Tuning a Transmitter.—Fig. 62 shows a complete wiring diagram of a transmitter with a hot wire ammeter inserted in the aerial circuit.
To tune such a transmitter place both contacts A and B together on a turn of wire near the center of the helix. Set the transmitter in operation by pressing the key and move both contacts together along the various turns of wire until the meter shows a maximum reading. Then vary B alone until the reading is still higher. Shifting A varies the inductance in the closed circuit and B that of the open circuit.
Fig. 65 illustrates the circuit of a loosely coupled transmitter. To tune such a circuit it is necessary to connect a small needle spark gap between the aerial and the ground.
Fig. 66. United Wireless Helix, Spark Gap and Condenser.
A suitable spark gap for this purpose may be made of two darning needles mounted in double binding posts.
Set the clips, A and B, at the center turns of their respective coils. Press the key to operate the transmitter and adjust the clip, A, until the best spark is obtained in the small needle gap. Then adjust B until the spark is still better.
If several wires seem to give the same results, move the secondary a little further away from the primary and try again. After securing the best spark it is possible to obtain, remove the needle gap, which is only used in testing. The hot wire ammeter should then be placed in series with the aerial. By slowly and carefully adjusting the clips and varying the coupling, the hot wire ammeter will indicate the proper "tune."
The secondary of the loosely coupled tuning coil produces a "kick back" effect on the primary, due to induction, and unless the two coils are somewhat separated, the wave emitted from the transmitter will have more than one "hump" and a person receiving the wave will be able to tune it in on two places on his tuning coil.
A loosely coupled tuning coil does not radiate so much energy as a closely coupled helix, but will, when properly tuned, emit a wave which is not so highly damped. Therefore as far as distance is concerned the loosely coupled type is perhaps the most efficient.