Every allocation boundary tells a story. The one at 400 MHz is particularly instructive. Below it lies the upper portion of the most consequential block of military radio spectrum in the world. Above it, the allocation landscape fragments almost immediately into meteorological satellites, navigation systems, and civilian services — a hard transition that reflects decades of deliberate international coordination. Understanding the 318 to 400 MHz band means understanding how the top tier of the NATO military aviation block was built, who uses it, and what sits waiting on the other side.
The band in context
The 318 to 400 MHz range completes the 225 to 400 MHz military aviation block that has been reserved for defence use since the post-World War II reorganisation of radio spectrum. The previous two articles in this series covered the 243 to 270 MHz and 292 to 318 MHz sub-bands; this upper segment is the most heavily used portion of the three, carrying the densest concentration of tactical voice channels, the most operationally critical SATCOM infrastructure, and the most active telemetry traffic.
Wavelengths across this range run from roughly 75 to 94 centimetres — well into sub-metre territory, and well-suited to blade antennas on fast jets, shipboard whip antennas, and vehicle-mounted hardware that must survive the operational environment without the fragility of a dish or directional array. The US Navy has used the military UHF band spanning 300 to 400 MHz for satellite communications since the launch of the first Fleet Satellite Communications satellite in 1978. That four-decade legacy of infrastructure shapes everything about how the upper portion of this band is allocated and contested today.
Tactical voice and air traffic control
Military agencies, National Guard units, the Coast Guard, and the FAA are the heaviest users of the 335 to 399.9 MHz portion of the band, using it for critical air-to-ground and ground-to-air communications systems including training flights and air traffic control functions such as ground control and approach control. This is the workhorse segment of the entire 225 to 400 MHz block — the range where the majority of day-to-day military aviation voice traffic actually flows.
Channel spacing is 25 kHz with AM modulation, consistent with the rest of the military aviation block. The AN/ARC-182 radio, used across 35 countries, covers the full 225 to 400 MHz military UHF range and supports HAVE QUICK II anti-jam waveforms alongside simultaneous guard channel monitoring on 243 MHz. Its successor, the AN/ARC-210, extends coverage further still while maintaining backward compatibility with the existing 225 to 400 MHz infrastructure. These radios are the physical embodiment of NATO interoperability in the air — a pilot from any allied nation can reach any other using the same channel conventions across the entire block.
The US Navy and Coast Guard also use this segment for ship-to-air-to-ship and ship-to-ship maritime communications, for both clear and secure voice, as well as secure data links. The Navy additionally uses the band for flight testing of military aircraft and for telemetry. That last use — telemetry — is significant. Test ranges rely on this band to downlink performance data from experimental aircraft and weapons systems in real time, a requirement that demands low-latency, wideband links across line-of-sight distances that can exceed 100 kilometres.
MUOS: the satellite layer that changed the band
The most significant infrastructure development in the upper military UHF band over the past two decades is the Mobile User Objective System. MUOS is a network of orbiting satellites and relay ground stations operated by the US Space Force and Navy, providing secure beyond-line-of-sight UHF communications for mobile forces across more than 67,000 military user terminals globally.
MUOS transmits five wideband CDMA channels between 360 and 380 MHz from five geosynchronous satellites. This places the primary MUOS downlink segment squarely within the upper portion of the band covered by this article. MUOS uses wideband code-division multiple-access technology — the same underlying approach as 3G commercial cellular networks — and enables communications to handheld terminals, laptops, and personal communications units, providing more than ten times the system capacity of the legacy UHF SATCOM architecture it replaces.
The legacy system MUOS is replacing — the UFO (UHF Follow-On) constellation — used narrow 5 kHz channels spread across the 225 to 400 MHz range. The transition from that narrowband architecture to MUOS’s wideband WCDMA channels represents a fundamental shift in how military satellite communications work within the same band boundaries that have existed for half a century.
HAVE QUICK across the upper block
Since the end of World War II, US and Allied military aircraft have used AM radios in the NATO-harmonised 225 to 400 MHz band for short-range air-to-air and ground-to-air communications, with the HAVE QUICK frequency-hopping system developed to protect these communications against jamming and interception. The upper portion of the block, from 318 to 400 MHz, carries the densest concentration of HAVE QUICK nets, precisely because it hosts the most tactical voice channels. The hopping pattern spans the full military aviation block, but the majority of channel dwells occur in this upper tier where voice traffic is heaviest.
The adjacent bands
Below 318 MHz, the 292 to 318 MHz sub-band — covered in the first article in this series — carries military tactical voice, additional SATCOM uplinks, and radar-adjacent services. The character of the spectrum is continuous across this lower boundary; the distinction between the sub-bands is largely an engineering convenience rather than a sharp allocation change.
Above 400 MHz, the transition is sharp and consequential. The 399.9 to 403 MHz range carries navigation, positioning, time and frequency standard, mobile communications, and meteorological satellites — with the 400 MHz region serving as a companion downlink band for satellites that transmit uplinks near 150 MHz. This dual-frequency pairing exploits the relationship between the two bands to support Doppler-based positioning systems, a technique predating GPS that is still used in some oceanographic and scientific satellite systems.
Immediately above, the 400.15 to 401 MHz segment carries meteorological satellite downlinks, radiosonde transmissions, mobile satellite services, and space research allocations. Radiosondes — the sensor packages carried aloft by weather balloons — transmit atmospheric pressure, temperature, and humidity data on these frequencies continuously from hundreds of launch sites worldwide. The proximity of this civilian scientific infrastructure to the top of the military aviation block makes the 400 MHz boundary one of the more carefully managed transition zones in the UHF spectrum.
Further above, the 406 to 406.1 MHz band carries Cospas-Sarsat distress beacon signals from personal locator beacons and EPIRBs — the satellite-processed emergency alerting system that replaced 121.5 MHz and 243.0 MHz beacon monitoring in 2009. And above that, the 420 to 450 MHz amateur radio allocation opens the 70-centimetre band to licensed ham operators — the first significant civilian amateur allocation since the spectrum below 225 MHz.
The end of a reservation
The 400 MHz boundary matters beyond the specific allocations that sit on either side of it. It marks the upper edge of the largest contiguous block of government-primary military spectrum in the UHF range — 175 MHz of bandwidth held essentially intact across NATO and allied nations for the better part of a century. Spectrum reviews in the US, Europe, and elsewhere have periodically examined whether portions of the 225 to 400 MHz block could be shared or reallocated, particularly given the growth of commercial pressure on UHF frequencies generally.
The answer has consistently been no, and the MUOS investment illustrates why. When a single satellite communications architecture serving 67,000 terminals operates within a defined block, relocating even a portion of that block would require either replacing all terminal hardware or operating a transition period during which two incompatible systems share the same physical infrastructure. The military aviation block will remain intact for the foreseeable future — and the 318 to 400 MHz upper tier will remain its most active and most closely guarded segment.
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The 292 to 318 MHz band: a quiet stretch of spectrum with serious neighbours