CHAPTER 8 · REFERENCE DEPTH

Frequency Bands & Their Aviation Uses

Chapter 7 told you what a frequency is. This chapter tells you what each slice of the spectrum is used for — and this is where RTR papers harvest most of their marks. Examiners love precise numbers: which band carries ATC voice, which one the glide path, what the transponder replies on, where the emergency frequency sits.

SYLLABUS MAP

Part II (ii) Radio frequency bands · principal aviation frequencies · polarization

Learning objectives — by the end of this chapter you will be able to…

8.1 The spectrum, band by band

8.2 VHF — the ATC workhorse

8.3 HF — reaching the ocean

8.4 The aviation frequency master table

8.5 Channel spacing & capacity

8.6 Polarization & antenna length

VHF Antenna slicing through clouds
The aircraft's antennas are the physical link between the electrical circuits inside and the invisible frequency bands outside.

8.1 The spectrum, band by band

FIRST PRINCIPLES — WHY THE SPECTRUM IS SPLIT INTO DECADES

The radio spectrum is divided into bands, each a factor of ten in frequency above the one below. This matters because the dominant way a wave travels — and therefore what it is useful for — changes with frequency. Low frequencies have long wavelengths and bend around the Earth; high frequencies have short wavelengths and travel in straight lines. Aviation lives mainly in the HF, VHF and UHF bands.

The Radio Spectrum Bands
Band Frequency Wavelength Dominant propagation Typical aviation use
VLF3–30 kHz10–100 kmGround/wave-guide(Very long range; rare)
LF30–300 kHz1–10 kmGround waveNDB (low end)
MF300 kHz–3 MHz100 m–1 kmGround waveNDB; 2182 kHz maritime distress
HF3–30 MHz10–100 mSky waveLong-range / oceanic voice (SSB)
VHF30–300 MHz1–10 mSpace (line-of-sight)ATC voice, VOR, ILS LOC, markers
UHF300 MHz–3 GHz0.1–1 mLine-of-sightILS glide path, DME, SSR, GPS, mil voice
SHF3–30 GHz1–10 cmLine-of-sightRadar, radio altimeter, satcom
EHF30–300 GHz1–10 mmLine-of-sight(Advanced / experimental)
Mnemonic — the band order

"Very Long Modems Help Various Useful Signals Excel" → VLF, LF, MF, HF, VHF, UHF, SHF, EHF. Each band is ×10 the previous.

8.2 VHF — the ATC workhorse

The VHF aviation communication band

118.0 – 136.975 MHz, using Amplitude Modulation (AM), vertically polarized, propagating by line-of-sight (space wave). Range is limited to roughly the radio horizon — higher you fly, farther you reach.

WHY AM AND WHY VHF

VHF gives clear, static-free, short-range communication ideal for an airport's busy frequencies. AM is used so that two overlapping transmissions produce a tell-tale heterodyne squeal — alerting everyone that two stations transmitted at once, rather than one silently capturing the channel as FM would (the "capture effect", Chapter 10). AM's "you'll hear the clash" behaviour is a safety feature.

8.3 HF — reaching the ocean

The HF band in aviation

3 – 30 MHz, using Single Side Band (SSB) for efficiency, propagating by sky wave (refracted off the ionosphere). This bounces signals thousands of miles — essential over oceans and polar routes where VHF line-of-sight fails. Crews change HF frequency between day and night (Chapter 9).

Exam trap

Tie the band to its job: VHF = AM, short-range, line-of-sight; HF = SSB, long-range, sky wave. Swapping AM/SSB or the propagation path is the classic trap.

