distances, are of the damped wave classi- 

 fication. It will be best, therefore, first to 

 consider the damped wave transmitters in 

 detail. 



The Simple Spark Transmitter 



The "plain aerial" transmitters repre- 

 sented by Fig. 31, also reproduced from 

 last month's article, are not 

 much used at present. In the 

 original forms there were no 

 loading inductance coils L, and 

 as a result the groups of waves 



i 



T 



Fig. 31: The plain aerial transmitter is not 

 of a type that is used at the present time 



emitted were highly damped (that is to 

 say, died out very quickly) and therefore 

 were not suitable for sharp turning. 

 Even when the loading coil is added to 

 increase the persistence of the wave 

 groups (i. e., to make them die away less 

 rapidly), the various power limitations, 

 as well as the difficulty of securing high 

 electrical efficiency, helped to bring about 

 the disuse of this type of spark trans- 

 mitter. 



The main objection to the "plain 

 antenna" sending arrangement is that 

 the spark-gap itself is directly in series 

 with the antenna-to-ground circuit. 

 Consequently, all the oscillating current 

 of the aerial circuit must pass and re-pass 

 through this gap as it oscillates back and 

 forth between antenna and ground. 

 Since the spark-gap possesses a moder- 

 ately high resistance, radio frequency 

 energy passing through it is wasted in 

 producing heat. A further objection is 

 that the power available for producing 

 oscillations is limited by the capacity 

 and insulation of the antenna, and that 

 any leakage in the aerial insulators puts 

 a sharp restriction upon the ability to 

 store power before each spark passes and 

 each train (or group) of oscillations starts. 



By using the coupled two-circuit trans- 

 mitter of Fig. .32, these difficulties are 

 overcome wholly or in part. Obviously, 



the spark gap is no longer in the aerial 

 circuit, and therefore a large portion of 

 the losses due to that arrangement are 

 eliminated. Further, the ability to store 

 power before each spark passes is de- 

 termined by the capacity and insulation 

 of the secondary condenser C, and hence 

 the amount of energy in each oscillation- 

 group is no longer dependent entirely 

 upon the antenna. 



All of this will perhaps be made more 

 clear by considering successively the 

 several circuits in the transmitter, both 

 as to their arrangement and operation. 

 Two general arrangements of the power 

 circuits are much used. In the first, 

 there is an alternator located at the radio 

 station and forming part of the radio 

 equipment. This is the usual practice 

 in commercial stations. The second ar- 

 rangement has alternating current power 

 furnished over long lines from a distant 

 central power station, in which case the 

 alternator supplies a general lighting and 

 power load, and is not strictly a part of 

 the radio outfit. 



The Power Circuits 



Since the vast majority of commercial 

 stations, and nearly (if not quite) all mili- 

 tary and naval plants, have 

 special radio generators at the 

 transmitting points, this type 

 should be taken up first. It is 

 of little importance how the al- 



Fig. 32: The coupled two-circuit transmitter 

 overcomes difficulties from the plain antenna 



ternating current generator is driven. An 

 electric motor on the same shaft is the 

 most common arrangement, but some- 

 times steam turbines or gasol'ne engines, 

 or even geared Jiand-drives, ure used. In 

 every case, some mechanical power is 

 provided for the purpose of rotating the 

 moving part of the alternator, and alter- 

 nating current of the voltage and fre- 



