 inner and outer conductor (E = 1 for air) Solution Using Equation 2.2.2 Example 2.2.1 You will often find two types of flexible coaxial cables: one with a characteristic impedance Z of 50 S which is used mainly for RF instrumentation and the other has a characteristic impedance of 75 O used mainly for an- tennas. The inner diameter of the outer conductor is the 2.2.4.3 Microstrip line characteristic same in both cables. How would you distinguish the impedance (Zo) impedance of the two cables using only your eye? Before we start, it is best to identify some properties that Solution are used in the calculations on microstrip. These are In general, to save money, both cables are normally made shown in Figure 2.2.7 where w = width of the microstrip, with the same outer diameter. This is even more evident h = thickness of the substrate, t = thickness of the 3 BNC is an abbreviation for 'baby N connector'. It is derived from an earlier, larger threaded connector, the Type N connector, named after Paul Neill, a Bell Laboratories engineer. BNC uses a bayonet type fixing. There is also a BNC type connector which uses a thread type fixing; it is called a TNC type connector. Transmission lines CHAPTER 2.2 Top strip Substrate Ground Substrate plane Fig. 2.2-7 (a) Microstrip line; (b) end view of microstrip line. metallisation normally assumed to approach zero in these Synthesis formulae calculations, Er = dielectric constant of the substrate. In the synthesis case we know Z and Er and want to find Note that there are two dielectric constants involved in w/h and Ee. the calculations, the relative bulk dielectric constant E For narrow strips, i.e. Z > (44-2er) and the effective dielectric constant Ee. The effective dielectric is inevitable because some of the electric field (2.2.5) passes directly from the bottom of the strip width to the ground plane whereas some of the electric field travels via air and the substrate to the ground plate. where There are many expressions for calculating microstrip properties4 but we will use