Radio mirror (Jan-Oct 1923)

RADIO DIGEST ILLUSTRATED 11 A. B. C. Lessons for Radio Beginners IX THE previous chapter we were told about the Radio receiving station, how it consists of an antenna for intercepting the Radio waves as they pass througti space, a device known as the tuner by means of which the station can be adjusted or tuned to the desired wave length, and a detector by means of which the incoming Radio frequency oscillations are ren sttoete TM" BRASS WOO^ TO&E WITH WWOINS Figure 19 dered capable of affecting the human ear after they have been passed through a pair of telephone receivers. In this chapter we will interest ourselves with the process of tuning, as well as with the construction and operation of their various devices by means of which tuning is effected. Tuning the Radio Station Every antenna together with its leadin and ground wire has its own natural oscillation frequency. In other words, every antwnna system will respond more readily to an incoming wave of a certain length than to waves of other lengths. This natural wave length of an antenna depends upon the length of wire used and the height of the antenna above the ground. The longer the antenna or the higher it is elevated above the ground, the longer will be the natural wave length. It is evident, then, that an antenna system could be adjusted to any desired wave length by altering the length of the wire used, or by changing the height above the ground. But since either of these methods is generally not very convenient, it is common practice to add to the receiving apparatus an arrangement of coils and condensers by means of which the same effects can be produced as by altering the dimensions of the antenna. If the natural wave length of an an *3j~ DETECTOR. Slides ** 6R0UNti Figure 28 tenna system is less than the length of the desired incoming waves, the receiving station can be adjusted to the desired wave length by introducing additional inductance into the system. This is accomplished by means of some form of inductance coil connected in series with the antenna system. Such a "tuning coil" may be in the form of a one or two-slide tuner, a loose coupler, a variocoupler, or a variometer. Single-Slide Tuning Coil A single-slide tuning coil, as is illustrated in Figure 19, consists of a coil of wire wound in a single layer around a cardboard or fiber tube about four inches in diameter. The coil consists of about 150 turns of No. 23 or 24 insulated wire. cotton or enamel insulation being satisfactory. By means of the slider any number of turns can be cut into the circuit, and hence any desired amount of inductance can be obtained from maximum value of the coil to mimimum. A wiring diagram illustrating how a one-slide tuning coil is connected into a receiving station is illustrated in Figure 20. As shown, the antenna is connected to the binding post leading to one end of the coil, while the ground wire, as well as the other wire leading to the phones is connected ■* DETECTOR SLIDERS PHOMES — GROUND. Figure 21 to the slider. By moving the slider back and forth the necessary amount of inductance can thus be introduced into the antensa circuit until the station is tuned to the desired wave length. Two-Slide Tuning Coil A two-slide tuning coil is similar in construction to the single-slide coil except, as its name suggests, it is provided with two sliding contacts. A common circuit used for connecting a two-slide tuner into a receiving station is illustrated in Figure 21. As shown, the antenna is connected By Arthur G. Mohaupt CHAPTER VI to one end of the coil, the wire leading to the detector is connected to one slider and the wire leading to the ground is connected to the other slider. By moving the ground-connection slider back and forth the antenna system is adjusted to the wave length of the incoming signals, while by adjusting the other slider the detector circuit is thrown into resonance with the antenna circuit. The twoslide coil has the advantage in that it makes possible closer and sharper tuning. By closer tuning is meant the ability to tune in one station more accurately and to exclude all others. Loose Coupler A loose coupler is illustrated in Figure 22. The term coupler suggests that there are two inductively related or coupled coils. The loose coupler, as shown, consists of two coils, a primary and a secondary. The outer or primary coil, consists of a number of turns of wire wound on an insulating tube. It is also provided with a slider by means of which any number of turns can be cut in or out of the circuit. Sliding in and out of the primary is the secondary coil which also consists Figure 22 of a number of turns of wire wound on an insulating tube slightly smaller in diameter than the one used for the primary. The antenna is connected to one end of the primary coil, and the ground is connected to the slider. By adjusting the slider, the necessary number of turns can be introduced into the antenna circuit, so that it will have