Amplitude-Modulation Radio-Telegraph Systems

With amplitude-modulated systems, cither continuous (or undamped) waves, or interrupted-continuous waves, are employed. With the first method, "spurts" of radio-frequency waves are sent out from the transmitter. With the second method, the "spurts" of radio frequency are interrupted, for example, 1000 times a second.

The transmitters³⁶ are similar in many respects to amplitude-modulated radio-telephone transmitters, spark transmitters no longer being used. Methods are provided for keying the transmitter, that is, turning it on and off in accordance with the signals to be sent. The keying may be by hand, by printing-telegraph teletypewriters, or by tape transmitters. Amplitude-modulation telegraphy, which is being discussed now, is often called on-off modulation, or on-off keying.

There is an important trend^37,38 toward perforated tape methods of sending, receiving, and switching, comparable to that used with wire telegraphy (page 337). The extent of this is indicated by Fig. 21.

Figure 21. International radio-telegraph tape-printing relay network. (Reference 38.)

The radio-telegraph receivers usually are of the superheterodyne, or double-detection, type but are provided with a second oscillator (often called a beating oscillator, or a beat-frequency oscillator) that will introduce a wave into the intermediate-frequency amplifier so that continuous-wave messages can be received. These receivers are generally familiar, examples being in most radio laboratories, and are called communication receivers.