Radio signals do not exist in isolation. Every transmitter shares the spectrum with hundreds of others, and maintaining clear, interference-free reception requires both careful frequency planning and ongoing management. Understanding radio frequency interference, often abbreviated as RFI, helps explain both the regulatory structure of broadcasting and many of the reception problems listeners experience.
Types of Radio Frequency Interference
Interference comes in several forms. Co-channel interference occurs when two transmitters on the same frequency are both received at a listening location. Adjacent channel interference happens when a strong station’s signal spills into the frequencies immediately above and below it. Intermodulation distortion arises when two strong signals combine in a receiver or transmitter to produce spurious new frequencies. Electrical interference from man-made equipment — motors, switch-mode power supplies, LED lighting — can also affect reception at lower frequencies.
Frequency Planning and Coordination
National broadcast regulators assign frequencies to stations with interference in mind. For FM, adjacent channels are deliberately avoided in the same geographic area to prevent co-channel problems. For AM, medium-wave frequencies across Europe are coordinated under the auspices of the International Telecommunication Union to ensure that powerful stations do not cause unacceptable interference to each other across borders.
- The frequency spacing between FM stations in the same region is typically 200 kHz or more.
- Transmitter power is regulated to limit the interference radius from each station.
- Directional antenna arrays can shape transmission patterns to reduce interference in specific directions.
The Capture Effect in FM Reception
FM receivers have a useful property called the capture effect. When two signals arrive on the same frequency, the stronger one suppresses the weaker one rather than mixing with it. This means a relatively small difference in signal strength is enough for a listener to receive one station cleanly even in the presence of another on the same channel. This makes FM considerably more resilient to co-channel interference than AM.
Electrical Interference and its Sources
The proliferation of electronic equipment has increased the electrical noise floor in many environments. LED lighting, in particular, has been found to generate wideband interference that can affect AM and even FM reception. Some poorly designed switched-mode power supplies radiate noise across a wide range of frequencies. Broadcasting regulators set limits on emissions from electronic products specifically to protect radio reception.
DAB and Interference Immunity
DAB uses a modulation technique called COFDM, which spreads the digital signal across hundreds of closely spaced sub-carriers. This design gives it inherent resilience against many types of interference, including multipath reflections from buildings. However, DAB is not immune to interference; a sufficiently strong interfering signal can still degrade or break up reception, particularly at the edge of a coverage area.