Non-Ionizing Radiation
Electroservices has now started to provide a comprehensive service for the calibration of Non-Ionizing field strength probes and sensors traceable to national standards.
Electromagnetic fields (EMFs) arise whenever electrical energy is used. Example, in our homes from electrical appliances, from work processes such as radiofrequency heating and drying and generally in the world from radio, TV and Telecoms broadcasting masts and security detection devices.
Non-Ionizing radiation probes are widely used for Health and Safety surveys to ensure personnel are not exposed to harmful levels of RF or microwave radiation, a requirement that became law for occupational exposure on 30 April 2008.
Electroservices facilities, based in our Stafford Park Annex, include a TEM cell, anechoic chamber and Helmholtz coil.
The TEM cell is used to produce calculable electric and magnetic field strengths over the frequency range 10 Hz to 250 MHz. TEM cells are, in principle, coaxial lines expanded by input and output tapers to form a rectangular outer shielding conductor with a flattened inner conductor called the septum. They are operated in the transverse electromagnetic mode, so that both the E-field and H-field components generated between the septum and outer conductor have the characteristics of a wave propagating in free space.
The field strength can be calculated from the dimensions of the cell, its impedance at the measurement plane and the input power. TEM cells have an upper frequency limit, determined by the dimensions of the cell, below which, the fields set up in the central region are essentially uniform which makes them ideal for calibrating probes and sensors. However, once the upper frequency limit is exceeded, higher order modes can propagate, which give rise to large spatial variations in the fields. In the frequency range 250 MHz to 18 GHz measurements are carried out inside an anechoic chamber, where the calculable uniform field is generated using a calibrated coaxial system feeding a horn antenna. Knowing the power fed into the antenna and its gain, one can readily calculate the density of the field and hence field strength at a known distance from the horn antenna.
The Helmholtz coil is used to generate AC and DC magnetic fields in the frequency range DC to 100 kHz. A Helmholtz consists of two identical magnetic coils that are placed symmetrically one on each side of the test area along a common axis, and separated by a fixed distance equal to the radius of the coil. Each coil carries an equal electrical current flowing in the same direction. The magnetic flux density at the midpoint between the coils is calculated from the radius of the coils, the number of turns in each coil and the current flowing through the coils.