 For those unfamiliar with these SI prefixes (1, n, etc.), concerned with the highest levels present. see Table 2.5-1 which lists those actually used in every- Readers will also be familiar with the idea that day work. a current flowing in a conductor gives rise to a magnetic If the existence of two identical waveforms as shown field around it. When such a current is varying, it gives in Figure 2.5-3 is considered, it is possible for these to be rise to a similarly changing electric field. Similarly 1 cycle 1 second Frequency f 4 Hz Figure 2.5-2 Sine wave frequency and time relationship. CHAPTER 2.5 Introduction to RF and microwave radiation In the case of radio transmitters, however, the whole Table 2.5-1 The most commonly used International System (SI) intention is to transmit RF energy into free space and the prefixes. antenna used to do SO is specifically designed to achieve this Symbol Factor objective. If we consider the frequencies discussed above, the very low frequencies, e.g. mains power frequencies, do not give rise to any significant amount of radiation. However, as we increase the frequency then it becomes increasingly possible to radiate electromagnetic waves, given a suitable antenna to act as an efficient 'launcher'. 10 12 The electric and magnetic field quantities mentioned above perhaps need a little more elaboration. The electric milli (E) field at any point is defined as the force acting on micro a unit positive charge at that point. The magnitude of the electric field is expressed in volt per metre (V m - 1 The magnetic field at a point is also a force and is 10-12 defined as the force which would act on an isolated north pole at that point. The classic demonstration of this is a changing electric field will give rise to a magnetic field. that the earth's magnetic field exerts a force on a compass Unchanging fields of either kind will not result in the needle, to the great blessing of navigators. The ampere is production of the other kind of field. With changing defined on the basis of the ma