230LETTER 22
HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS
My Dear Experimenter:
This letter is to summarize the operations which must be performed in radio-telephone transmission and reception; and also to describe the circuit of an important commercial system.
To transmit speech by radio three operations are necessary. First, there must be generated a high-frequency alternating current; second, this current must be modulated, that is, varied in intensity in accordance with the human voice; and third, the modulated current must be supplied to an antenna. For efficient operation, of course, the antenna must be tuned to the frequency which is to be transmitted. There is also a fourth operation which is usually performed and that is amplification. Wherever the electrical effect is smaller than desired, or required for satisfactory transmission, vacuum tubes are used as amplifiers. Of this I shall give you an illustration later.
Three operations are also essential in receiving. First, an antenna must be so arranged and tuned as to receive energy from the distant transmitting station. There is then in the receiving antenna a current similar in wave form to that in the transmitting 231antenna. Second, the speech significance of this current must be detected, that is, the modulated current must be demodulated. A current is then obtained which has the same wave form as the human voice which was the cause of the modulation at the distant station. The third operation is performed by a telephone receiver which makes the molecules of air in its neighborhood move back and forth in accordance with the detected current. As you already know a fourth operation may be carried on by amplifiers which give on their output sides currents of greater strength but of the same forms as they receive at their input terminals.
In transmitting and in receiving equipment two or more of these operations may be performed by the same vacuum tube as you will remember from our discussion of the regenerative circuit for receiving. For example, also, in any receiving set the vacuum tube which detects is usually amplifying. In the regenerative circuit for receiving continuous waves by the heterodyne method the vacuum tube functions as a generator of high-frequency current and as a detector of the variations in current which occur because the locally-generated current does not keep in step with that generated at the transmitting station.
Another example of a vacuum tube performing simultaneously two different functions is illustrated in Fig. 120 which shows a simple radio-telephone transmitter. The single tube performs in itself both the generation of the radio-frequency current and its 232modulation in accordance with the output of the carbon-button transmitter. This audion is in a feed-back circuit, the oscillation frequency of which depends upon the condenser C and the inductance L. The voice drives the diaphragm of the transmitter and thus varies the resistance of the carbon button. This varies the current from the battery, BA, through the primary, T1, of the transformer T. The result is a varying voltage applied to the grid by the secondary T2. The oscillating current in the plate circuit of the audion varies accordingly because it is dependent upon the grid voltage. The condenser CR offers a low impedance to the radio-frequency current to which the winding T2 of audio-frequency transformer offers too much.
In this case the tube is both generator and “modulator.” In some cases these operations are separately performed by different tubes. This was true of the transmitting set used in 1915 when the engineers of the Bell Telephone System talked by radio from Arlington, near Washington, D. C., to Paris and Honolulu. I shall not draw out completely the circuit of their apparatus but I shall describe it by 233using little squares to represent the parts responsible for each of the several operations.