ve apply to direction. antennas as well. Because of its bi-directional properties, The forward light intensity of a car lamp can be fur- a radio antenna can be used for transmitting or receiving ther improved by using one or more lenses to concentrate signals. 13 If you have any doubts about the system being bi-directional, you should visit a lighthouse which uses a similar reflector and lens system. Curtains must be drawn around the system during daylight hours because sunlight acting on the system has been known to produce such high light and heat intensities that insulation meltdown and fires have been caused. CHAPTER 2.1 Basic features of radio communication systems 2.1.5.2 Radiating resistance The relationship, power (watts) = ts2/ohms), is used source for calculating power loss in a circuit. It is not always possible to apply this law directly to a radiating circuit Voltage because a physical resistor does not always exist. Yet we pattern cannot deny that there is a radiated power loss when a voltage is applied across a radiating circuit. To overcome this problem, engineers postulate an 'equivalent' resistor to represent a physical resistor which would absorb the same radiated power loss. This equivalent resistor is called the radiating resistance of the circuit. source The radiating resistance of an antenna should not be confused with its input impedance. The input impedance is the value used when considering the connection of an Voltage pattern antenna to a transmission line with a specified charac- teristic impedance. Antennas are bi-directional and it is Fig. 2.1-12 (a) Voltage standing-wave pattern on an open- not uncommon to use the same antenna for transmitting circuited transmission line; (b) open-circuited line forming a dipole. and receiving signals. do this effectively, most antennas have physical sizes Example 2.1.3 comparable to their operational wavelengths. A transmitter with an output resistance of 72 S and an Figure 2.1-12(a) shows a two-wire transmission line, r.m.s. output of 100 V is con