VHF Radio — Foundations
In civil aviation, VHF (Very High Frequency) is used for voice communication between cockpit and ground station. Standard since the 1940s.
Frequency range
Aeronautical Mobile (R) Service:
- 117.975 – 137.000 MHz for civil aviation voice radio (ICAO Annex 10 Vol V).
- Usable voice traffic in practice starts at 118.000 MHz; 117.975 is nominally the lower band edge.
- Subdivided by function (ATC, FIS, Tower, Ground, ATIS, etc.).
Source: ITU Radio Regulations, ICAO Annex 10 Volume V.
Physical foundations — electromagnetic waves
Speed of light / propagation speed of electromagnetic waves:
c ≈ 300,000 km/s = 3 × 10⁸ m/s
This constant applies to all electromagnetic waves in vacuum (light, radio, X-ray) — virtually identical in air.
VHF waves propagate "quasi-optically" — i.e. they propagate essentially like visible light:
- Straight-line propagation along the line of sight (LOS).
- No significant bending around obstacles or Earth's curvature.
- No ionospheric reflection (unlike HF / shortwave).
- Reflection off obstacles (mountains, buildings) possible → multipath.
→ Consequence: VHF needs line of sight between transmitter and receiver. Behind mountains, beyond the horizon, or in a radio shadow → no reception.
Modulation
Amplitude modulation (AM) — specifically A3E (double-sideband AM, voice):
- Carrier wave + upper and lower sideband.
- Equipment failure clearly audible as noise (typical "mosquito" sound).
- Capture effect: only one transmitter per frequency audible — simultaneous transmission (double-key) produces whistling or silence.
Channel spacing
Historically: 25 kHz spacing → 720 channels in VHF band.
Today (EU): 8.33 kHz channel spacing — mandatory since:
- 5 February 2018: for all aviation stations in EU airspace above FL195.
- 2 February 2020: extension to all areas.
In the lower airspace (PPL area) today a mix of 25 kHz and 8.33 kHz is in use — pilot must handle both.
→ 2160 channels in VHF band — mainly to relieve growing frequency scarcity in dense airspace.
Source: EU Implementing Regulation 1079/2012 ("eight-point-three-three").
Range — line of sight
VHF radio waves follow direct line of sight (LOS) — no ionospheric reflection. Range depends on altitude and topography.
Rule of thumb (Earth-curvature corrected):
R (NM) ≈ 1.23 · √h (ft)
| Altitude AGL | Range (NM) — flat terrain |
|---|---|
| 100 ft | 12 NM |
| 1000 ft | 39 NM |
| FL065 (6500 ft) | about 95–99 NM (see example) |
| FL100 | 123 NM |
| FL200 | 174 NM |
| FL400 | 246 NM |
Example calculation FL065 over flat terrain: R = 1.23 × √6500 = 1.23 × 80.6 = 99.1 NM ≈ 95 NM (with safety margin).
→ Practically achievable at cruise altitude FL065 about 95 NM to ground station in flat terrain.
Factors affecting VHF reception
- Altitude of transmitter and receiver — primary factor.
- Topography — mountains and hills reduce effective range.
- Weather — usually small effect; static during severe thunderstorms.
- Obstacles in the beam path — radio shadow.
- Antenna characteristics — vertically polarised in aviation.
VHF radio equipment requirements
For certain aircraft types, VHF radio equipment is mandatory:
| Aircraft type | VHF mandatory |
|---|---|
| Rotary wing (helicopters) | Yes (in most airspaces) |
| Powered gliders | Yes (for commercial operation, in controlled airspace) |
| Gliders | Yes in controlled airspaces, many sport fields |
| Airships | Yes |
| Free balloons | Yes when ascending into/through controlled airspace |
→ Generally: any aircraft in Class A, B, C, D, RMZ (Radio Mandatory Zone) requires VHF radio. In Class E/G without RMZ, radio is recommended but not always mandatory.
Source: ICAO Annex 6 Part II + national AIPs.
Aviation-specific sub-bands
| Frequency range | Use |
|---|---|
| 118.000–121.395 | ATC (Tower, Ground, Departure, Approach) |
| 121.500 | International emergency frequency — "Guard Frequency" |
| 121.600–121.975 | Aerodrome / Ground movement |
| 122.000–123.675 | Aerodrome / Air Traffic Services |
| 123.450 | "Air-to-air" frequency (informal pilot exchange) |
| 123.500 | Glider common |
| 123.700–128.825 | ATC en-route |
| 130.000–137.000 | Company / operations |
Source: ICAO Annex 10 Vol V, national frequency plans (BNetzA in Germany, OFCOM in Switzerland).
121.500 MHz — emergency frequency
- Listen watch is recommended for all pilots if a second radio is available.
- ELT signals (emergency locator transmitter) transmit on 121.500 MHz and 406 MHz.
- On primary-frequency radio failure: try 121.500.
Cockpit radio display
Typical:
- Active frequency: left/top — the one currently in use.
- Standby frequency: right/bottom — already set for the next change.
- Flip button: switches active ↔ standby.
Audio panel
Audio panel routes audio signals between radios, intercom, NAV audio. Pilot selects:
- COM1 / COM2 listen.
- NAV1 / NAV2 listen (for Morse identifier).
- PTT (push to talk): right pedal tap or yoke button for transmit mode.