Radio age (Jan 1927-Jan 1928)

RADIO AGE for January, 1927 denser may be shunted either around the primary or the secondary coil. Try it in both positions. Transformer Coupling CONNECTING the amplifier with the two-circuit crystal detector set, as described above, gives what is known as transformer coupling. The tuning coils can be placed in the antenna circuit and a radio frequency amplifying transformer used for coupling the amplifier to the detector circuit. It will give increased amplification. Connecting the amplifier with a singlecircuit crystal set, as stated, gives resistance coupling. Each type of coupling has advantages and disadvantages. Government experts have stated that resistance-coupled amplifiers seldom give full amplification below 1,000 meters. On the other hand, they save some troubles due to distortion. They require more "B" battery power than transformercoupled amplifiers, so they are less desirable for use in portable sets. One weakness of the radiofrequency transformer is that usually it covers a narrow band of wavelengths. The United States Bureau of Standards built several hundred of them, studied them exhaustively and produced a type that gave good amplification over a comparatively wide range. One of the men who worked on the problem resigned from the Bureau and manufactured transformers of this type, but the fact that he made them in plug-in form, so that one could be removed and another substituted easily, indicates that even the best do not give equal amplification over the entire broadcastin range of wavelengths. Some have found it difficult to understand how a radio frequency amplifier amplifies weak signals more in proportion than strong signals, while an audio frequency amplifier amplifies strong signals more than weak ones. The secret lies in the fact that the radio frequen cy amplifier amplifies the voltage applied to the detector and is not concerned with increasing the power output, while the audio frequency amplifier must amplify the power available to actuate the diaphragm of the phones or the loud speaker. Vibrating a diaphragm or cone and propagating sound waves One stage of radio frequency amplification resistance-coupled to single-circuit crystal detector set that must run through thousands of cubic feet of air and make themselves heard by many ears requires much more energy than it does to increase the grid potential of a tube. The small amount of energy used to produce the changes of grid voltage, or the voltage applied to the crystal detector, is used up in the tube and its circuits, while the energy in the The Magazine of the Hour 7 plate circuit of the tube comes entirely from the "B" battery. Grid Influences Plate VARIATIONS in the grid voltage which, in the case of the single radio frequency amplifying tube, are caused by the very small amounts of power gathered from the radio waves, influence the plate current much more than changes in the plate voltage itself. With the plate voltage at 40, for example, an increase of grid voltage from .04 to 1. increases the plate current from 430 to 530 microamperes, or 167 microamps for each volt. Increasing the plate voltage of one volt increases the plate current only 21.6 microamps. The increase of grid voltage is eight times more effective, therefore, than a corresponding change of plate voltage. This explains why the radio frequency amplifier can be used effectively with a crystal detector receiver, and give it twenty times as, great a range, while the best an audio frequency transformer could do would be to increase the volume on such stations as probably could be heard, at least faintly, with the crystal detector without amplification. VAR1A&UE CONDENSER. 00O25M.F.D DETECTOR UNIT In order to show the entreme simplicity of the crystal set, we are reprinting the diagram above which shows the coil, condenser and detector unit. Taps are shown on the coil, although they are not absolutely necessary — a honeycomb coil of 50 turns, or 50 turns of bell wire on a 3 inch form, will suffice. Full details of this simple crystal receiver appeared in the August issue of Radio Age (1926) page 17