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Lessons in Wireless Telegraphy

Chapter 20: LESSON SEVENTEEN. THE HELIX.
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 SEVENTEEN. THE HELIX.

The Helix supplies the greater part of the inductance to the closed circuit of the transmitter. It also acts as a transformer, serving to raise the voltage of the currents surging through the closed circuit and impress them upon the aerial system. The turns of the helix included in the closed circuit constitute the primary of the transformer, while those in the open circuit form the secondary.

A helix consists of a heavy conductor, either brass or copper, wrapped around a suitable frame of wood or hard rubber. Some forms consist of a spiral of copper ribbon clamped between two cross-shaped frames.

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FIG. 36. Helix.

Helixes are of two kinds, known as "close" or direct coupled and "loose" or inductively coupled. In an inductively coupled transmitter the primary and secondary are wound upon separate frames and are not connected together.

The U. S. Government Radio regulations place a limit on the amount of damping permissable in a transmitter.

It has already been explained in one of the previous lessons how the oscillations or surgings of the spark discharge rapidly die away. A spark which thus rapidly dies away is said to be rapidly damped. The damping of a loose coupled transmitting set is never as great as that of a close coupled set.

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FIG. 37. A Damped Oscillation.

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Close-Coupled Transmitter vs. Loose-Coupled Transmitter

For this reason the old style helixes are now practically obsolete and the loose or inductively coupled helix is the one most commonly used. Loose coupled helixes are also often termed oscillation transformers.

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FIG. 38. An Inductively Coupled Helix.

An ordinary transmitter tends to emit two sets of waves of different length. By carefully adjusting the coupling, pure trains of waves are formed by attracting the apices of the two sets of waves into one.