 function efficiently; they must carry signal with mini- Collecting and transposing terms gives mum loss, minimum interference and minimum re- flections. Minimum loss cables are made by using good conductivity materials such as copper conductors and (2.1.20) low loss insulation materials. Minimum interference is obtained by using coaxial cables whose outer conductor shields out interference signals. Reflections in the system This equation is all we need to calculate the value of the are minimised by proper termination of the cables. For matching resistors in Figure 2.1-23. proper termination and no reflections in the system, two conditions must be fulfilled: Example 2.1.6 the antenna and cable must be terminated in its char- acteristic impedance Zo; A 75 O aerial system is used to supply signals to two the source impedance (Zs) feeding each receiver receivers. Each receiver has an input impedance of 75 Q. must be matched to the input impedance of the re- What is the required value of the matching resistor? ceiver (Zin), i.e. Zs = Zin, otherwise there will be Given: 75 S aerial system, input impedance of each signal reflections and minimum cable transmission receiver = 75 S, no. of receivers = 2. loss will not be obtained. Required: Value of matching resistor. Basic features of radio communication systems CHAPTER 2.1 Solution By inspection Using Equation 2.1.20 with n = 2, we obtain Vout = R+Z n Example 2.1.7 A 50 O aerial receiving system is to be used under matched conditions to supply signal to four receivers, each of input impedance 50 O. If the configuration shown Therefore in Figure 2.1-23 is used, calculate the value of the resistor, R, which must be used to provide matching conditions. Given: 50 O aerial system, input impedance of each receiver = 50 S, no. of receivers = 4. Required: Value of matching resistor. Using Equation 2.1.20 and substituting R = [(n - 1) Solution the above equation From Equation 2.1.20 From the answers above, it would appear that an aerial Transposing, we find that system can be ma