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tained from the magnetic effect of the incoming waves applied directly to a suit- able air-core or self-excited telephone, since the magnetic force acting on the diaphragm depends upon the product of the two currents in the coils 6 and 14, and that in 14 from the local generator may be made quite large.
Figure 3 shows the duplex hetero- dyne system. Here the receiver just described has added to it, in series with the antenna, a radio-frequency alternator powerful enough to generate the strong waves used in sending. This transmit- ting alternator has its field coils, 76, sup- plied with power through the sending kev, 78, and also has connected across its armature terminals a circuit which is
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��Fig. 4. An improved form of heterodyne receiver
coupled to the receiver coil, 14, by way of transformer 68, 6p. The condenser, 27, may be inserted to tune the auxiliary circuit. All the other main elements of Fig. 3 are the same as shown in Fig. 2, except that variable condenser 22 may be added to make the diaphragm-coil circuit resonant.
When the sending key, 78, is open the sending generator, 26, does not generate and the system is entirely equivalent to that shown in Fig. 2, since all the receiv- ing portions are operative. When the key is depressed to make a ]^Iorse dot or dash, however, the generator field circuit is closed and intense radio-frequency currents are set up in the aerial system. These induce strong currents in the re- ceiver coil, 6, which might have so great an effect on the diaphragm as to make receiving from the distant station impos-
��Popular Science Monthly
sible. Closing the key, however, con- nects in the circuit 27, 68, and through the coupling transformer similar, but op- posed strong currents are set up in the receiver coil 14. The intensity and phase of these is adjusted so that their mag- netic field exactly neutralizes that of the transmitter currents in coil 6, and the diaphragm is therefore left undisturbed and in receiving condition even though the key is pressed down. Thus the aerial is used for sending at the same time it receives.
This duplex system makes possible the transmission of twice the normal amount of traffic betw^een two radio stations, for messages can pass both ways simultaneously. Since the same aerial is used both for sending and re- ceiving, there is no need for erection of separate sending and receiving sta- tions located some distance apart and connected by wire lines, as is done at the Marconi trans-oceanic plants.
The patent specification points out a number of variations of both simple and duplex heterodyne operation ; for instance, the telephone receiver may be mechanically tuned to the beat-note frequency, or the action of the dyna- mometer may control a microphonic- contact relay (/j. Fig. 3) operating an ordinary telephone 12 by varying the current from a local battery ii. It is also suggested that, instead of cur- rents, the voltages set up by the re- ceived waves may be used to interact with locally generated radio-frequency voltages, upon an electro-static tele- phone, to produce heterodyne beats and a musical signal.
An improvement upon the dynamo- meter heterodyne just described is the subject of 1915 U. S. Patent number 1,141,717, issued to J. W. Lee and J. L. Hogan, Jr. In principle this receiver is identical with the older forms of heterodyne, but instead of adding the effects of the incoming and locally generated currents mechanically upon a dvnamometer device, the two are combined electrically. As shown in Fig. 4, a normal receiving outfit is first set up. This may consist of the antenna A, having in series with it to ground F a loading coil B, the primary
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