Electromagnetic Interferences (EMI)
Every radio receiver operates in an electromagnetic environment that contains not only the signals it is intended to receive, but also a multitude of unwanted electromagnetic emissions. These unwanted signals and disturbances can significantly affect the quality of radio communication. In electromagnetic compatibility (EMC), this phenomenon is generally referred to as electromagnetic interference (EMI) or radio frequency interference (RFI).
According to the ITU Radio Regulations (Article 1.166), interference is defined as the effect of unwanted energy, resulting from one or more emissions, radiations or inductions, on the reception of a radio communication system. It manifests itself through a degradation of performance, misinterpretation or loss of information that could otherwise be received.
A particularly severe form is known as *harmful interference*. According to the ITU, this includes interference that endangers radionavigation or safety services, or seriously degrades, obstructs or repeatedly interrupts authorised radio communications.
It is important to distinguish between accidental interference, which is a consequence of unwanted electromagnetic emissions, and intentional jamming, where a transmitter deliberately generates signals to prevent or degrade the reception of another radio system.
Where do electromagnetic disturbances originate?
Electromagnetic disturbances can originate from virtually any electrical or electronic equipment. Typical sources include:
- Switching power supplies, computers and digital electronics.
- Solar inverters and photovoltaic installations.
- Electric vehicles and high-power charging systems.
- Electric motors, frequency converters and industrial machinery.
- High-voltage power lines and switching equipment.
- Radio transmitters operating outside their intended frequency range or with unwanted emissions.
The physical mechanisms behind these disturbances are diverse. Rapid switching of electrical currents generates electromagnetic fields. For example, a switching power supply operating at 100 kHz produces periodic current variations that can generate electromagnetic emissions at the fundamental frequency and its harmonics:
fₙ = n · f₀
For a switching frequency of 100 kHz, harmonics occur at 200 kHz, 300 kHz, 400 kHz and so on. Depending on the switching waveform and its rise and fall times, significant spectral components can extend far into the radio frequency spectrum.
Disturbances reach a radio receiver through three principal coupling mechanisms:
1. Conducted interference: Unwanted currents and voltages travel along power supply cables, signal cables or other electrical connections.
2. Capacitive and inductive coupling: Electric and magnetic fields transfer unwanted energy between neighbouring conductors and circuits.
3. Radiated interference: Electromagnetic waves propagate through space and are received by antennas, cables or other structures.
The physical impact on radio reception
A radio receiver cannot distinguish between a wanted and an unwanted signal simply by looking at the electromagnetic energy arriving at its antenna. Both contribute to the total received signal.
The decisive factor is therefore the relationship between the wanted signal and the disturbance, commonly expressed as the signal-to-interference ratio (SIR):
SIR = Pₛ / Pᵢ
In logarithmic units:
SIR (dB) = Pₛ (dBm) − Pᵢ (dBm)
Consider a radio amateur receiving a weak signal with a power level of −100 dBm. If an unwanted signal reaches the receiver at −90 dBm, the interference is 10 dB stronger than the wanted signal. The resulting SIR is −10 dB. Depending on the modulation and receiver characteristics, reliable communication may become impossible.
Even much weaker disturbances can be significant. The thermal noise power density at room temperature (approximately 290 K) is:
N₀ = k · T ≈ 4.0 · 10⁻²¹ W/Hz
This corresponds to approximately −174 dBm/Hz.
For a receiver bandwidth of 2.4 kHz, the theoretical thermal noise power is:
N = −174 + 10 · log₁₀(2400) ≈ −140.2 dBm
A receiver with a noise figure of 6 dB therefore has an input-referred noise level of approximately −134.2 dBm.
This illustrates why even extremely weak unwanted emissions can affect the reception of weak radio signals, particularly in sensitive HF communication systems.
The electromagnetic environment and EMC
The electromagnetic environment is not restricted to industrial facilities or densely populated urban areas. It exists everywhere: in residential buildings, rural areas, industrial plants and around transportation infrastructure.
The purpose of electromagnetic compatibility is to ensure that electrical and electronic equipment can operate satisfactorily in this shared environment without generating intolerable disturbances and without being excessively sensitive to external interference.
Two fundamental requirements therefore apply:
- Emission: Equipment must limit the electromagnetic disturbances it generates.
- Immunity:Equipment must maintain its intended functionality when exposed to electromagnetic disturbances.
Ultimately, EMC is about maintaining a sufficient margin between unwanted emissions and the immunity of the affected system.
For radio amateurs, this is particularly important because their receivers are often designed to detect signals close to the physical noise limit. Understanding the origin, propagation and measurement of electromagnetic disturbances is therefore essential for understanding the practical limitations of radio communication.