CHAPTER 2.5 Introduction to RF and microwave radiation Figure 2.5-7 Pulse modulation. will be too many cycles of carrier in each pulse to actually illustrate them. For example a radar working at 1 GHz and with 2 ms pulses will have 2000 cycles of carrier in each pulse. There are many other methods of modulation and transmission which can be applied to radio equipment and which cannot be covered here but which need to be known to those doing safety surveys. Much power is wasted in amplitude modulation and various other forms which reduce the waste are widely in use such as single sideband (the two sidebands in AM contain identical intelligence) with a reduced carrier power, double sideband where the two sidebands carry different intelligence and again the carrier is reduced, etc. The recent development of digital radio and television will likely become widespread over the next few years. Ionising and non-ionising radiations Confusion between these two forms of radiation amongst the public has been mentioned earlier. There is also a surprising amount of misunderstanding amongst elec- tronics and radio engineers about the distinction between these two forms of radiation even amongst newly quali- fied graduate engineers, so that RF radiation is sometimes considered to be the same as ionising radiation. Ionising radiation, by definition, is radiation capable of ejecting electrons from atoms and molecules with the resultant production of harmful free radicals. There is a minimum quantum energy below which this disruption cannot take place. Since the human body is largely water, the water molecule is used to define this minimum level. Different reference sources give varying figures for this between 12 eV and 35 eV. The actual value does not matter for the purposes of this comparison. An energy of 12 eV corresponds to a wavelength of 1.03 Â 10 À7 metres (103 nm) which can be seen from Figure 2.5-8 lies just above the ultraviolet (UVc) spectrum. The highest RF frequency used in standards for RF safety is 300 GHz which corresponds to a wavelength of 10 À3 metres and lies in the EHF band of the radio frequency spectrum. If the calculation is done the other way round, 300 GHz corresponds to an energy of 0.00125 eV which, from the foregoing, is too small by about four orders to cause ionisation. However, in radio transmitters using very high supply voltages, ionising radiation in the form of X-rays are produced. It should be clear that this ionising radiation is not inherent in the RF energy but rather that both forms of radiation can co-exist inside equipment and the RF engineer or technician needs to be aware of the hazards involved. It is also the case that ionising radiation is, in most countries, subject to definitive legal provisions due to its hazardous nature. Explanation of terms used In this section those terms and units which are most frequently used in dealing with RF radiation are explained. The more formal definitions may be found in reference books. Other more specialised terms are introduced in the text as appropriate. 1 Transverse electromagnetic mode wave (TEM) An electromagnetic wave in which the electric and magnetic fields are both perpendicular to each other and to the direction of propagation (see Figure 2.5-4). Figure 2.5-8 Radiation wavelengths relative to 300 GHz (WHO). 146