Introduction

This chapter provides an introduction to the principles of radio communication systems. Radio waves are said to be polarised in the plane of the electric (E) field. Thus, if the E-field is vertical, the signal is said to be vertically polarised, whereas, if the E-field is horizontal, the signal is said to be horizontally polarised. Depending on a number of complex factors, radio waves can propagate through the atmosphere in various ways. These include: ground waves, ionospheric waves, space waves and tropospheric waves. The ionosphere provides us with a reasonably predictable means of communicating over long distances using HF radio signals. Much of the short and long distance communications below 30 MHz depend on the bending or refraction of the transmitted wave in the earth's ionosphere which are regions of ionisation caused by the sun's ultraviolet radiation and lying about 60 to 200 miles above the earth's surface.