devices which have no radiation Frequency is simply the number of cycles per unit function, for example, due to inadequate shielding, time and the international convention is 'per second'. unblocked apertures in metal cases, and similar short- The unit is the hertz (Hz) named after the German comings. Apart from any effects of leakage on people, it physicist, 1 hertz corresponding to 1 cycle per second. It also causes interference with other equipment. It is not follows that the time of 1 cycle in seconds is given by the surprising that the presence of SO much electromagnetic reciprocal of the frequency in hertz. Introduction to RF and microwave radiation CHAPTER 2.5 Angle - degrees Figure 2.5-1 Sine wave illustration. The AC mains supply frequency (50 or 60 Hz) is displaced along the time axis SO that whilst they are referred to as a low frequency whereas the frequencies identical in form, the starting points of the cycles may used for radio transmission are much higher frequencies. not be identical, i.e. there is a phase difference between The time T for the duration of a cycle at, for example, 50 the two waveforms. Hz is 1/50 S = 20 ms (twenty thousandths of a second) This may be expressed in angular terms, e.g. 90° phase whereas the time for higher frequencies is much shorter difference. If two such identical waveforms are exactly as shown in the examples below: in phase and are added, the amplitude of the resultant at any point will be twice that of either waveform alone. T (s) = 1/f (Hz) Conversely, if the two waveforms are 180° out of phase the sum will be zero. This becomes relevant when considering radiation surveys and it is necessary to Examples: consider the additive possibilities of radio waves 1. f = 100 )kHz T = 10 us (10-5s). reflected from the ground and from metal masses. 2. f = 1 MHz T = 1 us (10-6s). Obviously additions increase any potential hazards 3. f = 1000 MHz T = 1 ns (10-9s). whereas cancellations are less significant in this context since the safety measurement activity is essentially