Radio Broadcast (May-Oct 1926)

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262 RADIO BROADCAST ADVERTISER WHY SNUBBERS? Did you ever ride in an automobile without shock absorbers or snubbers? If so, you know what happened when you hit a bump. The better the springs, the longer the up and down vibrations continued — hard on the rider and hard on the car. That's why automobiles require snubbers to damp out spring vibrations — — and that's why B-T UX sockets are designed to absorb vibrations as well as shocks. The new B-T socket is the result of years of intensive study of the problem of protecting the vital, delicate tube elements — it absorbs the shocks that cause damage to the tube and stops the vibrations that nun reception. UX Detector Socket. Kl .00 The Detector Type carries double absorbers — top and bottom — its efficacy has been proved by our year's experience with the B-T Silent Socket(still in use with Universal Base, SI. 25). The spring contacts of the new UX are noteworthy — and show typical B-T efficiency. Long contact surface — soft and yielding to prevent side strain — with continuous flexible leads. YOUB TUBES MUST HAVE PROTECTION USE B-T SOCKETS IF YOU BUILD INSIST ON B-T SOCKETS IF YOU BUY HlSf WATCH I lie B-T Line and Profit Authorized dealers are now being franchised on the new Counter + phase Seven — when you hear the story you'll understand the rush. BKEMER-TULLY MFG. CO. .">:t2 South Canal Street Chicago, 111. Radio Broadcast Laboratory Information Sheet July, 1926 A Batteries THE DRY TYPE L70R the majority of receivers using type 199 tubes, a bank of dry cells can be used to supply the filament current. For portable sets such an arrangement is very convenient, and although, in general, the operation of these tubes will be found somewhat more expensive than storage battery tubes, their added convenience usually more than compensates for the greater cost of upkeep. The common type of dry cell usually consists of a zinc container (which also acts as one element of the battery) in which is placed the active material. The active material is usually a mixture of powde r e d carbon and manganese dioxide moistened with a solution of sal ammoniac. Between the zinc container and ~rj«44 volts-p ® the active material, there is usually placed a layer of blotting paper. The layer of paper acts, not only as an absorbent of some of the electrolyte but also as a separator which prevents the manganese dioxide from coming into contact with the zinc. If such contact does occur, an internal short-circuit takes place and the cell becomes useless. The zinc case of the cell forms the negative terminal, and the positive terminal is a carbon rod that is placed in the center. This carbon ( ^^>\ rod is insulated from the zinc shell and does not react , chemically with any of the ftmmeter © other substances used. The current from any one cell should not exceed one-quarter ampere. In the case of portable sets, it is not always possible to use that number of cells which would give greatest efficiency. In an installation in the home, arrangements should be made to use sufficient cells for most effective operation. For any receiver using up to four 199 tubes, only three dry cells are necessary, connected as is shown in A on the accompanying diagram. If the receiver uses more than four tubes, two banks of dry cells should be used connected as shown in B. Dry cells can be tested most easily by means of an ammeter. The instrument should be capable of reading up to about 50 amperes, and in testing the cell, it should be connected as in C. The cell should be thrown away if it reads less than five amperes. 15 THEORY OF OPERATION A LOOP antenna is quite commonly used in connection with multi-tube receivers, especially super-heterodynes. The action of a loop is not quite as simple to understand as is the action of a simple antenna. The theory of the operation of a loop is commonly explained in the following manner. Suppose we have two vertical wires separated by a distance of 200 meters, both of them insulated from each other and from the ground. Now, if a wave approaches from a direction perpendicular to the plane of the two wires, the wave will reach each wire at exactly the same time, and the voltages induced will be exactly in phase. If the wave approaches from some other direction, it will reach the two wires at different times and, therefore, the induced voltages will be out of phase with each other. If the wave approaches in the direction of the plane of the two wires and has a wavelength of 400 meters, the two induced voltages would be 180 degrees out of phase. Therefore, the voltage at the top of one wire will be a positive maximum when the voltage at the upper end of the other wire is at a negative maximum. Now, if the upper ends are connected together and the input to a receiver is connected across the lower ends, current will flow around the circuit, and if the circuit is tuned by a condenser, the currents will become comparatively large. The induced voltages will be greatest when the wave and the loop are Radio Broadcast Laboratory Information Sheet July, 1926 both in the same plane, since this will result in maximum phase displacement between the voltages induced in the front and rear wires of the loop. With regard to the design of loops, it will generally be found that the current induced in the loop varies directly as the area, directly as the number of turns, inversely as the resistance, and inversely as the length of the wave being received. The common type of loop antenna consists of several turns of wire wound on a rectangular form. The turns should be spaced about one-half or one inch from each other, so as to keep the capacity low. The distributed capacity of a loop also increases with the number of turns. This capacity increases rapidly with the first few turns, and then the rate of increase becomes slower. A very satisfactory loop for use with a 0.0005 mfd. condenser can be made by constructing a four-foot square form and winding on it six turns of No. 22 wire. Such a loop would have a range of from 1500 kc. (200 meters) to 600 kc (500 meters). Generally, for satisfactory operation, no connection to ground is necessary. However, somewhat louder signals can usually be obtained if the low potential end of the loop is connected to ground. When such a connection is made, it is likely that the loop also acts as a small antenna by reason of its capacity to ground. In this connection, it should also be pointed out that the inner end of the loop should always be at the lowest potential. No. 16 Radio Broadcast Laboratory Information Sheet Carrier Wave Analysis HETERODYNE INTERFERENCE July, 1926 RADIO waves travel with the speed of light — 300,000,000 meters per second. Now, in any wave motion, the frequency, or number of waves passing a given point per second, multiplied by the wavelength, gives the speed with which the waves are traveling. If a train of railroad cars passes a given point at the rate of two cars per second and each car is fifty feet long, the speed of the train is obviously one hundred feet per second A 5.000 tOWER SIDE BAND CARRIER UPPER SIDE BAND FREOUENCY Quite similarity, if the frequency of passing radio waves is one million per second, then the length of each wave must be 300 meters to make the speed come out the value stated above. Broadcasting stations have a frequency separation of 10 kilocycles to prevent heterodyning, and no uniform wavelength separation can be given that will be applicable throughoul t he broadcasting band. It we work with wavelengths, we must calculate anew the width of channel expressed in meters lor every different wavelength. Thus a 10-kilocycle channel at three-hundred meters wavelength is only a three meter channel, while at three thousand meters wavelength, it is a three hundred meter channel. There are about nine times as many 10-kilocycle channels available between the wavelengths 30 and 300 meters as there are between 300 and 30,000 meters. For very high quality music, all tones between about 30 and 5000 vibrations per second should be transmitted with equal efficiency. To transmit the former, we must transmit a frequency 30 cycles greater than the carrier and another 30 cycles less than the carrier, in addition to the carrier itself. To transmit the 5000-cycle note we must use the frequencies 5000 greater and 5000 less than the carrier, and to transmit all the intermediate tones, we must use the two bands of frequencies (called the upper and lower side bands) shown in the accompanying diagram. The whole range of frequencies used is called a "channel." In the case just described, the width of the channel is 10,000 cycles. The important thing about all this is thai broadcasting stations do not use only a single frequency or wavelength as might be supposed from the figure given at the lop of the newspaper radio programs (that figure is the frequency of their carrier wave in kilocycles per second), but they each require a channel of definite width, and hence only a rather small number can work at once without their channels overlapping. Overlapping results in a continuous whistling sound (of high pitch if the channels overlap only slightly, and of lower pitch if the overlapping is greater). -fa Tested and approved by Radio Broadcast if