Radio Waves, Modulation and Antennas

Every radio aid and radio in an aircraft, from the NDB to the weather radar, rests on the same physics. An alternating current in an antenna radiates radio waves; information is carried by changing, or modulating, those waves; and the size and shape of the antenna decide how well, and in which directions, a wave is sent and received. The frequency chosen fixes almost everything else: the wavelength, the size of the antenna, how the wave travels and what it can be used for.

The subject opens the EASA Radio Navigation syllabus and returns in Communications. Its exam questions are short calculations of wavelength, frequency and antenna length, the frequency bands and what uses them, and the meaning of modulation types and emission designators. They also explain much of what pilots meet in practice: why an ADF needs a BFO for some beacons, what codes such as A3E and P0N mean, and why a weather radar antenna can be small. How the waves then travel is covered in radio wave propagation.

Electromagnetic waves

An electromagnetic wave consists of an electric field (E) and a magnetic field (H) oscillating at right angles to each other, both at right angles to the direction of travel. When an alternating current flows in an antenna at a radio frequency (RF), energy is radiated as such a wave. In free space it travels at the speed of light, about 300,000,000 metres per second or roughly 162,000 NM per second.

Each wave is described by:

• its frequency, the number of complete cycles per second, measured in hertz (Hz): 1 kHz is 10³ Hz, 1 MHz 10⁶ Hz and 1 GHz 10⁹ Hz;
• its wavelength (λ), the distance travelled during one cycle;
• its amplitude, the strength of the field;
• its phase, the point in the cycle reached at a given instant, measured in degrees.

Phase matters in navigation because two signals of the same frequency can be compared: the VOR finds a bearing from the phase difference between two 30 Hz signals. Comparison only works between signals of equal frequency, since otherwise the difference would change continuously. Phase differences are quoted as positive values from 0° to 360°: with the reference at 110° and the variable signal at 315°, the difference is 110 − 315 + 360 = 155°.

Wavelength and frequency

Speed, frequency and wavelength are linked by the wavelength-frequency relationship:

λ = c ÷ f, which with c = 300,000,000 m/s becomes λ (metres) = 300 ÷ f (MHz).

Wavelength falls as frequency rises. Worked examples:

The radio frequency bands

The radio spectrum runs from 3 kHz to 300 GHz and is divided into eight radio frequency bands, each ten times higher in frequency, and so one tenth of the wavelength, of the one below.

NDBs work in the low and medium frequency (LF/MF) range, 190 to 1,750 kHz in ICAO Annex 10, which is why the two bands are often treated together; in the United States they normally use 190 to 535 kHz. The VHF aeronautical allocations sit side by side: FM broadcasting just below 108 MHz, radio navigation from 108 to 117.95 MHz, and voice channels from 118.000 to 136.975 MHz. In UHF, DME uses 960 to 1,215 MHz, SSR interrogates on 1,030 MHz and replies on 1,090 MHz, and GPS transmits its L1 signal on 1,575.42 MHz. In SHF, radio altimeters share the band 4,200 to 4,400 MHz, the MLS uses 5,031 to 5,090.7 MHz and airborne weather radar typically 9,375 MHz, a wavelength of about 3.2 cm.

Carrier waves and modulation

A plain radio wave carries no information. The transmitted wave is a carrier wave, and modulation is the process of adding information to it by varying one of its properties in step with the signal to be sent. Aviation uses keying (Morse), amplitude, frequency, phase and pulse modulation.

AM, FM, pulse and continuous wave

Amplitude modulation (AM) varies the amplitude of the carrier in step with the audio signal while its frequency stays constant. The result is the carrier plus two sidebands, one each side of it. VHF aeronautical voice is double-sideband AM, on channels spaced 25 kHz apart or 8.33 kHz in busy airspace. Single sideband (SSB), used for long-range HF, suppresses the carrier and one sideband, which roughly halves the bandwidth and puts the transmitter's power into the useful signal.

Frequency modulation (FM) varies the frequency of the carrier while its amplitude stays constant. Because noise and static mostly affect amplitude, FM gives cleaner reception, which is why broadcasters use it. In a conventional VOR the reference signal is frequency-modulated on a subcarrier and the variable signal amplitude-modulated; a Doppler VOR reverses the roles (see VOR).

Pulse modulation transmits the carrier in short, discrete bursts; the information lies in the timing, spacing and coding of the pulses. It is the basis of primary radar, DME and the SSR transponder. In a primary radar the pulse repetition frequency limits the maximum range and the pulse length the minimum range.

A continuous wave (CW) is an uninterrupted carrier. Unmodulated, it is what an NDB radiates between identifications. Keyed on and off, it sends Morse. A radio altimeter transmits a frequency-modulated continuous wave (FMCW), sweeping its frequency and measuring height from the difference between the frequency being transmitted and the one returning; because it transmits and receives at the same time, it needs separate transmitting and receiving antennas.

An unmodulated or keyed carrier makes no sound in an AM receiver. The beat frequency oscillator (BFO) mixes it with a locally generated signal offset by an audio frequency, producing an audible tone.

Emission designators

An emission designator describes a transmission in three symbols: how the main carrier is modulated, the nature of the modulating signal, and the type of information sent.

NDBs are listed as NON A1A or NON A2A (properly N0N A1A and N0N A2A). The FAA's AIM describes beacons whose carrier is modulated at 400 or 1,020 Hz for identification, the A2A case.

Polarisation

The polarisation of a radio wave is the plane in which its electric field oscillates, and it follows the orientation of the transmitting aerial: a vertical aerial radiates a vertically polarised wave, with the E field vertical and the H field horizontal. For the strongest signal, the receiving aerial must lie in the same plane. NDBs radiate vertically polarised waves, which favour the surface wave at LF and MF; the ADF's aerial senses them correctly only with the wings level, which is why it suffers dip error in a bank.

In circular polarisation the electric field rotates as the wave advances. Radar uses it against weather clutter: a circularly polarised wave reflected by a round raindrop comes back rotating in the opposite sense and is rejected by a receiver set for the original sense, while an aircraft, with its complex shape, returns a mixture that can still be received.

Antennas and polar diagrams

An antenna (aerial) works most efficiently when its length is matched to the wavelength.

• The half-wave dipole is a conductor half a wavelength long, fed at the centre. At 125 MHz it is 2.4 ÷ 2 = 1.2 m long. Mounted vertically, it radiates equally in every horizontal direction and least along its own axis.
• The quarter-wave (Marconi) antenna is a vertical conductor a quarter of a wavelength long, working against the ground, a counterpoise or the aircraft's skin, which supplies the missing half. At 125 MHz it is 60 cm; for an NDB on 300 kHz, λ = 1,000 m and a quarter wave is 250 m, so NDB aerials are large masts, often guyed.

A polar diagram plots an antenna's relative field strength or sensitivity in each direction.

A vertical aerial radiates little straight upwards, which is one reason for the cone of silence over an NDB. Directional antennas concentrate energy into a beam. A parabolic dish fed at its focus sends the rays out parallel and in phase, forming a narrow, high-gain beam that gives both range and angular resolution. For a given antenna size the beam narrows as the wavelength shortens, which is why radars use VHF and above: an airborne weather radar's pencil beam is typically 3° to 5° wide, and a narrower one would need an antenna too large for the nose. Similarly, the SSR ground antenna radiates a narrow beam that scans in azimuth, while the aircraft's transponder antenna replies in all directions.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.