pecialised terms are ejecting electrons from atoms and molecules with the introduced in the text as appropriate. resultant production of harmful free radicals. There is a minimum quantum energy below which this disruption 1 Transverse electromagnetic mode wave cannot take place. Since the human body is largely water, (TEM) the water molecule is used to define this minimum level. An electromagnetic wave in which the electric and Different reference sources give varying figures for magnetic fields are both perpendicular to each other and this between 12 eV and 35 eV. The actual value does not to the direction of propagation (see Figure 2.5-4). IONISING ULTRAVIOLET VISIBLE LIGHT INFRARED Radio lonising visible infrared frequencies radiations Table 2.5-3 WAVELENGTH 780 nm 300 GHz Figure 2.5-8 Radiation wavelengths relative to 300 GHz (WHO). Introduction to RF and microwave radiation CHAPTER 2.5 2 Power e.g. watt per kilogram. If the substance is not human The rate of doing work in joules per second. The unit is tissue, it should be specified. Note that a specific the watt (W) which corresponds to 1 Js-1 Sources of absorption rate (SAR) limit may be expressed in this standard form but be limited to a maximum mass of RF energy are rated in watts. Both the kilowatt (kW) and the megawatt (MW) are common in radio work, the tissue e.g. 10 Wkg-1 (10 g) should be interpreted as an latter typically for very high power equipment such as SAR of 10 Wkg-1 in any 10 g of tissue. radar equipment. 9 Frequency 3 Mean power The number of cycles of an alternating current per unit The r.m.s. power supplied or generated averaged over time where the international period is 1 second. The unit a period of time which is long compared with the longest is the hertz. 1 Hz = 1 cycle per second. period of any modulation component. 10 Pulse repetition frequency (p.r.f.) 4 Power flux density (power density) In a system which uses recurrent pulses, the number Power flow per unit area through a surface normal to the of pulses occurring per unit time.