Electromagnetic Compatibility, EMC and RF Exposure Safety
EMC principles
EMC has two sides: emission (what your station radiates or conducts) and immunity (how equipment resists it). Interference paths are conducted (mains, cables) and radiated (free space). Mitigation includes filtering at the source, shielding, bonding, cable routing and separation. Know that a resonant antenna near wiring couples energy into it, and that common-mode chokes on feedlines are highly effective. Traps include assuming a shielded cable alone solves interference when the shield is not properly bonded.
Exposure safety
RF exposure limits are expressed as power density (W/m² or mW/cm²), electric field strength (V/m) and specific absorption rate. Limits vary with frequency and are stricter for the general public than for controlled environments. Near-field versus far-field matters: the far field begins roughly at 2D²/λ for an antenna of dimension D. Average power over a defined time (often six minutes) is used. Know that exposure falls with distance and that antennas should not be touched while transmitting.
Practical measures
Assess before transmitting, restrict access, use time averaging, reduce power, increase distance and orient antennas away from people. Keep high-voltage supplies safe: discharge capacitors and observe lockout procedures.
Worked example
If power density at a point is 10 W/m² and the limit is 10 W/m², you are at the limit; doubling distance reduces power density by about four in the far field, giving 2.5 W/m².
Radio Regulations, Licence Conditions, Operating Procedures and Frameworks
Regulatory framework
Know the roles: the International Telecommunication Union coordinates spectrum internationally through the Radio Regulations; national administrations license amateurs and set conditions. The three ITU regions matter for band plans. Amateur allocations are secondary in many bands, meaning amateurs must not cause harmful interference to primary services and must accept interference from them. Traps include assuming an amateur allocation is primary and confusing national licence conditions with international recommendations.
Licence conditions
Licence classes define power limits, permitted bands and modes. Callsigns identify the station and licence class. Third-party traffic, international operation, reciprocal arrangements and maritime/emergency communication rules are commonly examined. Know the meaning of CEPT permits, the prohibition on commercial traffic, and the requirement to identify at intervals and at the end of contact.
Operating procedures
Use correct phonetics, Q codes, RST reporting, band plans, calling frequencies and split operation. Avoid interference, listen before transmitting, and keep a station log where required. Emergency communications follow declared procedures.
Worked example
If a band is allocated to amateurs on a secondary basis, an amateur receiving interference from a primary user must accept it and must not claim protection; reducing power or changing frequency is the correct response.