Radio Broadcast (May-Oct 1922)

38 RADIO BROADCAST receiving outfits is the impression that a "loud speaker" attached to a simple crystal receiving set will provide sound great enough to fill an entire room. Except at very short ranges this is not true. Radio "loud speakers" operate on precisely the same principle as the phonograph. A great vibration of the diaphragm produces a great sound and a comparatively smaller vibration of the diaphragm results in the correspondingly decreased sound volume. With phonographs this is brought about by the use of loud and soft tone needles, although the same record may be employed. With radio, the volume of sound produced by a loud speaking device depends upon the energy received from the transmitting station. With a simple crystal receiving set "loud speakers" cannot be employed directly, unless the receiver is located very close to the transmitting station. Where a vacuum tube detector is employed, the distance from the receiving station may be increased somewhat, but even this method is not recommended for general use. Regardless of what type receiver is employed, where the distance from a broadcasting station is more than a few miles, it is necessary to use an amplifier where a "loud speaker" is desired. An amplifier is a combination of units, which, working together, build up signal energy from the original energy absorbed from the air by the receiving aerial system. In general, amplifiers comprise one or two vacuum tubes with the necessary connecting equipment and controls, operated by a 6 volt, 40 to 100 ampere hour storage battery and two or three 22.5 volt " B " or plate batteries. Where two tubes are used, the amplifier is called a double stage amplifier, and means is generally provided for making instantaneous connection to the first or second stage at will, thus regulating the volume of the signal produced. The action of this character of amplifier is quite simple. The incoming radio signal is passed through the tuning elements of the receiver to the detector tube or crystal as the case may be and from here it is carried into the first amplifier tube. This amplifier tube functions as a valve and this valve is controlled by the intensity of the incoming waves. When the energy of the incoming wave is great, the valvular action of the tube is great; when the incoming energy is small, the reverse is true. This valvular action draws a current from the plate battery which is passed through the telephone receivers or "loud speaker" as the case may be. For every variation in the antenna current there is a very considerably augmented variation in the plate battery current, so that the resultant signal is very greatly increased. Where two stages of amplification are employed, the signals from the first stage are used to control the valvular action of the second amplifier tube and the resultant energy is carried from that plate circuit to the telephones or "loud speaker." With each stage of amplification of the signal, audibility is increased from six to ten times. More than two stages of this character of amplification are not recommended for ordinary use because there is a tendency to amplify disturbances from the atmosphere as well as disturbances from local trolley, power, and telephone lines, causing the production of parasitic noises which interfere with the reception of speech or music. Although the range in miles over which the various classes of receiving outfits may be counted upon to function satisfactorily cannot be judged closely, the following table may be found of value, and where a dealer recommends the use of apparatus listed therein for the accomplishment of greater work, the consumer should investigate very thoroughly before concluding that the information is correct. There are exceptional cases, when. the ranges will be found to be greatly extended, but they cannot be considered as standard. For instance, the range for any receiver is much shorter during the day than it is at night, and it is a common thing for stations having a normal range of 15 or 20 miles to receive from distances up to 50 or 60 miles. — 1. Simple crystal receivers with outdoor aerial 15 miles, with indoor aerials 3 to 5 miles, with loop aerial i to 3 miles, with outdoor aerial and "loud speaker" about 2 miles. 2. Vacuum tube receiver operated by dry cell with outdoor aerial — 75 miles, with indoor aerial 30 miles, with loop aerial 5 to 10 miles, with a "loud speaker" and outdoor aerial 5 miles. 3. Vacuum tube receiver with two stage amplifier and "loud speaker" 75 miles, with telephones 150 miles. Loop aerials are not recommended for use except by persons having a thorough knowledge of their capabilities and limitations. A method has been devised for increasing