There are bands in the UHF spectrum that carry a single dominant function — a military block, a broadcast allocation, a cellular reservation. The 406 to 430 MHz range is not one of them. Across just 24 MHz of spectrum, this band carries one of the most protected allocations in radio regulation, a major federal government land mobile block, a secondary radio astronomy allocation, and high-powered space surveillance radar. Understanding it requires treating each sub-range almost independently, because the services sharing this space have almost nothing in common except their frequency coordinates.
406.0 to 406.1 MHz: the most protected 100 kHz in the UHF band
The 406.0 to 406.1 MHz segment is reserved internationally for Cospas-Sarsat distress beacons — the satellite-processed emergency alerting system used by personal locator beacons (PLBs), emergency position-indicating radio beacons (EPIRBs) for maritime use, and emergency locator transmitters (ELTs) for aviation. There are three types of 406 MHz beacons: ELTs used by the aviation community, EPIRBs used in the maritime environment, and PLBs used by individuals in wilderness activities. When activated, all three transmit a distress signal on the 406 MHz frequency, which has been designated internationally for distress use only.
The technical specification for these beacons is precise to a degree unusual even in tightly regulated spectrum. The carrier frequency of beacons operating in the primary 406.025 MHz channel must be set within 2 kHz of that centre frequency and must not drift more than 5 kHz over a five-year period. That stability requirement exists because Cospas-Sarsat satellites use Doppler shift to calculate the position of an activated beacon — a frequency drift of more than a few kilohertz degrades the position solution. Each beacon also carries a unique 15-character hexadecimal identifier embedded in its digital message, allowing search and rescue authorities to identify the registered owner, associated vessel or aircraft, and emergency contacts before a rescue asset is even despatched.
The 100 kHz width of this allocation is deliberately narrow. Keeping the band small concentrates the satellite receiver’s sensitivity on a well-defined window and reduces the risk of interference from adjacent services. Regulatory proposals have sought to prohibit new frequency assignments in the 405.9 to 406.0 MHz and 406.1 to 406.2 MHz guard zones, and require that radiosonde operators select frequencies above 405 MHz that account for transmitter frequency drift to avoid encroaching on the protected 406 to 406.1 MHz window. The protection extends both below and above the allocation — an acknowledgement that even low-power meteorological transmitters represent a threat to the reception of distress signals when the receiver is designed for signals of this sensitivity.
406.1 to 420 MHz: federal land mobile with a radio astronomy constraint
Immediately above the distress beacon allocation, the 406.1 to 420 MHz band is one of the major UHF land mobile allocations in the United States, serving federal agency systems alongside the VHF low band, VHF high band, and the 450 to 512 MHz UHF block. Federal users in this range include the US Coast Guard, Department of the Army, Department of Commerce, Department of the Interior, Federal Aviation Administration, Department of Homeland Security, Department of Energy, and Department of Agriculture. The breadth of that list reflects the band’s role as a shared federal narrowband resource rather than a dedicated single-agency allocation — individual agencies coordinate frequency assignments within the block through the NTIA Government Master File.
The 406.1 to 410 MHz sub-segment carries an additional constraint that makes it unusual among land mobile bands: a secondary radio astronomy allocation. Fixed and mobile operations in the 406.1 to 410 MHz band are subject to local coordination to ensure that harmful interference is not caused to radio astronomy services, with prior coordination required near observatories including the National Radio Astronomy Observatory at Socorro, New Mexico, and the former Arecibo Observatory in Puerto Rico. Transmitter output power in the 406.1 to 410 MHz band is limited to 125 watts, and non-Federal use is currently restricted to four designated Hydro channels at 406.125 MHz, 406.175 MHz, 409.675 MHz, and 409.725 MHz. The Hydro channel designation reflects historic use by utilities and hydroelectric operators — one of the more specialised civilian land mobile sub-categories.
The 410 to 420 MHz portion of the federal land mobile block also carries a space research allocation in the space-to-space direction — meaning communications between orbiting spacecraft, not between ground and space. The space research service in this band must not claim protection from, nor constrain the use and development of, stations of the fixed and mobile services. In practice, this allocation supports extravehicular activity communications from the International Space Station and similar manned space operations.
420 to 430 MHz: radiolocation and the limits of amateur access
The 420 to 430 MHz segment carries a radiolocation allocation alongside fixed and mobile services. In the United States, this range is used for high-powered government radars engaged in critical national defence operations. The 420 to 450 MHz bands are used in some countries for high-powered radars that detect and track earth-orbiting satellites and space debris, and these radars also aid in identifying potential space debris hazards that could damage the International Space Station.
The amateur radio service has a footnote allocation in the 420 to 450 MHz range in the United States and some other ITU Region 2 countries, but it comes with a significant geographic restriction. In the 420 to 430 MHz band, the amateur service is not allocated north of Line A — the regulatory boundary running roughly along the US-Canada border — and amateur operations are excluded from areas within 160 kilometres of specific military radar sites including Clear, Alaska; Concrete, North Dakota; and Otis Air Force Base, Massachusetts. Those exclusion zones are not arbitrary — they protect the radar receivers from interference that would degrade their space surveillance mission.
Adjacent bands and the transition to 70cm
Below 406 MHz, the previous article in this series covered the 400 to 406 MHz MetSat and MetAids band — radiosondes, the Argos data collection system, and the meteorological satellite downlink segment. The 406 to 406.1 MHz Cospas-Sarsat allocation sits at the precise boundary between that meteorological infrastructure and the land mobile and radiolocation uses that follow.
Above 430 MHz, the 70-centimetre amateur band begins at 430 MHz in most ITU regions and extends to 440 MHz. This is one of the most active UHF amateur allocations globally, supporting FM voice repeaters, digital modes, amateur satellite operations, and weak-signal work. The 420 to 430 MHz radiolocation allocation effectively functions as a buffer zone between the federal government radar infrastructure and the amateur band, with coordination requirements that prevent amateur operations from encroaching on sensitive radar receivers in proximity-restricted areas.
A band defined by what it protects
The 406 to 430 MHz range is as much a story about protection as allocation. The 100 kHz Cospas-Sarsat window is protected from radiosonde drift below and guard bands above. The radio astronomy allocation constrains transmitter power in the federal land mobile block for dozens of megahertz around it. The radiolocation radars exclude amateur operations across entire geographic regions. Every sub-band here exists in a negotiated relationship with its neighbours — a reflection of how densely the UHF spectrum between the end of the military aviation block and the start of the 70-centimetre amateur band has been divided among services that have almost no tolerance for interference with one another.