gnetic force exerted when fields the magnetic field and electric field are thus a current flows in a conductor and magnetic field strength inextricably linked. Hence alternating currents and is measured in ampere per metre (Am-1). voltages do, by definition, involve time-varying fields. Being forces, both quantities are vector quantities It is easy to imagine that from any source of such having magnitude and direction. The normal Ohm's law fields some energy may be unintentionally released equations for power when the voltage and current are in (transmitted) into free space, causing interference with phase (plane wave conditions) can be used in an receivers or other equipment, without necessarily analogous way and with the same phase qualification to understanding the phenomenon. This is because such calculate power density. 'interference' has been experienced by most people in Plane wave conditions involve the concept of 'free their everyday lives. Perhaps the most common example space impedance" which is given by the expression is the motor car ignition system which can also prove to be a rudimentary example of the spark transmitter! Zo(s) = (100/&0) Angle - degrees Figure 2.5-3 Phase difference between two waveforms. Introduction to RF and microwave radiation CHAPTER 2.5 where HO is the permeability of free space and E0 is the There is another approach to RF radiation whereby permittivity of free space. the concept of particles (photons) is used to describe the Hence Z = J(4 X 10-7/8.854 19 10-12) = radiated signal. However, for the purposes of this work, 376.7 S (taken as being 3770). the wave concept seems to serve the purpose best and is For plane wave conditions, Z = |E/H| where E and generally so used. Hare field values in Vm-Superscript(1) and Am respectively. Hence, under the same conditions, S (W m - 2 =E2/Zo= where S is the power flux density in Wm-2. Frequency and wavelength In the USA the most common unit used for S is mW cm-2 and being the larger unit, is numerically one tenth of the quantity expre