 end (see Figure 2.1-15) is connected to earth. 2.1.7 Antenna distribution systems Unbalanced antenna Occasions often arise where it is desired to have one Transformer antenna supply signal to several television and radio re- Unbalanced ceivers. A typical example is that of an apartment block, Unbalanced where a single aerial on the roof supplies signals to all the coaxial cable apartments. Another possible use for such a system is in your own home where you would like to distribute sig- nals to all rooms from a single external aerial. In such cases, and for maximum efficiency, an aerial distribution Unbalanced system is used. There are many ways of designing such antenna Unbalanced a system but before discussing them, it is best to un- to balanced transformer derstand some of the terms used. Balanced 2.1.7.1 Balanced and unbalanced Unbalanced coaxial cable systems Examples of balanced and unbalanced aerials and distri- Fig. 2.1-22 Unbalanced antenna system and (a) unbalanced bution lines are shown in Figures 2.1.21 and 2.1.22. You distribution system; (b) balanced distribution system. CHAPTER 2.1 Basic features of radio communication systems A balanced line (Figures 2.1-21(a) and (b)) is a trans- mission line where the impedance between earth and each conductor is identical. A twin pair cable is an ex- TX line ample of a balanced line because the impedance between earth and each conductor is the same. An unbalanced line (Figures 2.1-22(a) and 2.1-22(b)) is a transmission line where the impedance between earth and each con- Antenna ductor is not equal. A coaxial cable is an example of an n receivers unbalanced line because the impedance between earth and the outer shield is different to the impedance be- Fig. 2.1-23 Aerial distribution system for n receivers, each with an tween earth and the inner conductor. input impedance of Zin. The key to the connections in Figures 2.1.21 and 2.1.22 is the balanced/unbalanced transformer. These trans- In Figure 2.1-23, an aerial of characteristic impedance formers are careful