Interference, EMC and safety
Interference, EMC and safety tests practical operating responsibility. You must identify sources of interference, apply suppression at the right point, and protect people from RF and electrical hazards. Greek licence conditions require you to operate without causing harmful interference and to comply with exposure limits.
Causes of interference
Radiated interference comes from harmonics, spurious emissions, poor filters and antennas coupling into nearby wiring. Conducted interference travels along mains leads and coax shields. Common-mode current on a feeder often causes TVI or audio breakthrough. Intermodulation in a nearby receiver or in your own transmitter can create signals on unrelated frequencies.
Suppression and EMC
Suppress at the source first: low-pass filters on transmitter output, ferrite chokes on feed lines, shielded enclosures, and good bonding. For received interference, common-mode chokes and ferrites on mains and signal cables help; differential-mode filters handle power-line noise. Do not use ferrites as a substitute for a proper antenna balun. Keep transmitting antennas away from domestic wiring and use the minimum power needed.
RF exposure
RF exposure depends on frequency, power, distance and duty cycle. Power density from an isotropic radiator is S = EIRP/(4πd²). Compare the result with the applicable ICNIRP reference level. At 144 MHz the general-public reference level is 2 W/m²; at 100 W EIRP and 10 m, S = 100/(4π×100) = 0.0796 W/m², well below the limit. Antenna gain, near-field effects and duty cycle still matter.
Electrical safety
Treat mains as lethal: isolate before working, use RCD protection, and do not rely on a switch. Ground antenna masts and coax shields properly, but never create a ground loop that carries fault current. High-voltage valve equipment and large batteries need discharge and insulation checks. RF burns occur at antenna terminals and open feeders; never touch a live antenna.
Traps that cost marks
- Suppressing the victim’s equipment instead of the source, or fitting ferrites without identifying common-mode current.
- Assuming low power is automatically safe; proximity and antenna gain change exposure.
- Using an earth connection as a neutral or relying on a water pipe as an RF ground.
- Ignoring duty cycle: SSB and CW exposure is lower than continuous FM at the same PEP.
- Forgetting that interference can enter through the mains, not only the antenna.