Every word spoken between a pilot and an air traffic controller in civil aviation happens in a 19 MHz slice of spectrum. From the moment a pilot calls for an instrument clearance on the ground to the moment they check in with the tower on final approach, every transmission travels through the 118–137 MHz VHF communications band. It is the voice of global aviation — a remarkably narrow channel for a remarkably large task.
Understanding this band in detail means understanding not just which frequencies carry what traffic, but why the system is built the way it is, how it has been stretched to serve an air transport network vastly larger than its designers anticipated, and what pressures are now accumulating that will eventually force a fundamental change.
The Boundary That Defines the Band
The 118 MHz lower boundary is not arbitrary. It was chosen to sit immediately above the radionavigation sub-band (108–117.975 MHz), where VOR and ILS signals occupy 200 narrowband channels. Navigation and voice communications required different signal structures and different receiver designs, so a clean boundary was established between them. Aviators and engineers conventionally refer to the 108–137 MHz range as the “airband,” but when pilots speak of their comms radio — the one they use to talk — they mean 118–137 MHz.
The upper boundary at 137 MHz similarly reflects a deliberate allocation decision. Immediately above it, 137–138 MHz hosts satellite downlinks for meteorological and mobile satellite services. Above that, 138–144 MHz is reserved in the US for exclusive federal government and military use. The 118–137 MHz band is therefore bounded on both sides by allocations that constrain any expansion. This compression has been a defining engineering and regulatory challenge for decades.
Amplitude Modulation: The Deliberate Choice
The entire 118–137 MHz communications band uses amplitude modulation (AM). This surprises many people encountering the band for the first time, because the rest of the VHF spectrum — FM broadcast, marine radio, public safety, land mobile — uses frequency modulation, which delivers cleaner audio and better noise rejection. The aviation band’s continued use of AM is not technological conservatism. It is a safety decision made and remade with full understanding of the alternatives.
The critical issue is simultaneous transmission. In any active airspace, multiple aircraft share a single frequency. A tower controller may be assigned one frequency for all aircraft using the runway. Multiple aircraft, calling at overlapping moments, will occasionally transmit simultaneously. What happens at the receiver when two AM signals arrive at the same time depends on their relative power: the stronger signal partially suppresses the weaker one, but both are partially audible. The receiver produces a garbled, distorted mix — clearly recognisable as simultaneous transmission, audible enough that the controller knows someone tried to call, and usually intelligible enough that a partial clearance or alert can be reconstructed. Most importantly, both pilots and controllers recognise the characteristic squeal or scratchy quality of a stepped-on transmission as a signal to wait and re-transmit.
FM behaves differently. FM demodulators exhibit a phenomenon called the capture effect: when two FM signals arrive simultaneously on the same carrier frequency, the stronger one fully suppresses the weaker. The weaker signal is simply not heard. In an aviation context, this is dangerous. A controller issuing a critical instruction on FM could be entirely blocked by a nearby aircraft’s stronger transmission, with neither the controller nor other aircraft aware that any suppression occurred. The weaker transmission has not just been degraded — it has disappeared without a trace. AM’s partial degradation is visible and audible; FM’s capture is silent and invisible. For air traffic control, the failure mode matters as much as the nominal performance.
The modulation standard in the airband is specifically A3E — double-sideband, full carrier AM. This is the same modulation type used in medium-wave broadcast radio. It is spectrally inefficient compared to single-sideband (used in HF aviation communications) or digital modulation, but it is demodulated correctly by virtually any AM receiver without synchronisation, its failure modes are predictable, and its degradation under interference is audible rather than silent.
Channel Architecture: 760 Channels and Counting
The 118–137 MHz communications band contains 760 channels at 25 kHz spacing — a plan that reached its current configuration in 1990 when the 136–137 MHz segment was added to the existing 118–136 MHz block. The evolution from the band’s origins illustrates just how tightly the channel plan has been managed.
