Radio age (May 1922-Dec 1923)

10 RADIO AGE— "THE MAGAZINE OF THE HOUR" Arrangement of loud-speakers in Congress Hall, Pageant of Progress, Chicago. (By courtesy of Greater Chicago Magazine) Expert Explains Radio Frequency Amplification By CHARLES KILGOUR, Engineer, Crosby Mfg. Co. RADIO Frequency Amplification is regarded at present as the most interesting subject connected with wireless telephony, and Charles Kilgour, who is in charge of the engineering department of the Crosley Manufacturing Company, Cincinnati, Ohio, operators of the radio broadcasting station WLW, has prepared the following interesting explanation of it. Mr. Kilgour has dealt in terms of the layman and made his explanation so simple that a beginner may understand every word of it. Mr. Kilgour said in part: "A radio enthusiast is not satisfied with a mere definition of radio frequency amplification. He wants a plan of construction, for a great part of radios fascination is due to the ease with which it is possible to try out various schemes for making audible the infinitesimal waves of the ether which constantly are lapping upon our aerials. "The first essential of a radio frequency amplifier is a proper vacuum tube. Any standard amplifier tube will serve. Upon the grid of this tube is impressed incoming alternating current. This is accomplished by connecting one side of the secondary coil to the grid and the other to the filament circuit. No grid con denser is used because the tube acts as an amplifier and not as a rectifier or detector. "To cause a vacuum tube to amplify properly the voltage impressed upon its grid, it is necessary to place an impedance, or resistance, in the plate circuit, which is the connection between the plate and the filament. It is also necessary to hold the plate at a positive potential of about 45 volts with respect to the filament. This is accomplished by the familiar 'B' battery. "The high impedance required in the plate circuit may be obtained in several ways. A high ohmic resistance may be used, but as this has a high resistance to direct current it opposes the action of the 'B' battery, thus introducing difficulties. "An inductance or coil may be used to set up the necessary impedance. An inductance may have very low ohmic resistance and so not interfere with the proper action of the 'B' battery and at the same time, due to its reactance offer high impedance to an alternating current such as we wish to amplify. At the high frequency handled a condenser or capacity effect is always present in a coil. This is equivalent to connecting a condenser across the terminals of the coil. This capacity, together with the induct ance of the coil, forms a closed circuit which has a natural period of oscillation or is resonant at a certain frequency. It is a peculiar quality of such a circuit that it' offers a very high resistance to an alternating current of the natural frequency of the circuit. "In other words such a coil introduced in the plate circuit of a vacuum tube will have a high impedance to one frequency and will cause currents of that frequency to be greatly amplified. It is essential, however, that the amplifier works properly on various wave lengths. For this reason the ohmic resistance of the coil may be increased, broadening the range of the amplifier but reducing its efficiency. "By far the best solution of the problem is the use of a rather small inductance with a variable condenser connected across its terminals. The same sort of a circuit is formed as in the last case, but the variable condenser makes it possible to change the natural period of the circuit and so amplify a signal of any desired frequency within the range of the condenser and coil. The ohmic resistance of such a condenser and coil may be very low and paradoxically the impedence at resonance as a consequence will be extremely high.