ow attenuation and ra- read the above paragraphs again just to ensure that you are fully cognisant of the terms shown in bold print. diation losses but it is expensive because of its metallic construction (usually copper). It is also relatively heavy and lacks flexibility in use because special arrangements 2.2.3 Types of electrical must be used to bend a transmission path. One variant of transmission lines the waveguide is known as the coplanar waveguide (Figure 2.2-3(b)). Many of the terms introduced in the last section also apply to electrical transmission lines. However, you should be 2.2.3.2 Coaxial and strip lines aware of the great difference in the velocity of sound waves (331 ms-1 and the velocity of electrical waves Another way of carrying high frequency signals is to use (3 X 108 ms-1 in air). There is also a great difference in a coaxial transmission line similar to the one that con- frequency because audible sound waves are usually less nects your TV set to its antenna. The coaxial line is than 20 kHz whereas radio frequencies are often in tens of shown in Figure 2.2-4(a). This is merely a two wire line GHz. For example, satellite broadcasting uses frequen- but the outer conductor forms a circular shield around cies of about 10-12 GHz. Since wavelength = velocity/ the inner conductor to prevent radiation. frequency, it follows that there will be a difference in One variation of the coaxial line appears as the wavelength and that in turn will affect the physical size of strip line (Figure 2.2-4(b)). The strip line is similar to Conducting plane Metallic waveguide Ground plane Fig. 2.2-3 (a) Metallic waveguide; (b) coplanar waveguide. The term 'transmission line' is often abbreviated to 'tx lines'. Transmission lines CHAPTER 2.2 Outer conductor Inner conductor Metallic plates Inner conductor Fig. 2.2-4 (a) Coaxial cable; (b) strip line. a 'flattened' coaxial line. It has the advantage that it can monolithic microwave integrated circuits (MMICs) be- be easily constructed with integrated circuits. cause 