VHF aviation communications began with 200 kHz channel spacing in the immediate post-World War II period, providing 70 usable voice channels from 118 to 132 MHz. As air traffic grew through the 1950s, channel spacing was halved to 100 kHz, then halved again to 50 kHz, extending the upper limit to 135.95 MHz and providing 360 channels. The shift to 25 kHz spacing in 1972 brought the count to 720 channels. The 136–136.975 MHz extension in 1990 added the final 40 channels to reach the current 760.
Each step required retrofitting or replacing equipment across thousands of aircraft and hundreds of ground stations simultaneously — a coordinated international effort managed through ICAO. The retrofit cycle for 25 kHz radios, still ongoing at US airports into the late 1980s, gave regulators a detailed picture of how difficult it is to change the channel plan once it is established. That difficulty explains the conservatism surrounding further changes.
In European and some other airspace, a third halving of channel spacing — to 8.33 kHz — has tripled the theoretical channel count to approximately 2,280 within the same 19 MHz. The implementation began in 1999 for flights above FL245 in European core airspace and has been progressively extended. Since 2007, all IFR aircraft flying above FL195 in the ICAO European region have been required to carry 8.33 kHz-capable radios, with lower altitudes following through subsequent mandates.
The 8.33 kHz system introduces a detail worth noting for anyone designing or testing equipment: the channel designator and the tuned frequency are not the same. The 8.33 kHz plan uses a six-digit numbering scheme derived from the old 25 kHz grid. A channel labelled “118.010” actually tunes to 118.008333 MHz — offset by one-third of 25 kHz. This is not an error; it is a designed feature that allows the 8.33 kHz channel plan to coexist with the legacy 25 kHz plan in mixed-equipage airspace. Equipment that correctly implements the standard handles the offset transparently, but any receiver or spectrum analyser monitoring this band without awareness of the offset will see channels at unexpected frequencies.
The Functional Organisation of the Band
Within 118–137 MHz, traffic is organised by function, not by sub-band. Unlike spectrum where different frequency ranges serve entirely different services, the aviation communications band assigns specific channels to specific functions based on local and regional planning — a tower at one airport might operate on 118.3 MHz while a tower at another airport uses 133.9 MHz. The frequency assignment is a product of coordination rather than allocation.
That said, certain frequency ranges within the band have conventional uses that provide a rough structural map.
118.000–121.950 MHz is primarily used for controlled airspace ATC services: clearance delivery, ground control, tower, approach and departure. These are the frequencies where controllers actively manage aircraft movements. Assignment within this range is dense at major airports and sparser at smaller facilities.
121.500 MHz is the international aeronautical emergency frequency — Guard. Its structure within the band is unique: there are no assigned channels between 121.400 and 121.600 MHz. The 100 kHz exclusion zone around 121.5 MHz exists because the frequency must be receivable without interference from adjacent-channel signals. No other frequency in the band receives this protection.
121.975–123.650 MHz hosts UNICOM, MULTICOM, flight service station frequencies, and Common Traffic Advisory Frequencies (CTAFs) used at non-towered airports. UNICOM is a licensed non-government advisory service — essentially a radio-equipped operator at a small airport who can provide wind, traffic, and runway information on request. Common UNICOM frequencies include 122.7, 122.8, 122.9, 123.0, and 123.05 MHz. At airports without towers, the UNICOM frequency typically serves as the CTAF, where pilots broadcast their position and intentions to coordinate with other uncontrolled traffic.
122.750 MHz is the designated national air-to-air frequency in the US, used for direct pilot-to-pilot communication that is not appropriate on a controlled frequency. It is the aviation equivalent of a common-room channel — used for coordination between pilots who are not currently in contact with ATC, or for brief exchanges that have no place on a busy tower or approach frequency.
122.100 MHz is used in the US to contact Flight Service Stations (FSS) while monitoring a VOR frequency — a legacy arrangement from the era when pilots on cross-country flights would check in with FSSs for weather updates and flight following.
123.100 MHz is designated internationally for search and rescue, and 123.450 MHz is widely used informally as a pilot-to-pilot air-to-air frequency — sometimes called the “Unicom in the sky” — for en-route communications between pilots who recognise each other’s callsigns or who want to exchange trip reports.
