Radio Broadcast (May 1928-Apr 1929)

360 RADIO BROADCAST OCTOBER, 1928 +B67k FIG. 5 The circuit of a stage of tuned impedance r. f. amplification using a screen-grid tube per stage could be obtained. The method of measuring this gain was to put in a signal from an r.f. oscillator of o. 1 volt, as measured on a Rawson Thermal Multimeter, and to measure the voltage developed across points 3 and 4 by means of a vacuum tube voltmeter. The results are shown in Fig. 3. In curve 1 voltage amplification is plotted against wavelength. The theoretical amplification as calculated from equation 1 is shown by curve 2. The discrepancy between measured values and calculated ones is probably due to the capacity between primary and secondary windings of the r.f.t., as the effect of any capacity would be more pronounced on the short than on the long wavelengths. As a matter of comparison the amplification of a 201A tube used in conjunction with a well-designed transformer is shown by curve 3. Not only does the screen-grid tube with the transformer described above give more amplification per stage, but furthermore a number of stages may be used without neutralization, whereas with the 20 1 a, careful neutralization is necessary. Before considering the amplification given by a tuned impedance, let us consider the selectivity obtained with the transformer and the Ceco a.c. 22 tube. The selectivity depends primarily upon the resistance, R2, inherent in the coil and condenser in the secondary circuit. However, this resistance is increased due to the effect of the primary. Instead of considering the resistance R itself let us consider . — which is nearly constant over the wave band and gives directly the sharpness of tuning of the circuit. The smaller this factor the sharper the circuit tunes. For a given amount of amplification the selectivity of the radio-frequency transformer as a whole depends upon the coefficient of coupling so that when it is increased to obtain amplification the selectivity of the transformer is also increased. The solid curve in Fig. 4 shows the resonance curve of the transformer at 400 meters where amplification is plotted against capacity of the tuning condenser above and below resonance. The calculated curve falls for the most part on the FIG. 6 The equivalent schematic circuit of Fig. 5 measured curve except that the theoretical curve is slightly higher as shown by the dotted line. Let us now consider the amplification and selectivity of the screen-grid tube using tuned impedance as shown in Fig. 5, with the equivalent circuit Fig. 6. In this case the gain is given by Eg itf Rp + R2 Uj so that the smaller the resistance in the secondary circuit and the smaller rl2 the larger the amplification. Fig. 3, curve 4, shows the amplification measured while using a 2" coil. The calculated curve, 5, in this case is quite close to the measured values, 4. However, the apparent selectivity of the tuned impedance amplifier as shown by the dot-dash curve in Fig. 4, is not as good as in the case of the transformer. There seems also to be another disadvantage in using tuned impedance which is due to the tendency of the circuits to oscillate when using only two stages. The writer has been able to build with careful shielding a two-stage r.f. amplifier using the transformers described without the slightest tendency to oscillate. Regeneration on the detector was possible with the result that tremendous signal strength and fine selectivity were obtained, while with tuned impedance considerably more care was necessary to get two stages to be stable, and even then it seemed as if the signal strength were no greater than with the transformers plus regeneration, while the selectivity of the two systems was not to be compared. Wireless Direction Finding and Directional Reception. By R. Keen. Iliffe & Sons, Ltd., London. 490 pages. 1927. 21s. THE statement that a given work is indispensable to those interested in the subject is a much misused cliche of technical book-reviewing, but in the case of Keen's "Wireless Direction Finding and Directional Reception" it is merely the literal truth. This book was first published in 1922 under the title of " Direction and Position Finding by Wireless." In the second edition the title was changed to include directional aerial systems, which had in the meantime assumed importance. Keen's work is an important contribution to the specialized literature or r,adio. It is a serious technical job and not intended for those to whom radio is a plaything. The mathematics is fairly simple, but the vectorial and diagrammatic treatment is very thorough and obviously designed for the attention of engineers and engineering students. After an introduction, which includes an impartial historical treatment of the subject, directional transmission and reception are discussed. The wave antenna of Beverage, Rice, and Kellogg is described at the end of Chapter 2. The third chapter is devoted to " Frame Aerial Reception." This is discussed in detail, such topics as " Elimination of Vertical" (the antenna effect of a loop, which plays a part in reception) being treated. The theory of practical systems of this type is comprehensively stated and the chapter closes with a discussion of " Fallacies in Heart-Shape Circuits," this being a study of difficulties found in cardioid-reception circuits using a combination of loop and antenna pick-up. The following chapter describes the characteristics of rotating loop installations of the following types: Radio Communication Company, Ltd., Societe Francaise Radio-Electrique, Gesellschaft fur Drahtlose Telegraphie (Telefunken), Siemens Brothers and Company, Ltd., U. S. Bureau of Standards, Washington, Marconi's Wireless Telegraph Co., Ltd., Radio Corporation of America, and the Federal Telegraph Company. The descriptions are quite extensive and well illustrated with diagrams and photographs. Chapter 5 is devoted to an analysis of the Bellini-Tosi system, which uses large fixed loops for directive transmission and reception. Another chapter goes into the theory and practice of map drawing. The radio engineer will have his hands full with such terms as "The Gnomonic Graticule," "The Retro-Azimuthal Chart," "The Orthomorphic Cylindrical Projection"; in this chapter and the one following, on "Position Finding and Wireless Navigation," he will have it brought home to him that radio direction finding is as much a branch of geography and navigation as of wireless communication. Chapter 9, on "Night Effect and Other Freak Phenomena" is of general interest to students of the vagaries of radio transmission. In the following two chapters the discussion returns to types and characteristics of apparatus on shore and afloat. Theory, testing, calibration, and operation are considered in turn. Chapter 12 tells about "The Aircraft D. F. Installation"; it is as complete as possible, but in his preface the author says that this chapter "is still necessarily very curtailed. Although there has been much activity in this direction, there are few concrete designs of aircraft D. F. available for inclusion here." For this state of affairs the difficulties encountered in such installations, as well as the indifference of many airmen, are responsible. Two more practical chapters, on " Fault Clearing and Maintenance," and "Notes on Field and Nautical Astronomy" complete the work. A bibliography of 374 references and an index are included. If anything material on the subject has been omitted it has escaped the present reviewer.