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Lessons in Wireless Telegraphy

Chapter 23: LESSON TWENTY. AERIAL SWITCHES.
By Alfred Powell Morgan
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About This Book

This work presents a compact, lesson-based introduction to the physical principles behind wireless telegraphy, aiming to give operators a clear theoretical grounding. Arranged in discrete lessons, it starts with magnetism, magnetic induction, and basic electrochemical cells, then develops circuit laws and the components and methods for producing, radiating, and detecting electromagnetic signals. Emphasis is on clear explanations, illustrative diagrams, and simple experiments to show how transmitters, receivers, antennas, and tuning interact, while detailed engineering and maintenance procedures are deferred to a companion volume.

LESSON TWENTY. AERIAL SWITCHES.

Since the same aerial is used both for transmitting and receiving, some method of quickly connecting it to either the transmitter or receiving apparatus must be provided. This is accomplished by means of an aerial switch.

The best and most efficient switch adopted generally by the commercial stations is the "T" type, consisting of a double pole, double throw switch having very long blades. One set of contacts is mounted on the switch base and the second are carried on a "T"-shaped support from which the switch derives its name. The aerial and ground are connected to the blades of the switch.

The lower contacts lead to the transmitting apparatus and the upper ones to the receiving instruments. By simply moving the switch up or down the aerial and ground may be connected to either the transmitter or the receptor at will.

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FIG. 43. Aerial Switch.

A third blade, much shorter than the other two is usually provided and connected by means of an insulating bar to the other blades so that when they are moved it also moves. It connects with a contact arranged so that when the switch is thrown into position for transmitting the two come together. This blade and contact are made a part of the circuit supplying current to the primary of the coil or transformer so that in case the key should be accidentally touched while receiving the powerful discharge of the transmitter would not destroy the adjustment of the detector.