123.675–128.800 MHz returns to standard ATC use: approach, departure, and en-route sector frequencies.
128.825–132.000 MHz is reserved for aeronautical operational control (AOC) — airline company communications. This is where flight crews contact their company dispatch centres, maintenance control, and operations teams. It is not ATC. The frequencies are managed by Aviation Spectrum Resources, Inc. (ASRI) in the US on behalf of airlines and operators. A flight crew calling their operations centre with a fuel state, maintenance squawk, or estimated arrival time does so somewhere in this range.
132.025–136.475 MHz is a second block of standard ATC frequencies, used for en-route centres and approach control sectors.
136.500–136.975 MHz is a second block of airline operational control frequencies, again managed by ASRI. The boundary between ATC and airline ops frequencies in this range reflects both historic planning and current operational practice — en-route controllers hand off aircraft to airline operations while simultaneously handling ATC on adjacent frequencies.
136.975 MHz is the worldwide VDL Mode 2 Common Signalling Channel — the single most important digital datalink frequency in the airband, used by aircraft to establish initial contact with ground stations for ACARS-over-VDL and CPDLC links.
The Guard Frequency in Practice
121.5 MHz occupies a position in the airband that no other frequency does. Every ATC facility with VHF equipment is required to monitor it continuously during hours of service. Every aircraft equipped with more than one VHF radio is encouraged — and in many national regulations, expected — to keep a second receiver tuned to 121.5 MHz whenever the primary radio is on an assigned ATC or operational frequency.
The operational nickname “Guard” reflects the concept of a dedicated monitoring watch — an always-open receiver that catches distress traffic regardless of what the primary radio is doing. Airlines vary in how strictly they enforce the practice: some mandate continuous guard monitoring as company policy, others leave it to crew discretion, and operational reality means the volume on the guard receiver is sometimes turned low enough that it might be missed. Military operations treat guard monitoring more strictly, with the AN/ARC-210 and similar military radios providing a dedicated hardware guard receiver that monitors 121.5 MHz and its UHF equivalent (243.0 MHz) simultaneously with the primary channel, without requiring any crew action.
The practical uses of 121.5 MHz extend well beyond aircraft emergencies. ATC facilities broadcast on Guard to reach aircraft that have missed a frequency handoff — a common enough occurrence that controllers treat it as routine. A flight crew absorbed in a cruise checklist occasionally forgets to switch from one sector’s frequency to the next, finding themselves out of contact with the facility they were supposed to call. Controllers “call in the blind” on Guard knowing that any other aircraft within line-of-sight range, and likely also the errant aircraft itself, will hear the call. Pilots who hear their callsign on Guard know to check in immediately on the correct frequency.
The frequency is also used for interception procedures. An aircraft intercepted by military fighters in national airspace is instructed to contact the intercepting aircraft or the nearest ATC facility on 121.5 MHz. The universal awareness of the frequency across all aviation operators — civil, military, and government — makes it the logical channel for urgent communications that cross organisational boundaries.
ACARS and the Data Layer
The 118–137 MHz band carries not just voice but a persistent digital data layer that has become critical to airline operations. ACARS — Aircraft Communications Addressing and Reporting System — has operated in the band since the 1970s, initially as a simple text messaging system that automated routine messages between aircraft and airline ground systems.
The ACARS physical layer uses subcarrier FSK modulation at 2.4 kbps, transmitted on standard 25 kHz channels within the airband. In North America, primary ACARS frequencies cluster around 129.125 MHz and 130–131 MHz. The system is managed by two competing ground networks, ARINC (now Collins Aerospace) and SITA, whose receiver stations provide overlapping national and international coverage. An aircraft anywhere above low altitude over a populated landmass is almost always within range of an ACARS ground station.
ACARS messages carry automatic position reports, departure and arrival times (the OOOI sequence — Out, Off, On, In, denoting pushback, takeoff, landing, and gate arrival), fuel loads, engine performance data, maintenance alerts, weather updates, and free-text messages between flight crews and company operations. An airline operations centre can track every aircraft in its fleet in near real-time through ACARS without voice contact, and can exchange detailed operational instructions that would be impractical to read over a busy ATC frequency.
