Radio Broadcast (May 1929-Apr 1930)

STRAYS FROM THE LABOR ATOIIY A Radio Manual The second edition of Sterling's Radio Manual contains much material that was not in the first edition. It is now a book of nearly 800 pages on theory and practice. It has material not generally available, such as the circuit diagrams and operation data on Western Electric transmitters of the latest type, receivers used for shipto-shore traffic, and much other practical matter of interest and value to radio operators, engineers, servicemen, and others who want descriptions and other data on modern radio equipment. Airplane apparatus, beacons, automatic SOS transmitters, the International Laws relating to radio communication, the "Q" signals, tables of LC products, conversion tables of wavelength and frequency — all are in this big book with the soft covers. It is published by Van Nostrand and was edited by Robert S. Kruse. Causes of A.C. Hum The following statements are digested from an article in the General Electric Review, "The Operation of Radio Receiving Tube Filaments on Alternating Current," by Dr. K. H. Kingdon and H. M. MottSmith, Jr. Hum due to operating a tube filament on a.c. comes from three sources, 1. a fluctuating potential drop along the filament. 2. a fluctuating magnetic field about the filament, 3. fluctuations in filament temperature. Hum due the first cause is of two kinds, one having a frequency equal to the frequency of the supply voltage, and the other having a frequency twice that of the supply, e.g., for a 60-cycle supply, 60 and 120 cycles. The first can be balanced out by returning the grid and plate circuits to the proper place along the filament by means of a variable center-tapped resistor. The amplitude of the fundamental frequency hum is proportional to the ratios between the maximum value of the filament voltage and the approximate "lumped voltage" on the plate, while the second harmonic is proportional to the square of this value. Since this ratio is small, it is important to balance out the fundamental component of the hum. A tube adjusted to minimum hum when the supply is sinusoidal will not be balanced if the supply contains even harmonics, although it will be in balance for odd harmonics in the supply. If the double-frequency component has been reduced — by proper construction and operation of the tube — to one tenth some given value, the introduction of a second harmonic of an amplitude of five c?100 o o *t 90 80 50 < O 40 30 Z 20 10 per cent, of the fundamental supply will cause an increase in the hum of 100 per cent. If the filament transformer also supplies power to a plate-supply unit variations in the load on the rectifier may cause considerable corresponding variations in the amplitude of the hum. UJq; O Ul < trcr 11 iz St, 1 — 1 — 1 — 1 — SELECTIVITY GRAPHS RAOIOLAS 21 122 t "7 .10 00 1 r— I 14< 60 )0K OK C c 60 40 20 0 + 20 40 60 WAVE FREQUENCY IN KILOCYCLES Fig. 1 It happens, fortuitously, that the hum due the magnetic field is opposite in phase to the hum due the first cause, and so a tube can be constructed and operated so that these effects partially balance out. The third source of humoccursonly when the plate current is appreciable compared to the filament current, for example, if a 199-type tube with a 60-milliampere filament is operated from an a.c. supply. Calculations show that for a 226-type tube operated with a plate potential of 135 volts, the magnetic and potential ripple voltages will cancel out when the grid bias is minus 11.8 volts. However, in practice the hum voltage is of the order of 7 millivolts across a 26,000-ohm load. ■> < >. , 1400 K ■o \ s / / \ 6C )0 KC y\ 1—4 ( \ FIDELITY GRAPHS RADIOLAS 218.22 \ \ \ eo 100 .000 FREQUENCY IN CYCLES PER SECOND Fig. 2 The Radiola 21 In January, 1930, Radio Broadcast there appeared a description of the Radiola 21 and 22 battery-operated radio sets designed for the farm market. Figs. 1 and 2 on this page show the important characteristics of these sets; the selectivity at standard radio frequencies, 600, 1000, and 1400 kc; and the fidelity of response at two radio frequencies, 600 and 1400 kc. The Radiola 21 is a table model, uses 5 tubes, and costs $69.50; the Radiola 22 is a console model using the same equipment plus a Radiola 100b loud speaker. It sells for $135.00. All About Electricity An excellent series of articles on the nature of electricity, originally published in Power Plant Engineering in 1928 and 1929 by Andrew W. Kramer, editor of that magazine, has been collected into book form under the title of Electricity, What It Is and How It Acts. In a small volume the author seems to have collected more material than ordinarily goes into a book of double its size. It is most readable by those who think they know what it is all about and by those who admit they don't know much about electricity. In other words, it is a book that tells you all about electricity, whether you are a power engineer or a tyro in the field of communication. The illustrations are excellent and quite frequently are of the simple and graphic form that many serious writers seem afraid to use. We recommend the book highly. The publishers are Technical Publishing Company, 53 West Jackson Blvd., Chicago. Wire Communication A book of interest to those who work with wire communications instead of the more, in the popular mind, "wonderful" radio medium is Shea's Transmission Networks and Wave Filters. This is the most recent of the already-long series of books known as the Rell Telephone Laboratories Series. It deals with the calculation and design of telephone networks and wave filters. In addition mathematics and electrical theory have not been neglected; in fact, a radio man looking at this book will be amazed to learn how simple his own mathematics are compared to what a full-fledged telephone engineer must wrestle with. One entire section of th'e book (there are three in all) is devoted to transients. Others deal with terminal networks, losses due to impedance mismatching, filters of all types, Fourier series analysis, etc. The book is published by Van Nostrand. 10,000 • FEBRUARY 1 930 • • 223