OCR Text |
Show j (Edited by G. Douglas Wardrop. Editor of Radio Merchandising.) W fa j j To itiise Warea?eU nJ --X; TffLowcrhkvtkisth t "L 4 , I C fief or to Creufid I For a Better Four-Circuit Set. Introduction of Variometer and Proper Condenser Will Be of Great Help. By PETER MOMBELLO Those fans using a four-circuit tuner have noticed that the control of the antenna coil is not good. For 43 turns of wire there are only seven taps (1st, 3d, 7th, 13th, 21st, 31st, 43d) and the number of turns that are jumped when changing from one tap to another cannot afford as fine a control con-trol of the primary circuit as would be desirable. Also in the tapped coil only a portion of the windings is actually ac-tually in the circuit. This causes greater distributed capacity and resistance. re-sistance. The added capacity simply increases the wave length of the coil and the resistance dampens the circuit cir-cuit or decreases selectivity. All this is commonly known as dead end loss. This may not be extremely Important, as regeneration makes up for such losses, but as mentioned before, finer control is desirable. The only instrument that can give this finer control and, if properly selected, se-lected, will do away with dead end losses, is a variometer. In a variometer variom-eter the inductance is varied by moving the rotor within the stator, and with a good vernier on the variometer variom-eter dial this variation can be as fine as it is possible to get with any instrument. instru-ment. Then it will be noted that all the turns on a variometer are used at all times, and because of this no dead end losses can detract from the effi ciency of a set using a variometer. For these reasons it can be readily seen that the substitution of a variometer variom-eter for the tapped antenna coil will be a great improvement in the four-circuit four-circuit tuner. But to make this change worth while the variometer must be selected with great care. Three Important Things. There are three things that must be considered In variometer construction low resistance in windings, low internal in-ternal capacity and proper inductance value. Wooden variometers, if well made, will have a low capacity value, but shellac, varnish and poor rotor bearings will make them unsatisfactory. unsatisfac-tory. Molded variometers are good, but their high Internal capacity makes them also unfit for this use. For low internal capacity the best variometers on the market are those of self-supporting type. In single layer form, or still better, in lattice-wound form. The windings should be attached to the frame In such a way that the frame does not Itself act as a dielectric and thus create capacity effects. Excessive binding material on the windings should also be avoided. For low re- sistance it is merely necessary that the wire be large enough. To get the number of turns on a reserved space the manufacturer often resorts to liner gauge wire. This causes resistance and impairs fine tuning. The wire used should he No. 20 or larger. Practically Prac-tically all the self-supportig variometers variom-eters now on the market have a high enough ratio of inductance for this circuit, cir-cuit, so this is all that need be said about this matter. Now, in convening the set, the antenna an-tenna loading coil is removed from the regular coil, as it will only binder line operation. The variometer can be put in a small unit by itself and connected outside of the set, but It Is advisable and more efficient to rebuild the whole set on a new panel following the diagram dia-gram in Fig. 1. The variometer Is best placed at the left of the variable condensers, con-densers, near the ground binding post. It is to be placed so that the stator windings will be at right angles to the wluding of the coil and connected with the rotor toward ground potential. poten-tial. The coil should be placed as far away from the variometer as possible without having the connections to the variable condensers too long to cause a loss. The Fixed Condenser. The value and location of the fixed condenser shown at No. 1 and No. 2 cannot be given, since the inductance of Ihe variometer determines this factor, fac-tor, if the set tunes high place the condenser at No. 2. anil if it tunes low place it at No. 1. II is best, however, to place the condenser ni No. 2 and then experiment with its value. Anything Any-thing between .00025 and .OV'2 may be ideal. Perhaps the varioinoier selected can cover the necessary wave length band without any condenser whatever. In such a case the small fixed condenser con-denser should not he used. Hut If a condenser Is needed It should be of the rugged mica type that cannot he squeezed or easily bent. Also try the variometer connection shown in the dotted line A B in conjunction with the condenser, removing the connection between A and C. The set is tuned In exactly the same manner as before except that the variometer vari-ometer takes the place of the taps. Revolve it until the loudest point Is obtained ob-tained and then adjust the condensers Considering that dead end lossea are done away with and finer control Is obtained, this conversion Is well worth the trouble and expense made on It. New York Herald. |