VDL Mode 2 carries the same content but at 31.5 kbps on 25 kHz channels using D8PSK modulation — roughly thirteen times the throughput of analogue ACARS. More importantly, VDL Mode 2 carries Controller Pilot Data Link Communications (CPDLC): structured text exchanges between controllers and pilots that allow routine clearances — altitude changes, speed instructions, route modifications — to be issued and acknowledged digitally rather than spoken. A controller managing a high-density sector can issue a CPDLC clearance to one aircraft while speaking to another, the digital channel providing a parallel communications path that does not require the primary voice frequency. CPDLC reduces both frequency congestion and the risk of readback errors that can arise when pilots mishear a spoken instruction.
The Common Signalling Channel at 136.975 MHz is used by aircraft to find and connect to VDL Mode 2 ground stations. Once a connection is established, the session may transfer to a different working channel, but 136.975 MHz functions as the universal entry point that makes the system interoperable across different ground network operators and geographic regions.
Propagation Geometry and Frequency Reuse
The 118–137 MHz band propagates as line-of-sight VHF: signals travel in straight lines, attenuated by the curvature of the Earth and blocked by terrain, with a radio horizon slightly extended beyond the geometric horizon by atmospheric refraction. The range of an airband communication link is determined primarily by the altitude of the lower of the two stations. An aircraft at cruise altitude (35,000 ft) has a radio horizon of approximately 230 nautical miles. The same aircraft at 5,000 ft has a horizon of about 87 nautical miles.
This altitude-dependent range has a critical implication that shapes how the band is managed. A frequency assigned to a tower at a single airport becomes usable again at sufficient geographic distance, because the coverage area of any VHF transmission is physically bounded. ICAO regional frequency coordination groups maintain continental and international frequency plans that exploit this spatial reuse — the same 25 kHz channel can serve multiple facilities across a continent, provided their respective coverage areas do not overlap at the altitudes where both are active.
The practical range of air-to-ground communications at cruise altitude is typically 150–200 nautical miles from a reasonably sited ground antenna. At lower altitudes, particularly in mountainous terrain where VHF propagation is obstructed by ridgelines, coverage gaps exist and can require aircraft to climb before contact with an ATC facility is possible. Remote and oceanic airspace where VHF ground stations are absent is served by HF radio (using ionospheric propagation) or, increasingly, satellite datalinks — but within line-of-sight range of a suitable ground station, VHF remains the primary voice and data medium.
Tropospheric ducting, caused by temperature inversions in the lower atmosphere, occasionally extends VHF propagation far beyond the normal radio horizon. During ducting events, a frequency assigned to a facility on one side of a continent can be received, at normal signal strength, by ground stations or aircraft on the other side. This creates interference between normally separated assignments that the frequency plan assumed could not see each other. Ducting events are weather-dependent and transient, but they are a known source of interference reports that frequency managers must account for.
The Congestion Problem and What Comes Next
The 760-channel airband plan that reached its final form in 1990 was designed for the traffic levels of its era. The European airspace, in particular, has outgrown it. Air traffic control centres in dense airspace open new sectors as traffic increases — each new sector requires a new frequency, and the supply is finite. The 8.33 kHz channel subdivision has temporarily eased European capacity pressure, but the projection is unambiguous: at current traffic growth rates, even the 2,280-channel 8.33 kHz plan will reach saturation in European core airspace within the coming decade. ICAO and EUROCONTROL estimate VDL Mode 2, the digital channel, is approaching capacity crunch by 2028–2030.
The response being developed is a system called LDACS — L-band Digital Aeronautical Communications System. LDACS operates in the aeronautical L-band (960–1164 MHz) rather than VHF, using OFDM modulation to deliver data throughput of 500 kbps to 2.6 Mbps — nearly 200 times faster than VDL Mode 2. It is designed to carry both ATC data and airline operational communications, supporting CPDLC and advanced trajectory-based operations that future air traffic management concepts require. Rohde & Schwarz has been the primary industrial developer, working with DLR (the German Aerospace Centre) and DFS (the German air navigation service provider) in a research programme funded through the German LuFo aviation research initiative. Product development was set to begin in 2025, with market launch targeted for 2028.
