Page:Popular Science Monthly Volume 92.djvu/494

 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

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��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-

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