148 



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 





Hi|i|i|i — ' ^ 



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 