8.4 The aviation frequency master table

Master Frequency Allocations
System / use Frequency Band
VHF ATC voice118.0 – 136.975 MHzVHF
VHF emergency (guard)121.5 MHzVHF
UHF military emergency243.0 MHzUHF
SAR scene-of-search (aux)123.1 MHzVHF
ELT (satellite / homing)406 MHz / 121.5 MHzUHF / VHF
VOR108.0 – 117.975 MHzVHF
ILS Localizer108.10 – 111.95 MHzVHF
ILS Glide Path329 – 335 MHzUHF
Marker beacons75 MHzVHF
DME962 – 1213 MHzUHF
SSR transponderInterrogate 1030 / Reply 1090 MHzUHF
NDB / ADF≈ 190 – 1750 kHzLF/MF
Radio altimeter4200 – 4400 MHzSHF
Maritime distress2182 kHzMF

8.5 Channel spacing & capacity

FIRST PRINCIPLES — FITTING MORE AIRCRAFT INTO ONE BAND

The VHF band is a fixed width, so the number of channels it holds depends on how closely they are spaced. Traditionally channels were 25 kHz apart; to fit more in the crowded European band, 8.33 kHz spacing was introduced — tripling the number of channels.

Worked example — channel capacity

The VHF band is 118.0 to 136.975 MHz, a width of 18.975 MHz = 18,975 kHz.
At 25 kHz spacing: 18,975 ÷ 25 ≈ 760 channels. At 8.33 kHz spacing: 18,975 ÷ 8.33 ≈ 2,280 channels — roughly three times as many.
This is why 8.33-spaced frequencies read like 120.275 or 118.905 rather than ending in tidy tenths.

8.6 Polarization & antenna length

Definition & the aviation fact

Polarization is the plane of the wave's electric field — vertical or horizontal. The antenna must match it: a vertical wave needs a vertical antenna. Aviation VHF communication is vertically polarized, which is why the COM antenna (the blade) is mounted vertically.

Why higher-frequency aerials are short

An efficient antenna is sized to the wavelength — commonly a half-wave or quarter-wave. Since λ shrinks as frequency rises, VHF aerials are short blades while HF aerials are long wires or probes. From Chapter 7, a 120 MHz λ is 2.5 m, so a quarter-wave whip is about 0.6 m — a practical blade. An HF λ of 100 m needs a long-wire or a tuned coupler.

Cockpit reality

The little vertical blade on the fuselage is your VHF COM antenna, matched to the vertical polarization of the signal. Mismatch the polarization and received signal strength drops sharply — one reason antennas are mounted exactly as they are.

Figure 8.1: The Radio Spectrum
Figure 8.1 — The radio spectrum and where aviation lives.

☆ Numbers to memorise

Essential Facts for Chapter 8
Fact Value
Band order (×10 each)VLF · LF · MF · HF · VHF · UHF · SHF · EHF
VHF ATC voice118.0–136.975 MHz · AM · vertical · line-of-sight
HF voice3–30 MHz · SSB · sky wave · oceanic
Channel spacing25 kHz (≈760 ch) and 8.33 kHz (≈2280 ch)
VOR / ILS LOC108.0–117.975 / 108.10–111.95 MHz (VHF)
ILS GP / DME329–335 MHz / 962–1213 MHz (UHF)
SSR transponder1030 MHz interrogate · 1090 MHz reply
Emergency121.5 (VHF) · 243.0 (UHF) · 406 (ELT) · 123.1 (SAR aux)
Aviation VHF polarizationVertical
Question bank

Part A — MCQs (click an option to check)

