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Q&A High frequency electromagnetic fields

28. February 2018

High frequency electromagnetic field immunity

 IEC 61000-4-3

Today we are surrounded by a wide variety of high frequency electromagnetic fields. Radio, television and mobile communication technologies transmit a multitude of high frequency signals. These fields are useful for modern communication, but can adversely affect other electronic devices. Conductor tracks and wires of the devices act as antennas and the coupled electromagnetic energy can, depending on the field strength and circuit, negatively alter the performance characteristics of a device or directly destroy components. For example, we all know the “crack” of radios, when a mobile phone is nearby. In case of power supplies, the negative impact of the electromagnetic field could express itself in form of a drop in the output voltage.
To prevent this from happening, the EMC standard IEC 61000-4-3 regulates tests regarding the immunity of devices against high frequency electromagnetic fields. This article describes the general regulations of the standard, gives an overview on electromagnetic fields with their effects and names some measures to increase the immunity of electronic devices.

What are high frequency electromagnetic fields

High frequency electromagnetic fields are located in the electromagnetic spectrum in the frequency range between 100 kilohertz (kHz) and 300 gigahertz (GHz). They are generally radiated from an antenna and can transmit energy and information over long distances. Due to the wide range of possible uses of high-frequency electromagnetic fields, especially for modern communication today, (e.g. radio, television, mobile communications, cordless cellphones, WLAN and Bluetooth applications) people are surrounded by a multitude of different transmission devices that operate with different transmission powers and frequencies. The frequency and wavelength of electromagnetic fields are linked by the propagation velocity (in free space this is the speed of light c) and describe the wave character of the fields. At high frequencies f, the wavelengths λ (lambda) are small and correspondingly larger at low frequencies. When propagated in free space, the wavelengths are between 3 kilometers and 1 millimeter.[1]

Table: Frequency bands and wavelengths
Unit of measurement of the electric field strength

The intensity or strength of the fields is indicated either in the form of the electric field strength (unit: volts per meter, V / m), or the magnetic field strength (unit: amps per meter, A / m), or in the form of power flux density (unit: watts per square meter, W / m2).[1]

Propagation of high-frequency electromagnetic fields

As the distance from a transmitter increases, the field strength decreases rapidly. In free space, the power flux density decreases with the square of the distance, which means, with the double distance the flux density decreases to a quarter. Because many antennas radiate with certain preferred directions due to their design, the intensity at locations in the vicinity of a transmitter can be very different, despite identical distances to the source. As a rule, it is not possible to deduce the field strengths at a particular location alone from the distance. High-frequency electromagnetic fields can also be reflected or absorbed by objects that are in the direction of propagation. Which mechanism predominates depends, among other things, on the material properties of the respective object. Therefore, the propagation of high-frequency fields in the real environment often differs significantly from the simple case given above; the propagation in free space.[1]

Effect on humans

Humans contain many electrically charged particles and polar molecules. Although polar molecules, such as the water molecule, are electrically neutral as a whole, they carry a negative charge at one end and a positive charge at the other. Electric and magnetic fields exert a force on electrically charged or polar particles to move. In a high-frequency electromagnetic field, the particles move very fast in time with the frequency. They rub together and heat is created. If the fields are very strong, entire cells can move due to the force effect. They align themselves in the field or migrate. Such non-thermal effects cannot be triggered by fields of radio applications, since their field strength is not sufficient for this.
Decisive for the biological effect of high-frequency fields is the energy absorbed by the body. The basis for this is the Specific Absorption Rate (SAR, unit of measure: Watts per kilogram, W / kg). It indicates the power (energy per time) absorbed per kilogram of tissue. If the body is only heated locally, the blood can dissipate the extra heat. If the whole body is heated, the skin is supplied with more blood and the heat is released by evaporation on the skin surface (sweating). Health effects can be expected if certain thresholds are exceeded and the body’s thermoregulation is disturbed.[1]

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