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.
[Illustration: Fig. 61. Closely Coupled Helix.]
The helix is raised above the level of the operating bench or table by
three small feet fastened to the under head, 120 degrees apart.
[Illustration: *Plate III. Transmitting Circuits.*]
If it is desired to use this helix with the one quarter K.W.
transformer, the diameter of the wire should be increased to 5/16 inch.
The two K.W. transformer will require seven turns of 1/2-inch hard drawn
brass wire two feet in diameter.
[Illustration: Fig. 62. Prague Transmitting Helix.]
When setting up a transmitting station, it is a good plan to coil up a
long piece of the wire to be used on the helix around a rough form. The
adjacent turns should be the same distance apart as they are to be on
the finished instrument. Tune up the station with this improvised helix
and ascertain the amount of inductance required. It is then easy to
design a helix containing the proper amount of wire. Four or five feet
extra should be included to allow plenty of range in case it is ever
necessary to make any changes in the aerial or condenser.
[Illustration: Fig. 63. Closely Coupled Tuning Circuit.]
Plate III illustrates the transmitting circuits of the various
commercial systems. The circuit used will determine the number of
binding posts and movable clips necessary. Fig. 63 gives the diagram of
a very good circuit.
*Loosely Coupled Helix*. The heads of the primary coil of the loosely
coupled helix illustrated in Fig. 64 are circular pieces of hard wood 1
inch thick and 18 inches in diameter. Six uprights 3/4 x 1/2 inch and 11
inches long are set into notches in the upper head and fastened to the
base so that the space between the heads for winding the wire is 10
inches.
[Illustration: Fig. 64. Loosely Coupled Transmitting Helix and Contact
Clip.]
A square wooden pillar 1 1/2 x 1 1/2 inches and 32 inches long passes
through a square hole 1 1/2 x 1 1/2 inches in the center of the heads
and projects 20 inches above the upper one. The heads of the secondary
coil are each 12 inches in diameter. The distance between them is also
12 inches. A square hole 1 1/2 inches x 1 1/2 inches in the center of
each of the heads permits the whole secondary coil to slide up and down
on the pillar. Several 1/4 inch holes bored 1 inch apart in the pillar
admit a small peg of the same diameter. The coupling between the two
coils is varied by changing the height of _S_ above _P_ and inserting
the peg to hold the upper coil in position.
The primary winding of the transformer should consist of about 20 turns
15 inches in diameter, and may range in size of wire from No. 2 to No. 8
B. S. gauge.
The secondary is 10 inches in diameter and will require about 40 turns
of No. 10 to No. 16 B. S. gauge.
The largest sizes of wire are for the one-quarter and one-half K.W.
transformers while the smaller sizes are best suited to small induction
coils.
Fig. 64 also illustrates a clip for making connections to the turns of
the helix. The handle is a piece of hard rubber rod 2 1/2 inches long
and 5/8 inch in diameter. A saw slot 1 inch deep is cut down the center
of one end. Two strips of spring brass 2 inches long, 5/8 inch wide and
3/64 inch thick are bent as shown in the illustration. The straight ends
are slipped in the slot in the handle and clamped together by boring a
hole and passing a machine screw through. The upper end may be fitted
with a nut or a binding post to facilitate connection. Flexible copper
ribbons or stranded wire should be the only conductor used for the
tuning leads.
[Illustration: Fig. 65. Loosely Coupled Transmitting Circuit.]
*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.
[Illustration: 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.