Radio Broadcast (May-Oct 1925)

Making Radio Receivers More Selective Practical Instructions on How to Improve the Selectivity of Various Popular Circuits in Use — A Clear Explanation of the Theory Involved in the Changes By KEITH HENNEY HpHE problem that has been bothering many radio listeners recently, is one of * the selectivity of their receivers. Mr. Henney, in this article, has discussed the whole question of selectivity. There is such a variety of circuits and sets to be considered when one tries to solve the problem of increasing the sharpness of tuning that a general consideration such as this,' we think is the best way to help the individual. This is distinctly not a how-to-make-it article, but the reader will find all the necessary constructional information given. The individual can apply this information to suit his own problem. The suggestions here given are more in the nature of a remedy than a cure^ The real cure for the situation lies in a_readjustment of the broadcast transmitting situation. We believe that Mr. Henney !s discussion of the theory involved here will prove very helpful to the radio constructor.— The Editor ACCORDING to the average radio listener, the flaws in the present scheme of broadcasting are two: the ^ multiplicity of stations and the approach of "super-power." And in his peculiar dilemma of wanting to be in touch with all that goes on in the ether and yet to be exclusive, the listener must turn in but one direction, to increased selectivity. Whether the problem is to doctor a receiver now in operation or to build a set that will be sufficiently selective, the questions that face the radio listener are the same: What is selectivity? How may it be obtained? How much is necessary, or desirable? WHAT IS SELECTIVITY? SELECTIVITY is a relative term, and signifies the ability of a receiver to distinguish between several transmitting stations operating on frequencies that do not differ much from one another. To take an analogy from the phonograph field, let us suppose that the mechanism for recording music would respond only to those tones that lie between middle C and one octave above. Then no matter how many notes a pianist might play, the mechanism would record only those between the proper limits. In other words, the recording apparatus would be selective, and other tones would not bother it. Broadcasting stations in Class B are now stationed 10,000 cycles apart. A receiver sufficiently selective will respond to only one station at a time. Frequencies 10,000 cycles different from that to which the receiver is tuned will not be heard to any marked degree. 799 800 801 FREQUENCY IN KILOCYCLES FIG. I The current in a receiving circuit increases as the resonance point is reached, as shown in this Figure. This receiver would be too sharp since it will respond to a band of frequencies only two kilocycles wide and would lose the higher musical notes entirely. A good receiver should have a resonance curve io kilocycles wide in order to get all of the notes broadcast from the transmitter