the correct natural wave length to receive the desired signals. Across the # terminals of the secondary coil is connected the detector circuit The incoming oscillations in flowing through the stationary or primary coil set up around it a pulsating magnetic field which induces a current of corresponding nature and frequency in the secondary coil. The amount of this inductive influence upon the secondary can be altered by adjusting the position of the coil. The signals will be heard loudest in the telephone receivers when the secondary or detector circuit is in resonance with the primary circuit. Variocoupler Another form of tuning device used is the variocoupler. This, like the loose coupler, also consists of two coils, a pri Figure 23 mary and a secondary, but differs in that the secondary rotates within the primary. The variocoupler is somewhat more efficient than the loose coupler and has the additional advantage that it can be conveniently mounted on a panel. As illustrated in Figure 23, it consists of a primary coil with a number of taps brought out, by means of which the necessary number of turns can be cut in or out of the circuit. These taps are electrically connected to a number of switch points mounted on the panel and over which a rotating switch lever moves. The secondary consists of a number of turns of wire wound on an insulating tube somewhat smaller in diameter than that used for the primary. The secondary is mounted on a rotating shaft which extends through the front of the panel and by means of which the secondary can be adjusted to any degree of inductive coupling desired. On the end of the shaft, projecting through the front of the panel a graduated dial can be mounted so that the position of the second ary can readily be observed. The antenna is connected to the end or first tap of the primary coil, while the ground connection is made to the shaft of the rotating switch, which moves over the contact points to which the taps of the primary are connected. The detector circuit is connected across the secondary or rotor of the coupler. STATOI?. SHAFT tfOTOC Figure 24 To operate a variocoupler, the antenna is first tuned to the desired wave length "by adjusting the rota'ting switch, and after this has been done the secondary circuit is thrown into resonance with the primary until the signals can be heard loudest in the telephone receivers. In the coupler illustrated in Figure 23, the secondary is mounted at an angle of 45 degrees with the rotating shaft The advantage of this construction is that it enables the rotor to be adjusted through a greater range of coupling. Variometer The variometer is another type of variable inductance. The variometer, like the variocoupler, consists of two coils, a stator or primary coil and a rotor or secondary coil. It differs from the variocoupler, however, in that there are no taps brought out on the primary coil and no variation in the length or amount of wire used is made. In a variometer the two coils, that is, the primary and secondary, are connected in series and the amount of inductance of the two windings is varied by changing the position of the secondary with respect to the primary. When the rotor and stator are in such a position that their mag netic fields act in the same direction the inductance of the variometer will be a maximum; while if the rotor is turned through an angle of 180 degrees to this position the two magnetic fields will oppose each other and the inductance will then be a minimum. By setting the rotoT at any intermediate position any desired amount of .inductance can be secured. The common type of variometer used in Radio receiving apparatus has the stator coil wound in two sections on the inner surface of two wooden blocks which are turned out to form a hollow sphere. The rotor coil is wound on a wooden ball which is mounted on a shaft and which rotates within the stator coil with a minimum amount of clearance between the two. Such a form of variometer is illustrated in Figure 24. In another type of variometer, the supports for the stator and rotor coils are made of bakelite or some other insulating compound. Such variometers are called moulded variometers and are somewhat neater in appearance, although there is still a question as to whether their operation is better than, or even as good as the operation of the wooden type. Receiving Circuit In Figure 25, is illustrated a -very efficient receiving circuit employing a vario* {Continued on page 12) \ TNOVCK B" BATTERIES Ik 1 NOTE THE INSULATED, .BINDING POSTS AND* Tin. WIRE CONNECTOR o*vo "M'-r'G CO. \ ■WesscO' W. E. SUPPLY AND SERVICE CORPORATION 18 MURRAY STREET, New York 2 Bloeks South of Chambers, between Broadway and Church St. OUR PRICES SPEAK LOUDER THAN WORDS FREE FREE FREE WITH EVERY $10.00 AND OVER PURCHASE A $1.50 22%-VOLT VARIABLE B BATTERY Mail orders must include postage. TEEMS: — Money Orders with Orders — checks not accepted. 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