ken together these two different types of radiation. This is known as the elec- fields produce an electromagnetic field which at any in- tromagnetic spectrum. stant varies in intensity along the length of the line. The electromagnetic field pattern is, however, far from stationary. Like the voltage on the line, it propagates 2.1.4.2 Free-space radiation from end to end with finite velocity which - for an air spaced line - is close to the velocity of light in free space. 9 Introduction The flow of power from source to load is then regarded as At operational frequencies, where the operational wave- that of an electromagnetic wave propagating between the lengths are comparable in size to circuit components, conductors. any circuit consisting of components connected by Strictly speaking 'free space' is a vacuum. However, the velocity of propagation of electro-magnetic waves in the atmosphere is practically the same as that in a vacuum and is approximately 3 X 108 metres per second. Wavelength (2) is defined as the ratio, velocity/frequency. Transmission lines have impedances because they are constructed from physical components which have resistance, self-inductance, conductance and capacitance. Generally taken to be the case when the operational wavelength is about 1/20 of the physical size of components. Basic features of radio communication systems CHAPTER 2. 1 Infrared Power and audio signals Radio signals Microwaves and visible X-rays X-rays-> 10 ¹2 - 10 15 10 15 - 10 17 Fig. 2.1-9 The electromagnetic frequency spectrum. conductors will tend to act as an imperfect transmission said to be vertically polarised; if horizontal, the wave is line. As a result, there will always be some loss of r.f. horizontally polarised. A wave is circularly polarised if its energy by way of radiation. In other words, the circuit will electric field rotates as the wave travels. Circular polar- tend to behave like a crude radio transmitter antenna. isation can be either clockwise or anti-clockwise. It follows that for minimal radiatio