1. The VHF aeronautical communication band is:
  • 108.0–117.975 MHz
  • 118.0–136.975 MHz
  • 3–30 MHz
  • 329–335 MHz
Answer: 118.0–136.975 MHz. VHF ATC voice = 118.0–136.975 MHz. 108–117.975 is VOR/ILS-LOC.
2. Each radio band differs from the next by a factor of:
  • 2
  • 5
  • 10
  • 100
Answer: 10. Bands run in decades — each is ten times the previous in frequency.
3. Aviation VHF communication uses which modulation?
  • AM
  • FM
  • SSB
  • PCM
Answer: AM. VHF voice uses AM; HF long-range uses SSB.
4. Long-range oceanic voice communication uses:
  • VHF AM
  • HF SSB (sky wave)
  • UHF
  • VLF
Answer: HF SSB (sky wave). HF (3–30 MHz) with SSB propagates by sky wave for long range.
5. The dominant propagation of NDB (LF/MF) is:
  • Ground wave
  • Sky wave
  • Line-of-sight
  • Satellite
Answer: Ground wave. LF/MF travels mainly by the ground (surface) wave.
6. An SSR transponder replies on:
  • 1030 MHz
  • 1090 MHz
  • 121.5 MHz
  • 75 MHz
Answer: 1090 MHz. Interrogation is 1030 MHz; the reply is 1090 MHz.
7. The ILS glide path operates in which band?
  • VHF
  • UHF (329–335 MHz)
  • HF
  • MF
Answer: UHF (329–335 MHz). The localizer is VHF; the glide path is UHF (329–335 MHz).
8. ILS marker beacons transmit on:
  • 108 MHz
  • 75 MHz
  • 329 MHz
  • 1090 MHz
Answer: 75 MHz. All markers share a 75 MHz carrier, differing in tone/code.
9. Moving from 25 kHz to 8.33 kHz channel spacing roughly:
  • Halves the number of channels
  • Triples the number of channels
  • Has no effect
  • Doubles the band width
Answer: Triples the number of channels. 8.33 kHz is one-third of 25 kHz, so about three times as many channels fit.
10. Aviation VHF communication is:
  • Horizontally polarized
  • Vertically polarized
  • Circularly polarized
  • Unpolarized
Answer: Vertically polarized. Aviation VHF comms are vertically polarized — hence the vertical blade antenna.
11. Higher-frequency antennas are physically shorter because:
  • They use less power
  • Wavelength shrinks as frequency rises, and antennas are sized to the wavelength
  • They are horizontally polarized
  • They carry less data
Answer: Wavelength shrinks as frequency rises, and antennas are sized to the wavelength. Antennas are a fraction of a wavelength; shorter λ at high f means a shorter aerial.
12. The radio altimeter operates in the:
  • VHF band
  • HF band
  • SHF band (≈4200–4400 MHz)
  • LF band
Answer: SHF band (≈4200–4400 MHz). The radio altimeter works around 4200–4400 MHz in the SHF band.
13. The maritime distress frequency in the MF band is:
  • 121.5 MHz
  • 2182 kHz
  • 406 MHz
  • 243.0 MHz
Answer: 2182 kHz. 2182 kHz is the maritime distress frequency (MF).
14. The international VHF emergency frequency and the UHF military one are:
  • 121.5 / 122.5 MHz
  • 121.5 / 243.0 MHz
  • 118.0 / 136.975 MHz
  • 406 / 121.5 MHz
Answer: 121.5 / 243.0 MHz. 121.5 MHz (VHF guard) and 243.0 MHz (UHF military guard); 243 = 2 × 121.5.
15. VOR operates in the:
  • VHF band (108.0–117.975 MHz)
  • UHF band
  • HF band
  • MF band
Answer: VHF band (108.0–117.975 MHz). VOR is VHF, 108.0–117.975 MHz (sharing 108–111.95 with ILS-LOC).
16. DME operates in the:
  • VHF band
  • UHF band (962–1213 MHz)
  • SHF band
  • LF band
Answer: UHF band (962–1213 MHz). DME is UHF, 962–1213 MHz.

Part B — Oral / viva (tap to reveal model answers)

State the VHF communication band and its characteristics.
Model Answer:
118.0–136.975 MHz, amplitude modulated, vertically polarized, propagating line-of-sight (space wave) for short range.
Why is HF used for oceanic communication?
Model Answer:
HF (3–30 MHz) propagates by sky wave, refracting off the ionosphere to reach thousands of miles where VHF line-of-sight cannot. SSB is used for efficiency, and the frequency is changed between day and night.
Name the bands and uses for the ILS components.
Model Answer:
Localizer — VHF, 108.10–111.95 MHz (azimuth); glide path — UHF, 329–335 MHz (vertical); marker beacons — 75 MHz (distance).
On what frequencies does an SSR transponder interrogate and reply?
Model Answer:
Ground interrogation on 1030 MHz; airborne reply on 1090 MHz.
What is channel spacing, and why was 8.33 kHz introduced?
Model Answer:
The frequency gap between adjacent channels. 8.33 kHz spacing was introduced to fit about three times as many channels into the crowded VHF band as the older 25 kHz spacing.
What is the polarization of aviation VHF, and why does it matter?
Model Answer:
Vertical — the COM antenna is a vertical blade to match it. If the antenna polarization is mismatched, received signal strength drops sharply.

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