13.4(a) Radio fundamentals and communication systems (ATA 23)

Scope

13.4 is the biggest block of Module 13 (level 3 for B2; the B2L COM/NAV rating exam covers 13.4(a)). This note covers radio fundamentals and communication systems; navigation is in the next note and surveillance in the one after.

Radio-frequency bands

Band	Frequency	Aviation uses
LF / MF	30 kHz–3 MHz	NDB/ADF
HF	3–30 MHz (aviation HF comm uses about 2–30 MHz)	Long-range voice, HF data link
VHF	30–300 MHz	VHF comm (118–136.975 MHz), VOR/LOC (108–117.95 MHz), marker beacons (75 MHz)
UHF	300 MHz–3 GHz	Glideslope (329–335 MHz), DME (962–1213 MHz), transponder/TCAS (1030/1090 MHz), GNSS, SATCOM (L-band)
SHF	3–30 GHz	Radio altimeter (about 4.3 GHz), weather radar (about 9.375 GHz), MLS (about 5 GHz)

Wavelength λ = c ÷ f. Example: 125 MHz → λ = 3 × 10⁸ ÷ 125 × 10⁶ = 2.4 m; a quarter-wave VHF antenna is therefore about 0.6 m long.

Propagation

Wave	Behaviour	Bands
Ground (surface) wave	Follows the Earth's curvature; attenuated faster at higher frequencies	LF/MF (NDB)
Sky wave	Refracted by the ionosphere back to Earth; long range, with a skip zone between the end of the ground wave and the first sky-wave return	HF
Space (direct) wave	Line of sight; passes through the ionosphere	VHF and above

Ionospheric layers (D, E, F1, F2) vary with sunlight: the D layer absorbs HF by day; at night it disappears, so lower HF frequencies propagate further at night. Higher HF frequencies are used by day.

Approximate VHF line-of-sight range: 1.23 × √(height in ft) nautical miles for one end at height and the other at ground level (e.g. 10,000 ft → about 123 NM).