Radio theory

062 Radio NavigationChapter 1

Basic terms

Formula — Speed, frequency and wavelength

c = λ × f
c = 3 × 10⁸ m/s (speed of light)
T = 1 / f   (time period)

Key fact

Frequency is the number of complete cycles per second (Hz). Wavelength (λ) is the distance travelled in one cycle. Low frequency = long wavelength; high frequency = short wavelength.

Unit	Value
1 kHz	10³ Hz
1 MHz	10⁶ Hz
1 GHz	10⁹ Hz

Antenna length and polarisation

Key fact

Ideal antenna length is ½ the wavelength. If not possible, ¼, ⅛ etc. will work. Polarisation is the plane of the electrical component (E). Transmission from a vertical aerial gives vertical E and horizontal H; horizontal aerial gives the reverse. Circular polarisation spins both components about the axis of advance (used by GPS helical antennas).

Key fact

Electrical field E is in the same direction as the aerial. Magnetic field H is perpendicular. A dipole is the simplest antenna — receiver and transmitter must be on the same plane.

Polar diagrams

Type	Characteristics
Omnidirectional	Equal radiation in all horizontal directions
Directional	Main lobe plus unwanted side lobes

Applies to both transmitter and receiver aerials. Flat (slotted) plate antennas generate fewer side lobes than parabolic dishes.

Modulation types

Definition — Keying (NON A1A)

Interrupting the carrier wave to transmit Morse code. Does not change frequency or amplitude. Will temporarily interrupt nav-aid output.

Definition — AM

Amplitude of the carrier varies with the audio signal; carrier frequency stays constant. Oldest method after keying. Small amplitude areas give weak signals prone to interference. Requires extra modulation power.

Definition — FM

Frequency of the carrier varies with audio amplitude; amplitude stays constant. Positive amplitude = higher frequency; negative = lower. FM transmitters are simpler and cheaper, need less modulation power, and are almost static-free on VHF. Horizontally polarised — suffers less weather static than vertical. Receivers are more complex and need a wider frequency band.

Definition — Pulse

Carrier switched on/off at regular intervals — used in radar. Effectively transmits 0s and 1s.

AM sidebands

Key fact

When a carrier is AM modulated by a frequency lower than itself, sidebands are created. Example: carrier 500 kHz, audio 4 kHz → output at 496 / 500 / 504 kHz. A passband filter removes unwanted frequencies. Single Sideband (SSB) transmits only one sideband — all TX power focused on one sideband increases range. HF comms and VOLMET use SSB (J3E). FM has many more sidebands than AM.

Emission codes (MSI)

Position	Meaning
1st	Type of modulation
2nd	Nature of modulating signal
3rd	Type of information transmitted

Wave behaviour

Definition — Refraction

Radio waves bend when travelling obliquely between media of different density. Low to high density = slows and bends toward the normal. Types: coastal (land to sea — fly higher or move beacon toward coast to reduce), atmospheric (density change with altitude), ionospheric (sky waves).

Definition — Reflection

Radio waves bounce off solid surfaces. Two signals arriving out of phase cause fading or temporary loss.

Definition — Diffraction

Energy scattered when a wave passes a solid object — allows reception behind mountains.

Definition — Attenuation

Weakening of the radio wave. Higher frequencies are more susceptible to surface attenuation (hit the surface more often). Ionospheric and atmospheric/radar attenuation also absorb energy — radar energy striking water droplets is both absorbed and reflected.

Doppler effect

Key fact

Positive Doppler shift: distance between source and receiver decreasing → received frequency appears higher than transmitted (more waves detected than if stationary). Negative shift: distance increasing → frequency appears lower. The actual wavelength stays the same.

Propagation paths

Band	Propagation
VLF / LF / MF / HF	Surface wave + sky wave
VHF / UHF / SHF	Direct (line-of-sight) wave

Direct wave

Formula — Maximum range (nm)

Max Range (nm) = 1.23 × (√HT + √HR)

Key fact

Direct wave is essentially line of sight. Range depends on height of Tx and Rx, transmitter power, and intervening terrain. Range can be reduced by lowering TX power. Space waves = direct + reflected + sky.

Surface waves

Key fact

Due to diffraction and surface attenuation. Attenuation slows the bottom of the wave, giving a forward tilt that follows Earth’s curvature. Attenuation is reduced over the sea — waves travel roughly twice as far. Lower frequencies have longer range but suffer from low-efficiency aerials, static, and high TX power requirements.

Sky waves

Key fact

Signals refracted by the ionosphere (approx. 50–500 km) sufficiently to return to Earth. Lower frequencies refract more. Ionosphere layers: F (HF), E (LF/MF), D (VLF). Anything VHF or greater passes straight through. At dawn/dusk, re-ionisation may cause no signal.

Definition — Night effect

At night: D layer disappears; E and F layers increase in height. An 8 MHz signal goes higher; to keep it in range, use approximately half the frequency so it refracts more. MF/LF sky waves are attenuated by day — no sky waves during daylight. At night, sky waves can interfere with surface waves (e.g. NDBs) so TX power may be reduced.

Term	Definition
Critical angle	Minimum angle for refraction back to Earth; less = no refraction; more = increased skip distance
Skip distance	Distance from transmitter to first sky-wave arrival point
Dead space	Area between surface-wave limit and first sky wave (mainly HF); minimised with lower frequency

Duct propagation

Key fact

Created by temperature inversion and/or rapid decrease in humidity with height. Causes super-refraction — VHF and above can have unexpected ranges. Layer normally no more than 1,000 ft thick.

Direction finding Q codes

Code	Meaning
QDM	Magnetic bearing to the station
QDR	Magnetic bearing from the station (radials go outwards)
QUJ	True bearing to the station
QTE	True bearing from the station

VDF (VHF Direction Finding)

Key fact

GDF mainly operates VHF/UHF; in civil aviation commonly called VDF. Uses Adcock or Doppler aerials (Doppler most common — direction calculated from Doppler shift phase). Aerials are vertically polarised. Frequency spacing: 25 kHz = frequencies; 8.33 kHz = channels.

VDF class	Accuracy
A	± 2° (not normally used)
B	± 5°
C	± 10°
D	Worse than 10°

Exam trap

QGH = ATC interpreted (heading and heights issued to maintain published pattern). QDL = pilot interpreted (series of QDMs). VDF range is limited by line of sight; accuracy affected by equipment errors, propagation (reflections, refraction, ducting), site errors, multipath, and crossed transmissions.