Radio broadcast .. (1922-30)

RADIO BROADCAST No. 304 RADIO BROADCAST Laboratory Information Sheet November, 1929 Distributed Capacity Measurements THE method commonly used in laboratories to determine the distributed capacity of a coil is to tune it to various wavelengths by means of a condenser and then plot a curve of wavelength squared against the capacity of the tuning condenser. The curve will be a straight line but will not pass through zero because of the distributed capacity of the coil. If the curve is extended so that it intercepts the line corresponding to zero wavelength, the intercept will give the distributed capacity of the coil. The method is simple and quite accurate pro- vided the individual measurements are care- fully made. If, however, there are {slight dis- crepancies in the various measurements it is necessary to estimate as accurately as possible the correct position for the curve. There is another method of graphically determining the distributed capacity which is not generally used but which is sometimes more accurate than the one described. This second method is illustrated on "Laboratory Sheet," No. 305. The general method of procedure is similar. The coil to be measured is connected across known capacities and the resonant wave- length is determined. Some sample data is given below: WAVELENGTH CAPACITY TO TUNE TO RESONANCE-MMFD. 300 315 247 200 134 0 The next step is to lay out a curve sheet as shown on " Labora toryf Sheet" No. 305. The left- hand axis is wavelength squared and the right- hand axis is the tuning capacity in micromicro- farads. Straight lines are now drawn being the various values of tuning capacity and the cor- responding values of wavelength squared. If all the measurements were perfect, these various straight lines would . li intersect at a common point but because of *i.*:ht inaccuracies they do not. As a result there is formed at the center a small polygon. The center of this polygon must now be estimated and between the center and the point corresponding to zero wavelength a straight line is drawn. This line will intersect the capacity axis at the point'corresponding to the distributed capacity of the coil. This latter line is shown dotted on the curve. No. 305 RADIO BROADCAST Laboratory Information Sheet November, 1929 Distributed Capacity Measurements No. 306 RADIO BROADCAST Laboratory Information Sheet November, 1929 Advantages of Automatic Volume Control T^HE use of automatic volume controls in ' receivers has certain definite advantages. The most obvious advantage is, of course, that such a control definitely determines the output of the receiver and maintains this output con- stant over wide variations in field intensity. Ordinarily as we tune from one local station to another the volume varies considerably, depend- ing upon the field strength obtained from the station, but in a set equipped with an automatic volume control, all stations will give approxi- mately the same volume. The second advantage of an automatic volume control is that it helps to some extent to reduce the effects of fading, since, as the signal begins to fade, the sensitivity of the set automa- tically begins to increase and in this way partially compensates the fading. A third advantage of this control system, is that by its use it is (possible to apply an input to the detector tube of a definite value of r.f. voltage. The set may be so designed that with this value of voltage applied to the detector, the distortion produced in the^detector circuit will be a minimum. The distortion ordinarily produced in detector circuits is a function of input voltage. It is high for small values of input voltage and also for very large values of input voltage. At some medium values deter- mined by the operating voltages of the detector tube, the distortion will be a minimum and it is of course advisable to operate the detector tube always under the conditions for minimum dis- tortion. These three advantages are responsible for the greatly increased use of automatic volume control systems and it is probable that in the future their use will become quite general. The automatic volume control tube generally works on the output of the r.f. amplifier and it automatically functions to control the output of the r.f. amplifier by varying its sensitivity. When the field strength is very high, the volume control tube causes a large reduction in the sensitivity of the amplifier and when the field strength is very low the tube functions to main- tain the radio frequency amplifierlatfmaximum sensitivity. THE ROCHESTER I. R. E. DISTRICT CONVENTION The following papers are scheduled to be given at the Eastern Great Lakes Dis- trict Convention of the Institute of Radio Engineers to be held at Rochester on November 18 and 19: "Considerations in Screen-Grid Receiver Design" by W. A. MacDonald of the Hazeltine Corpora- tion; "What Executives Expect of Engi- neers" by I. G. Maloff of the Valley Appliances Corporation; "Ultra High Frequency Transmission and Reception" by A. Hoyt Taylor of the Naval Research Laboratory; "A Broadcast Receiver for Special Purposes" by Paul O. Farnham of the Radio Frequency Laboratories; "Standardization in the Radio Vacuum Tube Field" by W. C. White of the Gen- eral Electric Research Laboratory; "The Engineer in the Radio Industry" by H. B. Richmond of the General Radio Company and president of the Radio Manufacturers' Association. Those attending the convention will be taken on inspection trips to Stromberg- Carlson plant, Kodak Park, and the Valley Appliance Corp. The Rochester, Buffalo-Niagara, Cleve- land, and Toronto Sections of the Institute are sponsoring the Convention, while the entire membership of the Institute will] take part in its activities. SELLING RADIO—THE BIGGEST SHOW ON EARTH (Concluded on page 23) But if you have made yourself an expert in your complete knowledge of broadcast- ing you are right on your customer's own street. You are a human being, therefore, you like to tell what you know. You know electricity and mechanics. Make yourself a student of the drama of radio, its comedies, its utilities, its music, and its bally-hoo—all the elements which make it a big show. Get full of that kind of knowledge. Inevitably, as a human being, you will want to talk it, to tell what you know. Then conversationally you find yourself on the customer's own street and your native sales ability gets a fifty per cent, better chance to do its work. You may even learn some things about broadcasting which the broadcasting sta- tions themselves don't .get, because your contact with the broadcasting audience becomes more intimate and more out- spoken. The radio business to-day is in the stage where both broadcasting and the receiving set trade do pretty well with the large numbers of people who really know they want to buy. Suppose the motor car business were on that same basis. I venture to say that the total number of cars in use in the country might possibly be equal to what this year's sales will be, might be less. Automobile makers, oil producers, re- tailers, highway builders, local boards of trade, state governments, all the resorts, merchants of all sorts, real estate people, all these and others are boosting the retail sale of automobiles for all they are worth, more cars, better cars, two to a family, then, three. No, it is not quite a parallel case. I've tried to admit it before you said it. But the opportunity of profitable cooperation is in radio to similar degree, and for lack of that cooperation, radio is in some danger of slipping into a we-all-have-it and a take-it-for-granted condition long before its novelty, its romance, its daily gift of pleasure have been appreciated fully. The romance of radio and the friendly gossip of radio are still very salable. 58 • • NOVEMBER 1929