tion in Figure 2.1-10. Measure- conductor with the velocity of light. ments at the radio receiver would then indicate that the oscillating electric field is acting all in one direction, the x-direction in Figure 2.1-10. The magnetic field is in- phase with the electric field but acts at right-angles to the Radio source electric field, in the y-direction. The two fields are thus at E-field right-angles to each other and to the direction of propa- gation. An electromagnetic wave with these characteris- tics is known as a plane wave. H-field Polarisation Propagation Provided there is no disturbance in the propagation path, the electric and magnetic field orientations with respect to the earth's surface will remain unchanged. By convention, Radio receiver the orientation of the electric field with respect to the earth's surface is called the polarisation of the electro- Fig. 2.1-10 Electric and magnetic field directions for an electro- magnetic wave. If the electric field is vertical, the wave is magnetic wave propagating in the z-direction. You can see this effect by looking at TV aerials mounted on houses. In some districts, you will see aerials mounted horizontally whilst in other areas you will find aerials mounted vertically. As a general rule, TV broadcasting authorities favour horizontal polarisation for main stations and vertical polarisation for sub- or relay stations. CHAPTER 2.1 Basic features of radio communication systems As stated earlier, the electric and magnetic fields idea that the power density falls off as the square of the in a plane wave are everywhere in phase. The ratio distance from the source, i.e. of the field strengths is always the same and is given (2.1.9) electric field strength Erms (V/m) where PD1, PD2 = power densities at distances D1 and D2 magnetic field strength Hrms(A/m) respectively. (2.1.8) This ratio is called the free-space wave impedance. It is Example 2.1.2 analogous to the characteristic impedance of a trans- If the data in Example 2.1.1 applies to a receiver located missi