Radio broadcast .. (1922-30)

RADIO BROADCAST. sitates careful design of the volume-control circuit. In these receivers the volume control has a double function; first, it varies the screen-grid potential of the r.f. amplifier tubes, and second, it varies the signal input from the an- tenna to the set. Detection The large r.f. signal strength made available by the high gain in the r.f. amplifier makes possible the use of plate rec- tification without loss in over- all sensitivity as compared with the usual grid-circuit rectifier used in conjunction with a lower gain r.f. ampli- fier, A 227-type tube is used as the detector. It is self biased to prevent overloading on strong signals. Such a detector will handle a very strong signal without any appreci- able distortion due to over- load. The resistor-capacitor combination in the detector circuit is of such a value that jio grid rectification occurs and it is utilized flierely to facilitate the connection of a •pick-up unit to the detector grid circuit for reproducing phonograph records elec- trically. Output Stage Two 245-type power tubes connected in push pull are used in the output stage. These tubes are self biased and are operated at their maximum recommended voltages, making it possible to obtain Selectivity Curves 600 K.C. 1000 K.a.HOOK.C. taken at Normal Output-50 Milli watts. Modulation 30% at 400 Cycles 1000 1020 1040 1340 CARRIER FREQUENCY. K.C. Fig. 3 1360 1380 1400 1420 1440 1460 approximately 3.2 watts of undistorted power output. Either a magnetic or dynamic loud speaker may be used on this set. The mag- netic loud speaker is connected by means of pup jacks in the rear of the chassis and the dynamic loud speaker is plugged in by means of a small tube base plug. This plug also connects the field coil of the dynamic to a source of rectified d.c. for magnetizing its excitation coil. The sensitivity of this receiver shown by Fig. 2, will be seen to be nearly a straight fine, starting at 5 microvolts per meter at the low-frequency end of the band and increasing to 12 microvolts per meter at the high-frequency end. Performance As mentioned previously, the constants of the r.f. sys- tem were chosen to give good selectivity. The band width in kilocycles at 10, 100 and 1000 times normal input voltage to give a standard output of 50 milliwatts, measured across a representative load in the out- put circuit, is a measure of the ability of the receiver to sepa- rate one station from another. Fig. 3 gives such data taken at three different radio frequen- cies. A band width of 12 kc. at ten times normal input, as shown in the curve for 600 kc., means that a station 12 kc. from the station to which one is listening must be ten times as strong as the station being received in order to have the same intensity in the loud speaker. A circuit diagram giving the details of the circuit is shown in Fig. 1. Note the phonograph pick-up jacks, television con- nection, and means for connecting either a magnetic or dynamic loud speaker. Provision is also made for long, short, or light-socket antenna connections. An automatic voltage regulator is placed in series with the primary of the power transformer. BOOK REVIEWS ENCYCLOPEDIE DE LA RADIO, by Michel Adam. 356 pages, 1550 figures. E. Chiron, 40 Rue de Seine, Paris, 50 francs. 1928. This illustrated dictionary of radio terms in the French should serve a useful purpose in many American radio libraries. The alphabetical arrangement is French, but in each case the German and English equivalents are given as well. The scope of the work is somewhat narrower than Sattelberg's Dictionary of Technological Terms Used in Electrical Communication, English-German, printed in Berlin by Julius Springer, since the Adam-Chiron effort is limited to T.S.F.—this crypto- graph, it should be explained, originally stood for "telegraphic sans fils," or "tele- graphy without wires," but has now been expanded to cover the whole field of radio. With a reading knowledge of French and German, the two works in combination should enable any normally trained radio engineer to steer his way through the bulk of the foreign literature of his art. The German volumes are getting a little old (1925) while the Encyclopedic de la Radio is up to date. The descriptions of the various terms are succinct and in most cases exact and informing. Where a general term, such as "modulation," is involved, a more exten- sive discussion is presented. The size of the book, with its eight by ten inch pages, makes it possible to include a formi- dable amount of material. As in every first edition of a work of this sort, there are a fair number of misprints. The number of serious errors is apparently small. Most of the blunders are amusing but inconsequential. Edwin H. Armstrong, for instance, is consistently referred to as Edwing H. Armstrong. The English of haubaner is given as anchor, instead of guy or stay (in relation to masts) which is rather comical in view of the fact that guy comes from the Old French gui, a guide, whereas anchor is Anglo-Saxon. But these are trivialities and the book should interest all communication engineers whose hori- zon extends beyond the U. S. A. RADIO OPERATING, QUESTIONS AND ANSWERS, by Arthur R. Nilson and J. L. Hornung. McGraw-Hill Book Co., New York. 367 pages, $2.00. 1929. This book by the two well-known radio educators, Messrs. Nilson and Hornung, is a second edition of their Radio Questions and Answers. It is intended as a companion volume to the same authors' Practical Radio Telegraphy, previously reviewed in these columns. Radio Operating is the immediate recourse of candidates about to take government license or civil service examinations, while Practical Radio Tele- graphy is a more extended text for those who have time to study the technology of radio communication more fully. The present volume starts with the standard request for a diagram of a com- plete commercial transmitting and re- ceiving equipment, which has exhausted so many aspirants for operators' tickets in the inquisitorial chambers of the U. S. Radio Supervisors. Formerly, the trans- mitter shown was a spark set; now tubes are the thing. The various parts of the equipment, such as the overload circuit breaker and the radio-frequency ammeter, are described individually. Tube, arc, and spark transmitters are treated in turn. A typical question is, "What causes overheating of a transmit- ting tube?" The answer is that the plate voltage may be too high, or the circuit may not be oscillating properly, there may be a punctured plate blocking condenser, or improper bias, or the tube may be defective internally, etc. The trouble with these enumerations is that so many would-be operators memorize them as mere verbiage, with little practical idea of what the terms mean and what physical realities underlie them. That is why a theoretical text, and experience with the actual equipment, must accompany a volume like Radio Operating to make it really useful to the students. The authors recognize this in the bibliographies following each chapter. Under "Receiving Apparatus" the venerable tikker is included as a means of receiving continuous waves, demonstrat- ing that for radio equipment, at least, there is a life after the grave. The same chapter contains a brief treatment of modern radio compass technique. Later chapters take up motor-generator sets used in radio transmission, the con- struction and care of storage batteries, the radio laws of the United States (brought up to date after the Washington convention of 1927), general theory, broadcasting, and amateur station opera- tion. The appendices add useful informa- tion on examination conditions, abbrevia- tions, wavelength allocations, etc. The book has an index. 56 NOVEMBER 1929