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CHAPTER XXIX RADIO RECEIVERS
(1) Radio receiving and reproducing apparatus is simpler than that used for television, although both operate on the same basic principles. According to the plan hitherto followed in these pages the fundamentals of common-type radio receivers will first be outlined, and will be followed by a description of how similar receivers are used for the more complex purpose of handling television signals.
(2) All radio broadcasting is picked up by some form of antenna, which may be regarded without too much inaccuracy as the secondary of a transformer of which the transmitting antenna is the primary. The antenna may be built into the radio, or if the radio is not perfectly shielded some of its r.f. transformer windings may act as an antenna. Every antenna is, of course, constantly exposed to an immense number of signals — may be regarded as the secondary of a transformer in which every transmitting antenna in the world is a primary. To select the signal desired from the multitude that are picked up by the antenna, frequency-selective filter circuits are used.
(3) In the simplest form of radio receiver the frequency filter circuits and the radio frequency amplifier are combined. The principle is diagrammed roughly in Figure 226, which represents an ordinary transformer coupled amplifier built around radio frequency air-core transformers. But the secondary coil of each transformer is bridged by a variable condenser and therefore that secondary and the condenser constitute a filter circuit which is also a part of the radio frequency amplifier circuit. The two condensers shown in Figure 226 are ganged as explained in Chapter XXVII, and both are
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