Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

APPARATUS REQUIRED IN MANY INSTANCES IN ORDER TO COMPLY WITH THE WIRELESS LAW.

OSCILLATION HELIX

The oscillation helix has almost become a necessity in order to comply with the regulations of the Wireless Law regarding wave form, except in those stations where a quenched gap is used.

Fig. 151. Amco Oscillation Helix.

The wave emitted by many stations is not pure. It is composed usually of two or three separate waves of different lengths instead of all the energy being confined to oscillations of one period. It is possible to tune such a wave in two or more places or "humps," as they are called, on the tuning coil. It is obvious that a wave possessing such humps cannot be closely tuned and is liable to interfere with the signals of another station. This is one of the principal causes of interference.

The reason for this phenomenon is simple. The action of a transmitter is to first charge a condenser. When the potential of the condenser rises to sufficient value it discharges across the spark gap and sets up oscillations in the closed circuit. These oscillations immediately induce oscillations in the open circuit or aerial system and part of the energy passes off into the ether as electro-magnetic waves. However, the oscillations in the aerial system do not immediately die away after the oscillations in the closed circuit cease during the interim until the next condenser discharge, but continue to surge and react upon the dosed circuit to sufficient extent to induce therein currents which surge back and forth long after the current from the condenser discharge has died away.

We might call the oscillations due to the condenser discharge primary oscillations and those induced in the aerial thereby secondary oscillations. Those which are then set up in the closed circuit by the reaction of the secondary currents are tertiary. This third train of oscillations persist after the secondary currents have died away, and induce another set of oscillations in the aerial which send out a second set of electromagnetic waves differing in length from the first.

The oscillations which take place after the initial surge in the closed and open circuits are naturally somewhat weak. By using an oscillation helix in which the primary and secondary are separated from each other it is possible to eliminate the third and fourth trains of oscillations and all others having a tendency to follow, by placing the circuits apart so that the weak oscillations are not strong enough* to react across the intervening space. The immediate oscillations set up by the condenser discharge are strong enough to act across the space and set up powerful oscillations in the aerial.