 two lines present difficulties in connecting external components to the inner conductor. The coplanar The characteristic impedance of transmission lines is waveguide is better in this respect and finds favour in calculated in two main ways: Conducting plane Ground Ground plane plane Microstrip Slotline Fig. 2.2-5 (a) Microstrip line; (b) slot line. CHAPTER 2.2 Transmission lines when the cables are terminated in a type of r.f. connector known as BNC. 3 Since these connectors have the same outer diameter, by using Equation 2.2.1 you can deduce that for Z = 75 S, the inner conductor will be smaller than that of the 50 O cable. In practice, you will be able to recognise this distinction quite easily. 2.2.4.2 Twin parallel wire characteristic impedance (Zo) Dielectric The expression for calculating the characteristic imped- Fig. 2.2-6 Twin parallel wire VHF cable. ance of the type of parallel transmission line shown in Figure 2.2-6 is from physical parameters and configuration; (2.2.2) from distributed electrical parameters of the line. Some relevant expressions for calculating the impedance where of these lines from physical parameters are given in the following sections. d = outer diameter of one of the identical conductors D = distance between the centres of the two conductors 2.2.4.1 Coaxial line characteristic E = relative dielectric constant = 1 for air impedance (Zo) The expression for calculating the characteristic Example 2.2.2 impedance of the coaxial transmission line shown in The twin parallel transmission line shown in Figure 2.2-6 Figure 2.2-4(a) is is separated by a distance (D) of 300 mm between the centre lines of the conductors. The diameter (d) of the (2.2.1) identical conductors is 4 mm. What is the characteristic impedance (Zo) of the line? Assume that the trans- mission line is suspended in free space, i.e. E = 1. where d = outer diameter of the inner conductor Given: D = 300 mm, d = 4 mm, E = 1. D = inner diameter of the outer conductor Required: Z. E = dielectric constant of the space between