nected via a matched line to open-circuited at one end and driven by a sinusoidal r.f. an antenna whose input resistance is 72 Q. Its radiation generator. Electromagnetic waves will propagate along resistance is also 72 Q. Assuming that the antenna is the line until it reaches the open-circuit end of the line. 100% efficient at the operating frequency, how much At the open-circuit end of the line, the wave will be reflected and travel back towards the sending end. The power will be transmitted into free space? forward wave and the reflected wave then combine to Given: Transmitter output = 100 V, transmitter form a voltage standing wave pattern on the line. The output impedance = 72 S, antenna input impedance voltage is a maximum at the open end. At a distance of = 72 S, radiation resistance = 72 S, antenna one-quarter wavelength from the end, the voltage efficiency = 100%. standing wave is at a minimum because the sending wave Required: Power radiated into free space. and the reflected wave oppose each other. Suppose now that the wires are folded out from the 2/4 points, as in Solution Figure 2.1-12(b). The resulting arrangement is called The antenna has an input impedance Zin = 72 S and a half-wave dipole antenna. provides a matched termination to the 72 O line. The r.f. Earlier we said that the electromagnetic fields around generator then 'sees' an impedance of 72 S, SO the r.m.s. the parallel conductors overlap and cancel outside the voltage applied to the line will be 100/2 = 50 V. The line. However, the electromagnetic fields along the two amount of power radiated is calculated using (2/4) arms of the dipole are now no longer parallel. Hence there is no cancellation of the fields. In fact, the two arms of the dipole now act in series and are additive. radiated power They therefore reinforce each other. Near to the dipole the distribution of fields is complicated but at a distance of more than a few wavelengths electric and magnetic where R = 72 S is the radiation resistance. The radiated fields emerge in pha