LDACS does not replace voice communication on the VHF airband. The 118–137 MHz band’s voice architecture, built around AM and a globally deployed equipment base of millions of radios in aircraft and thousands of ground stations, cannot be switched off on any schedule relevant to current planning. The transition path is that LDACS takes over the data load — CPDLC, flight management data exchange, trajectory optimisation — freeing VHF voice capacity for the human communications that remain in voice format. VHF voice and LDACS data are expected to coexist for decades, with the balance shifting toward data as LDACS ground infrastructure is built out.
What the Band Demands from RF Equipment
The 118–137 MHz communications band imposes specific requirements on any RF system designed to operate in or near it.
The 19 MHz span of the band sits immediately above the FM broadcast band (87.5–108 MHz), which terminates at the lower boundary of the VOR/ILS navigation sub-band. In a dense urban environment, FM broadcast transmitters may run effective radiated powers of tens of kilowatts from nearby tower sites. A receiver or preamplifier covering 118–137 MHz must reject FM broadcast energy that may be 40–60 dB stronger than the weakest in-band signals the receiver needs to detect. The transition between the FM broadcast band’s upper edge and the bottom of the airband at 118 MHz is the sharpest filtering challenge in this frequency region: the FM band ends at 108 MHz, the ILS/VOR navigation band begins there, and the communications band starts at 118 MHz.
Simultaneously, Digital Audio Broadcasting (DAB) in Band III (174–240 MHz) in regions where it operates, and television broadcast in Band III in others, contributes out-of-band energy from above. Digital television transition has generally reduced the power levels above the airband compared to the legacy analogue TV era, but in areas with nearby DAB transmitters the energy at 174 MHz is significant for a wideband monitoring receiver covering the full 108–137 MHz or 108–174 MHz range.
Within the band itself, the coexistence of narrowband AM voice channels, ACARS FSK bursts, and VDL Mode 2 D8PSK signals on different channels simultaneously requires a receiver front end with sufficient dynamic range to handle a full-power local ATC ground station on one channel without desensitising reception of weak aircraft signals on an adjacent channel. Airport areas present the worst case: a co-located ATC ground transmitter may operate at 50 watts or more while the system simultaneously tries to receive an aircraft at a marginal signal level.
For filter designers, the airband creates a clear product specification: a bandpass filter from 118 to 137 MHz (or 108 to 137 MHz to include navigation) with steep rejection below 108 MHz, adequate low-pass rolloff above 137 MHz, and low insertion loss in the passband to preserve signal-to-noise performance for weak signals. The FM broadcast rejection requirement — typically needing 60 dB or more of attenuation at 100 MHz in demanding urban installations — drives the filter toward higher order designs with steep skirts, while the narrow guard-frequency exclusion zone at 121.5 MHz argues for a flat, consistent passband response across the full communications sub-band.
A System Built on Global Trust
The 118–137 MHz aviation communications band functions because of something that rarely gets stated explicitly: every pilot and every controller in the world, operating under vastly different national regulations, languages, and procedures, agrees to use the same frequencies in the same way. The ICAO standards that define the channel plan, the modulation type, the emergency frequency, and the operational procedures are not enforced by any single government. They are adopted voluntarily by over 190 member states because every party benefits from the interoperability that results.
An aircraft diverted to an unfamiliar country’s airport can still contact the tower on a standard VHF frequency using standard phraseology and expect to be understood. A search and rescue coordinator can reach a distressed aircraft on 121.5 MHz without knowing its nationality, its airline, or its registration. A controller handing a flight to a neighbouring country’s airspace can do so on a standardised frequency with a standardised phraseology. This interoperability is not accidental. It is the product of decades of ICAO coordination, committee work, and national implementation — and the 19 MHz of spectrum that hosts it is the physical medium on which global aviation’s most fundamental communications run.