CHAPTER IX. TRANSMITTING HELIXES.
The transmitting helix or tuning coil supplies the greater part of the inductance which is so necessary for the production of electrical oscillations in the transmitting circuit. It consists merely of a few turns of heavy copper or brass wire wound in a helix around a form. Two or more movable contacts permit various amounts of the inductance to be inserted in the open or closed circuits.
The tuning helixes described in this chapter are offered principally to serve as guides. They have been designed as carefully as possible to suit the transmitters for which they are recommended.
There are factors, such as the aerial, etc., which vary greatly in stations of the same rated power and make it best to determine the length of the helix by actual experiment.
A certain amount of inductance is necessary in the closed circuit for the production of electrical oscillations and to transfer energy to the open circuit. Inductance beyond that necessary to receive energy from the closed circuit lessens the radiation and makes it necessary to increase the period of the open circuit by adding wires to the aerial.
The open and closed circuits of a tuning helix constitute an oscillation transformer, and the two circuits if they are very close together or intertwined are said to be closely coupled. When separated or far apart they are loosely coupled.
Closely Coupled Helix.—Fig. 61 illustrates a closely coupled tuning helix, suitable for ordinary induction coils giving sparks up to 3 or 4 inches in length.
The heads of the helix are circular pieces of hard wood 1 inch thick and 12 inches in diameter. Six rectangular notches are made at equal distances along the edges of the heads. Six uprights 3/4 x 1/2 inch and 8 3/4 inches long are fastened in the notches with small round headed brass screws so that the heads are separated a distance of 6 3/4 inches. Grooves are cut in the outside face of each of the uprights at a distance of 7/8 inch apart. The wire forming the helix is 1/4 inch brass. Brass wire is springy and retains its shape better than copper. About twenty-two feet will be required. The wire is wound in the grooves in the uprights and held in place by a few double pointed tacks placed judiciously. The adjacent turns of wire will then have a separation of about 5/8 inch.
Fig. 61. Closely Coupled Helix.