University of San Francisco - USF Don Yearbook (San Francisco, CA)

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THREE E LEM EXT YAC1TM TV BE 20 crophonic detector might be used for these types of waves but tlie same does not hold for continuous waves which are damped and which make no impression on any detector except at the beginning and end of such a wave and even then it is only evidenced by a click. When the crystal detector was in vogue it was customary to use some sort of mechanical interrupter in the circuit which was called a tikkcr. This tikker broke up the incoming continuous wave some three hundred to one thousand times a second, thus rendering the wave, when rectified, audible. This method was popular because there was no other. At the best it was crude and could be no better than the crystal detector that was used with it. The advent of the Vacuum Tube caused a hundred per cent improvement in both the detection of damped and undamped waves and in the method of rendering the 0. Y. audible after detection. The Vacuum Tube method entirely supplanted the old “Tikker” method. The system by which the ( AY. is rendered audible is sometimes referred to as “Beat reception” but is more properly called “Heterodyne.” The desired effect is obtained in the Vacuum Tube by supplementing the incoming radio frequency wave with a locally generated current which differs in amplitude by an amount which lies within the range of audibility. The resulting fortification and interference which takes place due to periodic synchronism gives rise to a third or “beat current” which, when properly rectified is audible. In this case as in detection the input is a trifle greater than the output due to the addition of the incoming wave to that locally generated by the tube. A brief account of the two methods, by which damped oscillations may be detected, follows: (1) The grid current is maintained with respect to the cathode, so that a negative grid charge will be followed by a slight decrease in the anode current or a positive charge on the grid will produce a relatively high increase in the plate current and the receivers are affected once for each

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28 TIIE IGXATIAX frequency would make absolutely no impression on the phones. A conversion is necessary to render the transmitted wave audible. These audio frequency currents are not necessarily direct currents in the strict sense of the word. They may be groups of separate direct current impulses but it does not make much difference provided that they affect the diaphragm of the phone under about fifteen hundred impulses a second. There are two general systems of wireless transmission— the continuous wave or C. Y. as it is called in the parlance of the profession, which sets in motion waves of continuous amplitude and undamped oscillation. These are of radio frequency. The discontinuous wave consists of oscillations which are not continuous in their passage through the ether. Furthermore the amplitude of the oscillations are not constant. Instead, after such waves have been given their initial start by the first power stroke of the transmitter, they rise to sudden height and gradually fall lower and lower in amplitude until they are finally damped out completely. The next stroke of the transmitter causes them to rise again and the rising and falling process keeps up indefinitely, depending on the amount of energy in back of it. Thus this sort of discontinuous or damped wave, as it is technically called, travels through space till it is exhausted; likewise its carrying powers are entirely proportionate to its initial energy. It is produced by the spark type of transmitter and has been in use since the inception of wireless communication Another phase of C. AA transmission is what is known as I. C. Y. or Interrupted Continuous 'Wave. I. C. Y. is practically the same form as C. AY. with the exception that an interrupter is inserted in the radiating circuit of the transmitter to break up the emitted waves so that they will be heard as universal audible tones at the receiving end; otherwise a special receiving circuit must be employed to render the waves audible as is done in C. AY. work. Damped and T. C. AV. waves may be detected by the ordinary detector and require no special hook-up for reception. Even a coherer or a mi-

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30 THE J ax ATI AS incoming wave group due to the flow of the rectified current. (2) This is the 44Grid condenser method. ’ With a properly constructed tube it is most sensitive because the plate potential varies at an audio frequency due to the condenser interposed somewhere in the grid circuit. Unidirectional current passage between the filament and the grid rectifies the impressed groups of radio oscillations and the condenser consequently receives an unidirectional charge which, in building up, causes the plate current to decrease. When the incoming oscillations have ceased the condensed charge dissipates through a high-resistance grid-leak and the plate potential returns to normal. The effect of inter-electrode capacities is next to be considered. Remembering the fact that a slight variation of the grid potential greatly changes the strength of the plate current, we can appreciate the fact that this phase of operation has two broad applications with respect to Wireless. Primarily it causes the Tube to be a self-contained amplifier which in no way interferes with its detective qualities, which may even be simultaneous, and secondarily, it is in this respect that the tube is used as an oscillation generator. These characteristics have been fully set forth by their discoverer, (’apt. E. H. Armstrong. In some types of circuits there is a marked difference in the action of the tube than what might have been expected. This is due to inter-electrode capacities. The effect of these is to cause a tube to produce oscillations even if there is no mutual inductance between the input and the output. It is also on account of these that there is an impedence between the cathode and the grid which depends both on these capacities and on the constants of the input circuit. When oscillations are impressed on such a circuit they die out at a rate which varies as the total resistance. If the impedence is negative the total resistance is reduced and there will be a smaller consumption of current in the input circuit and the tube will give an increase of amplification. This increase of amplifi-