Radio Broadcast (May 1928-Apr 1929)

56 RADIO BROADCAST ADVERTISER Here's the Answer to every question about the principles, methods, or apparatus of radio transmitting and receiving. THE RADIO MANUAL A Complete Course in Radio Operation In a Single Volume A Handbook for Students Amateurs Operators Inspectors For the first time an entire course of training in one book — the most complete and up-to-date work on radio. Developed simply and clearly from the elementary stage right through all phases of principles, practise, and apparatus so that a beginner with no knowledge of electricity get all he needs either for amateur operation or to qualify for a government license as operator or inspector. Prepared by Official Examining Officer The author, ©. E. Sterling, is Radio Inspector and Examining Officer, Radio Division, U. S. Dept. of Commerce. The book has been edited in detail by Robert S. Kruse for five years Technical Editor of QST., the Magazine of the Radio Relay League. Many other experts assisted them. 16 Chapters Cover: SnSSSfSS tors and Generators; Storage Batteries and Charging Circuits; The Vacuum Tube; Circuits Employed in Vacuum Tube Transmitters; Modulating Systems; Wavemeters; Piezo-Electric Oscillators; Wave Traps; Marine Vacuum Tube Transmitters; Radio Broadcasting Equipment; Arc Transmitters; Spark Transmitters; CommerciaJ Radio Receivers; Radio Beacons and Direction Finders; Radio Laws and Regulations; Handling and Abstracting Traffic. F« fAnn/iii'An never before available such mew information as a compiete description of the Western Electric 5 Kilowatt Broadcasting Transmitter; description and circuit diagram of Western Electric Superheterodyne Radio Receiving Outfit type 6004-C; Navy Standard 2-Kilowatt Spark Transmitter; etc., etc. Every detail up to the minute. Last Chance for Special Low Price "The Radio Manual" will be ready in a few days. Over 900 pages. Profusely illustrated. Bound in Flexible Fabrikoid. Regular price after publication will be $6.00. Orders received now will be accepted at the special advance price of $4.05 — an amazingly low price for the best Radio book available. Send for .free examination. Pay or return in ten days. Order on This Coupon \ D. Van Nostrand Co., Inc. 8 Warren St., New York Send me THE RADIO MANUAL for examination. Within ten days after receipt I will either return the volume or send you $4-95 — the special advance price.. (Radio Broadcast 11-28) Name St. & No City and State SHEETS THE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, set builder or service man. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, "Radio Broadcast's Data Sheets" may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. — The Editor. No. 233 Radio Broadcast Laboratory Information Sheet November, 1928 Balancing Radio Receivers AN EASY METHOD '"PHE change in the plate current of the detector ^ tube, when a signal is being received, may be utilized to balance the various tuned circuits in a single-control receiver. If the several tuned circuits in a multi-stage r.f. amplifier are not properly ganged, the set will be insensitive and the selectivity will be poor. It is essential, therefore, that the various stages be accurately aligned. How this can be done is the subject of this Laboratory Sheet. The method used is simple and is based on the action of a detec 1 tor when a signal is being received. If a milliammeter with a range of about 2 milliamperes is connected in series with the B-plus lead to the detector, as indicated in the diagram, it will be found to read about 1 mA. if the detector is of the grid leak and condenser type and about 0.2 mA. if a C-battery type detector is used. If a station is tuned-in, the plate cur lstA.F. rent of the detector tube will decrease if the former arrangement is used and increase with a C-battery detector circuit, the amount of the increase or decrease being proportional to the strength of the signal— the stronger the signal the greater the change in current. Therefore, when the set is accurately tuned and all of the condensers are perfectly aligned the deflection of the meter — and therefore the output of the set — will be greatest. Balancing therefore becomes a matter of tuning in some station, preferably one operating on a short wavelength, and then adjusting the various condensers, by whatever means are provided by the manufacturer, so that the greatest change is indicated on the meter in the plate circuit of the detector. When circuits have been adjusted so that the greatest current change is obtained, the set is balanced. It is best to make this adjustment with the set tuned to a short wavelength, for it is in this part of a receiver range that the greatest lack B + Det. of balance is liable to occur. No. 234 Radio Broadcast Laboratory Information Sheet November, 1928 The Audio Transformer operation at high audio frequencies TN LABORATORY Sheet No. 227, in the October number, we studied the characteristics of audio transformers and pointed out that the lowest frequency response depends upon the ratio of the reactance of the transformer to the plate resistance of the tube. Here we will consider the high frequencies. For convenience we have reprinted here the diagram from Sheet No. 227. At high frequencies the reactance of L is very large in comparison with C, and it may therefore be neglected. Essentially, we then have a circuit consisting of Rp, La and C in series. As Li and C come into resonance, the impedance of the circuit will decrease and more current will flow, thereby tending to increase the voltage across C, which is the voltage applied to the grid of the next tube. However, the voltage across C, for a given current, is inversly proportional to the frequency, and this will tend to lower the voltage across it at high frequencies. In some transformers, however, there is a marked peak at about 6000 cycles, corresponding to the resonant frequency of La and C in series, the output falling off rapidly beyond the point. If this upper resonant peak is very pronounced the gain of the entire amplifier will increase greatly at this point, tending to make the amplifiers oscillate. Good design requires that the peak be kept as small as possible. At frequencies higher than that at which La and C resonate, the reactance of La continues to increase, and the reactance of C to decrease; therefore, the voltage across C rapidly falls. If some transformer curves are examined, it will frequently be found that the curve drops rapidly beyond the upper reso>nant point. The problem of design is to adjust the leakage inductance. La, and the effective capacity, C, so as to give satisfactory high-frequency response.