474 



Popular Science Monthly 



Increasing the Frequency in 

 Radio-Telegraphy 



THE simplest way to set up currents of 

 very high frequency in a wireless 

 telegraph antenna is to connect a special 

 radio-frequency alternator in series with 

 it and the ground. If this special dynamo 

 generates current of 100,000 cycles per 

 second, and if the antenna system is tuned 

 to this frequency, intense waves of 3,000 

 meter wavelength will be radiated. It is a 

 very difficult mechanical problem to design, 

 build and operate alternating current 

 dynamos of large power at such high 

 frequencies as 100,000 per second. Such 

 machines are in use, however, giving out- 

 puts of some five kilowatts, and it is be- 

 lieved that much larger generators of the 

 same sort can be 

 built. But it is 

 much easier to 

 make dynamos 

 of lower frequen- 

 cy. If a 33.300 

 cycle generator 

 is connected to 

 an apparatus 

 which triples the An unusually simple 

 current frequen- method for greatly mul- 

 . . 7, tiplying the normal fre- 



cy. It is evident quency of an alternator 

 that the output 



terminals of this frequency transformer 

 could be led to an antenna circuit and that 

 the system would set up, in the antenna, 

 currents of 100,000 cycles per second fre- 

 quency. By using a large alternating current 

 dynamo of one-third the frequency its con- 

 struction is greatly simplified; and as a re- 

 sultof this fact, inventors have for some time 

 striven to produce efficient frequency trans- 

 formers to be used in radio transmitters. 



United States patent number 1,174,493, 

 recently issued to E. F. W. Alexanderson, 

 shows how a mercury-vapor tube may be 

 used to distort the wave-form of a high 

 frequency alternator and how the aug- 

 mented harmonics may be picked out and 

 used to excite antennas for wireless. The 

 figure is taken from the patent, and in it ^4 

 represents the high frequency machine 

 which supplies power to the system. Con- 

 nected across its terminals is the mercury- 

 vapor bulb B, which may consist of a 

 vessel having two pools of mercury at its 

 bottom and a third electrode at the top, 

 as shown. An electric arc passing between 

 the pools and fed with. current from the 

 auxiliary circuit C, comprising a battery 

 and resistance, serves to start the vacuum 



tube; current from the dynamo A keeps it 

 in operation thereafter. 



The effect of using such a vapor lamp as 

 a load for the alternator is to emphasize the 

 third and even the higher odd harmonics in 

 the fundamental current; that is to say, 

 when the lamp is lighted the generator is 

 caused to amplify one of these harmonics 

 which are always part of the entire current 

 it gives out. Each of these component 

 currents in an ordinary generator is a 

 direct odd multiple of the original fre- 

 quency, so that if the dynamo is built to 

 generate 30,000 cycles, the generator-and- 

 lamp system will produce principally cur- 

 rents of 90,000 cycles, or even higher 

 frequencies. 



Series circuits containing capacity and 

 inductance, such as D, E, and H, may be 

 connected across the line wires and "tuned" 

 to the high harmonic frequencies. When 

 this is done, comparatively large currents 

 of the tuned frequency will be produced in 

 each condenser and coil. A transformer pri- 

 mary, such as H, being part of the same 

 circuit, is coupled to an antenna and ground 

 system, as indicated at /. 



If the antenna is also tuned to the har- 

 monic frequency, it will have set up in it 

 currents of that frequency and will radiate 

 corresponding waves. Thus a compara- 

 tively low-frequency dynamo may be used 

 to generate very short waves by this 

 process of frequency transformation. If 

 the fundamental frequency is 25,000 per 

 second, the wavelength produced directly 

 would be 12,000 meters; by tuning the 

 circuit D, E, H to the third harmonic, and 

 adjusting the antenna to correspond, waves 

 only 4,000 meters long are sent out. 



Specially constructed alternators giving 

 unsymmetrical currents are now being 

 used with this system of amplification also. 

 Only unsymmetrical waves may contain the 

 even harmonics, so that the system can be 

 used with such an alternator simply to * 

 emphasize the second harmonic, thereby 

 doubling the frequency. 



An Emergency Aerial Using Door- 

 Bell Wires. 



HAVING some difficulty in getting an 

 aerial erected, I tried using one side 

 of the wires connected with the door-bell. 

 With this indoor antenna I am obtaining 

 very good results in receiving. It is only 

 necessary to disconnect one of the bell 

 wires and attach a wire to lead to connec- 

 tion on the tuner. — Richard A. Liddane. 



