Radio Broadcast (May-Oct 1922)

362 Radio Broadcast Musi different transformers be employed for various wave lengths? — J.B.IV., Oakland, California. THE essential difference between radio and audio fiequency amplification is this: With radio frequency, the very slight current produced in the receiving antenna system by passing waves from a transmitting system are caught and passed through amplifying devices designed to permit this cuirent to oscillate — that is, to How back and forth at the same frequency it passes through the ether. With audio frequency the current from the detector tube is passed through successive amplifying stages, not at the natuial frequency of the signal as it passes through the ether, but at a frequency very much lower, which is within the range of audibility. In the case of radio frequency amplification, the incoming signals are amplified by means of a local source of energy before they reach the detector tube, while audio frequency amplification takes place after detection. Detection requires a certain amount of energy for its proper functioning and it is obvious that several stages of audio frequency amplification would be valueless where the strength of the incoming signal was insufficient to produce detection. It is here that ladio frequency is valuable for it builds up the infinitely weak signal to a point where proper detection may take place, and from this point on it is possible to increase the signal audibility by the audio frequency amplification method. Radio frequency amplification alone ■\^ill not operate a loud speaker over any material distance. In fact, the general rule may be laid down that loud speakers may only oe employed where at least one or two stages of audio frequency amplification are employed. Radio frequency amplification has not been very popular in amateur circles until recently for the reason that different transformers were required for the various wave length ranges and the range of any one transformer usually covered but a few hundred meters. This difficulty has been materially reduced by the introduction of a new radio frequency transformer designed to function satisfactorily over a particularly broad range of wave lengths. This broad "range is made possible by taking advantage of the balancing effect found to exist when an iron core radio frequency transformer is employed. A transformer of this character having a wave length range of 200-5,000 meters may now be had and another transformer having a range of 5,000-25,000 may also be procured. For all practical purposes, the wave length ranges covered by these two transformers permit the operator to receive on practically all of the wave lengths now in use. A very significant fact regarding radio frequency amplifiers is that the results obtained by a single stage of radio frequency amplification and a vacuum tube detector nonregenerative circuit are approximately the same as those obtained by a vacuum tube detector alone, employed in a regenerative circuit of proper design. ■ JVill you please furnish me with a suitable circuit diagram for employing radio frequency arnplification with a regenerative receiver and two steps of audio frequency amplification? —0. H., Brooklyn. N. Y. THE accompanying diagram illustrates a suitable method for employing radio frequency with a regenerative receiver and two stages of audio frequency amplification. .As will be observed, a tuning coil and variable condenser are connected in series with the aerial and ground. An energy-absorbing circuit is thus formed, which feeds into the first radio-frequency amplifier tube. The second tube also acts as a radio-frequency amplifier and its transformer is of the tuned variety made by shunting a variable condenser across what would ordinarily be the antenna and ground connections of a vario-coupler or loose coupler, for the primary and the usual secondary circuit for the tuned secondary. Two potentiometers are used in this circuit; one to supply the correct plate potential for the detector tube and the other as a stabilizer for the radio-frequency amplifier tube. The transformer between the first and second stages of radio frequency is of the laminated iron core type having a wavelength range of 200 to 5,000 meters. When audio frequency is not desired, it is merely necessary to connect the telephones between the variometer in the plate circuit and the negative side of the "A" battery.