Today stations give out their frequency because nowadays this is far easier to measure. A frequency counter can be used to measure this very accurately, and with today's technology their cost is relatively low. It is very easy to relate the frequency and wavelength as they are linked by the speed of light as shown: "= 1= where l = the wavelength in metres f = frequency in hertz C = speed of radio waves (light) taken as 300 000 000 metres per second for all practical purposes Taking the previous example the wavelength of 1500 metres corre- sponds to a frequency of 300 000 000/1500 or 200 thousand hertz (200 kHz). Radio spectrum Electromagnetic waves have a wide variety of frequencies. Radio signals have the lowest frequency, and hence the longest wavelengths. Above the radio spectrum, other forms of radiation can be found. These include infrared radiation, light, ultraviolet and a number of other forms of radiation as shown in Figure 2.6. The radio spectrum itself covers an enormous range. At the bottom end of the spectrum there are signals of just a few kilohertz, whereas the top end new semiconductor devices are being developed which operate at frequencies of a hundred GHz and more. Between these extremes lie all the signals with which we are familiar. It can be seen that there is a vast amount of spectrum space available for transmis- sions. To make it easy to refer to different portions of the spectrum, Infrared Ultraviolet Visible light Radio waves Gamma ray and x-rays Frequency (Hz) Figure 2.6 Electromagnetic wave spectrum Radio waves and propagation 23 0.003 MHz Very Low Frequency (VLF) 0.03 MHz Low Frequency (LF) 0.3 MHz Medium Frequency (MF) 3 MHz High Frequency (HF) 30 MHz Very High Frequency (VHF) 300 MHz Ultra High Frequency (UHF) 3000 MHz Super High Frequency (SHF) 30 000 MHz Extra High Frequency (EHF) 300 000 MHz Figure 2.7 The radio spectrum designations are given to them as shown in Figure 2.7. It can be seen from this that transmissions in the long wave broadcast band (140.5 to 283.5 kHz) availabl