Meaning
Restricted frequency bands define a regulatory framework where intentional radio transmissions are prohibited to protect sensitive services from harmful interference. Federal Communications Commission part 15.205 lists specific spectrum windows including aeronautical and emergency radio services where only spurious emissions are permitted. These limits require that any radiated energy from a device falling within these protected windows must remain below the field strength levels defined in section 15.209.
Compliance involves testing the full harmonic output of a radio module to ensure no leakage occurs within these fixed segments.
Restricted Requirements
Manufacturers must account for harmonics and subharmonics of the fundamental operating frequency during the certification process. A radio design often uses filtering or shielding to suppress emissions that land inside these prohibited areas. Precise measurement setups detect unintended energy at these sensitive junctions to verify that the device meets the required attenuation levels.
Certification bodies examine these test reports to confirm that the hardware does not compromise essential radio navigation or communication links.
Operational Boundaries
Signal levels are measured at a distance of three meters from the radiator to determine compliance with the field strength limits. The regulation applies to all intentional radiators regardless of whether they operate in licensed or unlicensed spectrum. A device failing to maintain these emissions limits loses its authorization for legal sale or distribution in the market.
Each design modification necessitates a new evaluation of the emission profile to ensure that the hardware still adheres to these constraints.
Compliance Verification
Engineers conduct radiated emission tests in an anechoic chamber to isolate the device output from ambient signals. This process involves rotating the equipment and adjusting the antenna height to maximize the detection of spurious signals. The resulting data demonstrates the ability of the hardware to maintain signal integrity while avoiding interference in nearby protected frequency bands.
Strict adherence to these limits ensures that commercial radio hardware maintains coexistence with critical infrastructure systems.
