the global positioning system): the phase of the carrier wave is reversed to carry the information.
Pulses
Some aids transmit continuously; others, radar above all, transmit in bursts, and a pulse string has its own vocabulary, defined for the exam:
pulse length — the duration of one transmitted pulse;
pulse power — the power during the pulse itself;
continuous power — the power averaged over the whole cycle of pulse and silence.
A radar can have an enormous pulse power and a modest continuous power, because most of its time is spent listening, not transmitting.
Antennas on an aircraft
An antenna (or aerial) is an electrical device which converts electric power into radio waves, and vice versa. The simplest type is the dipole: a wire of length equal to one half of the wavelength it is built for, which is why antenna sizes track the band. The wave an antenna launches has an orientation: the polarisation of an electromagnetic wave describes the orientation of the plane of oscillation of its electrical component with regard to the direction of propagation, and a receiving antenna hears best when its own orientation matches it. When an aid needs to know direction, it needs a directional antenna. The common directional types, listed for the exam:
the loop antenna — used in old automatic direction-finding (ADF) receivers;
the parabolic antenna — the dish, used in weather radars;
the slotted planar array — the flat plate that replaced the dish in more modern weather radars.
Where an antenna sits on the airframe decides what it can hear. Antenna shadowing is the aircraft's own structure blocking the path between the antenna and a station: bank the aircraft or put the tail between antenna and transmitter, and the metal casts a radio shadow in which the signal weakens or vanishes. That is why antenna placement matters: each antenna is positioned so the aircraft shadows it as little as possible for the directions its aid must cover, which is why communication and navigation antennas are spread over the top and belly of the fuselage rather than gathered in one convenient place.
How waves travel
Three routes carry a wave from transmitter to receiver, and each has its definition:
space waves — the electromagnetic waves travelling through the air directly from the transmitter to the receiver;
ground (surface) waves — the electromagnetic waves travelling along the surface of the Earth, following its curve beyond the horizon before they fade;
sky waves — the waves riding the upper atmosphere.
The frequency decides which routes exist: radio waves in VHF, UHF, SHF and EHF propagate as space waves, essentially line of sight, while radio waves in LF, MF and HF propagate as surface (ground) waves and as sky waves.
The sky wave's mirror deserves its own paragraph. The ionosphere is the ionised component of the Earth's upper atmosphere, from approximately 60 to 400 km above the surface, vertically structured in three regions: the D, E and F layers, whose depth varies with time. Electromagnetic waves refracted back down from the E and F layers are called sky waves. The layers influence propagation differently: the low D layer mostly absorbs, taking energy out of anything sent up through it, while the higher E and F layers refract and return; by day the sun keeps the D layer strong and it absorbs much of what is sent up, and after dark the D layer fades, so more reaches the refracting layers and sky-wave reach grows at night. One more wave behaviour belongs in the toolkit: the Doppler effect, where the frequency of a wave increases or decreases when there is relative motion between the transmitter and the receiver; a later lesson's Doppler VOR takes its name from it.