Aircraft Systems

4Radio Propagation and Communication

4.1 Basic Properties of Radio Waves

As first verified by Heinrich Hertz in 1886, radio waves are electromagnetic waves. They travel at the speed of light and are the propagation of an oscillating electric and magnetic field oriented perpendicular to each other as illustrated in Figure 4.1a. This is an example of a linearly polarized wave, that is, the direction of electric field remains fixed and the direction of polarization is defined as being along the electric field. As shown in Section 4.4, this type of wave is emitted by a simple dipole antenna. Another type of polarization of radio waves used in aviation is shown in Figure 4.1b and is referred to as a circularly polarized wave. In this case, the electric field direction rotates once per cycle of amplitude variation so if an observer could detect the direction of the electric field it would rotate at the same frequency as the wave as it passed through the detector. For clarity, the magnetic field oscillation has been omitted from Figure 4.1b, but it is perpendicular to the electric field at all positions. The helicity of the wave is given by the direction of rotation of the electric field as it passes a fixed point, that is, right (clockwise) or left (anticlockwise). Different polarizations are utilized in aviation depending on the application and in the case of some radar applications the detected polarization is different to the emitted one to highlight specific features (see Chapter ...