Figure 34.10 shows a Hertz antenna (also known as a half-waveantenna, since its

Question

Figure 34.10 shows a Hertz antenna (also known as a half-waveantenna, since its length is lambda/2). The antenna islocated far enough from the ground that reflections do notsignificantly affect its radiation pattern. Most AM radio stations,however, use a Marconi antenna, which consists of the top half of aHertz antenna. The lower end of this (quarter-wave) antenna isconnected to Earth ground, and the ground itself serves as themissing lower half. What are the heights of a Marconi antenna for aradio station broadcasting at the following frequencies? Figure 34.10 (a) 650 kHz m (b) 1500 kHz m

Explanation / Answer

You are told the height of the antenna is a 1/4 of awavelength, so get the wavelength in each case: . (a)   = c/f = 3 x108 / 650 x 103 = 461.5m       antenna height = 461.5/4 =  115.4 meters . (b) = c/f = 3 x 108 /1500 x 103 =  200m       antenna height = 200/4 =  50.0 meters

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