t that at any point in the electromag- netic field the instantaneous values of the electric field Amplitude 2 (E) (volts/metre) and the magnetic field (H) (amperes/ metre) are related by Fig. 2.1-7 An eight level coded signal modulated on to a radio E (V/m) carrier. H = Zo (ohms) (2.1.7) (A/m) This method is also known as quadrature amplitude where Zo is the characteristic impedance of the trans- modulation (QAM). mission line. 10 It can also be shown that both approaches give identical results for the power flow along a matched line. 2.1.3.5 Summary of modulation systems In the two-wire transmission line shown in Figure 2.1-8, the parallel conductors produce electromagnetic fields In this section, we have shown you four methods by which overlap and cancel in the space beyond the con- which information signals can be modulated on to a radio ductors. The r.f. energy is thus confined and guided by carrier. the conductors from the source to its destination. If, however, the conductor spacing is increased SO that it 2.1.4 Radio wave propagation becomes comparable with the wavelength of operation techniques the line will begin to radiate r.f. energy to its surround- ings. The energy is lost in the form of free-space elec- tromagnetic waves which radiate away from the line 2.1.4.1 Properties of electromagnetic with the velocity of light. waves The 19th century mathematician James Clerk Max- well was the first to recognise that electromagnetic waves In Figure 2.1-8 we show the case of a radio generator can exist and transport energy quite independently of any feeding energy into a load via a two-wire transmission system of conductors. We know now that radio waves, line. The radio generator causes voltage and current heat waves, visible light, X-rays are all electromagnetic waves to flow towards the load. A voltage wave produces waves differing only in frequency. Figure 2.1-9 shows the a voltage or electric field. A current wave produces range of frequencies and the regions occupied by the a current or magnetic field. Ta