 point. It is expressed in volt per Smean X I/DF = Spk metre. Note that although pulse transmission often seems to 7 Magnetic field strength (H) at a point be uniquely linked to 'radar', pulse transmission is widely A vector quantity defined as the force acting on an used and radar is just one application. Note also the high isolated north pole at that point. It is expressed in values of Spk which are possible, depending on the duty ampere per metre. factor. 8 Specific absorption rate 13 Antenna (aerial) The rate of absorption of RF energy in a substance, The generally used term for any type of device intended normally human tissue, expressed in watt per unit mass, to radiate or receive RF energy. These range from simple CHAPTER 2.5 Introduction to RF and microwave radiation wires and rods to arrays (of which the television antenna running RF radiation safety courses shows that whilst is an example) to large microwave parabolic, elliptical many people work regularly with decibels, a surprising and rectangular aperture systems. number never have occasion to do SO and it is often Some antennas are dedicated to reception or trans- necessary to do a refresher session in this topic. mission whilst others do both. To most people the terms The bel and the decibel (one tenth of a bel) were antenna and aerial are synonymous. The English plural is originally used to compare sound intensities and are normally used for antenna. currently used in safety legislation to limit the exposure of people to intense sounds in the workplace. Some 14 Antenna, isotropic safety officers will be familiar with this method of noise control. A hypothetical, idealised, antenna which radiates (or In radio work, the decibel is used to compare powers, receives) equally in all directions. The isotropic antenna voltages and currents. The decibel is a dimensionless is not realisable but is a valuable concept for comparison number representing a ratio based on common loga- purposes. rithms. However, usage is such that the ratio is often referenced to