Tuning has been mentioned in several places but not explained in any
greater measure than was necessary to render a conception which would
enable the reader to follow the text intelligently in order not to
depart from the subjects under discussion there and consequently defeat
the purpose of clearness.
The great importance and value of properly "tuning" the circuit of
radiotelegraphic apparatus cannot be overestimated and for that reason
the subject can hardly be passed without some further explanation. Its
effects are two-fold. In the first place it is always desirable and
highly important that wireless messages should be, so far as is
possible, selective, inasmuch as there are often several stations in the
same immediate neighborhood operating at the same time. This result is
reached by tuning and it is possible for them all to transmit different
messages at the same time without confusion by the proper arrangement of
the wave length. The second effect is the transmission of messages over
long distances with the comparative consumption of small amount of power
by adjusting the "period" or electrical length of the circuits until the
oscillations "flow in harmony" with each other and resonance is secured.
Perhaps the only way that these results may be made clearly intelligible
is by resort to a graphical example. Suppose that a very heavy weight
were suspended from a chain as shown in the illustration and that it is
struck at regular intervals, once every second, with a hammer.
Every time that the hammer strikes the ball it will give it an impulse
and cause it to swing slightly. If the chain is short, the ball will
swing faster, while if it is long it will swing more slowly. We will
suppose that the ball is struck from such a direction that it starts to
swing over toward A. The ball is so heavy and the hammer so light in
comparison however that the ball does not swing very far and soon
commences a return journey. If it should return to the point B just as
the hammer delivers another blow the force of the blow will be expended
in stopping the ball rather than adding to its motion because they are
both traveling in opposite directions. However if the chain is
lengthened so that it has a period of swing lasting one second, the
succeeding blow will strike the ball after it has reached the point C
and is on its return journey, thus imparting fresh energy because both
the ball and hammer come together at the right time when they are both
swinging together. Proper adjustment of the length of the chain will
make it possible for the hammer to always descend at the right moment to
add its energy and motion to that previously given the ball. The result
will be considerable increase in the amplitude of the swing.
[Illustration: FIG. 86.—Chain and ball arranged to illustrate effect
of tuning.]
From this we may easily perceive how it is possible by shortening or
lengthening the period of an electrical circuit to so adjust it that
resonance is secured and each succeeding oscillation will take place at
the proper time to assist the previous one, not dying away after one or
two surges and becoming what is known in technical language as rapidly
"damped."
[Illustration: FIG. 87.—Loose coupled helix.]
The instruments for accomplishing these things consist as previously
explained, in the case of a transmitter, of the helix and in the
receiving station of various tuning coils and condensers.
[Illustration: FIG. 88.—Hot-wire ammeter.]
Helix and tuning coils are divided into the "inductive" or "loose" and
the "direct" or close coupled types. Inductive tuning coils are known as
"loose-couplers" and "receiving transformers." Inductive helixes consist
simply of two helixes, separated from one another as shown in the
accompanying illustration. The upper helix, called the secondary, can be
raised or lowered upon a central support. Varying the distance between
the primary and secondary is varying the "coupling." There are several
advantages derived by using loose coupled sending helixes, the chief of
which lie in the fact that it is possible to radiate larger amounts of
energy and also decrease the "damping."
[Illustration: FIG. 89.—The principle of the hot-wire ammeter.]
In order to tune a transmitter, the "hot-wire" ammeter is necessary.
This instrument makes use of the property which electrical conductors
possess to become heated and expand when a current is passed through
them.
The accompanying diagram serves to illustrate the principle of the
"hot-wire" meter. A piece of platinum wire is stretched tightly between
two rigidly fixed posts. A thread leads from the center of the "hot
wire" to a small spindle around which it passes once or twice. The
spindle is also connected to a spring which exerts a continual tendency
to turn the spindle but is prevented from so doing by the thread
attaching to the hot wire. Any tendency on the part of the string to
slacken a little, however, will immediately permit the spring to turn
the spindle. When a high frequency current is passed through the
platinum wire it becomes heated and expands. The expansion of the wire
allows the thread to slacken slightly with the immediate result that the
spindle turns. The spindle carries a pointer at the upper end which
shows the amount of turning. It is therefore easy to tell the
comparative strength of current flowing accordingly as the deflection is
great or small.
[Illustration: FIG. 90.—Diagram showing loose coupled helix in circuit.]
The meter is placed in series with the aerial and when the high
frequency currents pass through it they heat and expand a fine wire,
causing the needle to move over a graduated scale and indicate the
amount of current passing. The apparatus is "tuned" or in resonance when
the length of the spark gap, the condenser and the helix have been so
adjusted that the oscillations flow freely through the system and the
maximum amount of current is indicated by the ammeter.
[Illustration: FIG. 91.—Loose coupled tuning coil.]
[Illustration: FIG. 92.—Loose coupled tuner.]
The loose coupled tuning coil consists of two windings wound over two
concentric cylinders, forming a primary and a secondary. The secondary
is the smaller winding and slides in and out of the primary so that the
"coupling" is variable. The primary is adjustable by means of a slider
and the secondary by means of a multi-pointed switch. The slider is
usually connected to the aerial and one end of the coil to the ground.
The detector, etc., are connected to the terminals of the secondary.
Variable condensers may be added with good results to both the primary
and secondary circuits.
[Illustration: FIG. 93.—Diagram showing position of loose coupler
in circuit.]
Loose couplers also take the form of doughnut tuners in which the
secondary revolves instead of slides. The coupling is variable in such
an instrument by simply turning the secondary.
The wave emitted from a transmitter is in reality made up of two waves
of different lengths. The variation in the lengths of these two waves is
dependable upon a factor known as the coefficient of coupling. It is
almost impossible to clearly explain the phenomenon and in order not to
confuse and complicate by a rather lengthy explanation it may be well to
simply state that its effect is to make selective tuning difficult
unless the coupling of the receiving station can be varied to correspond
with that of the transmitter and ask the reader to take it for granted.
Varying the coupling adjusts the difference in the two wave lengths and
when properly accomplished renders the apparatus highly selective.
[Illustration: FIG. 94.–Fort Gibbons, Alaska, wireless station.]
[Illustration: FIG. 95.—Transmitting condenser (molded dielectric).]
Directive Wireless Telegraphy is an interesting phase of this new art
which is receiving considerable attention in the hands of investigators
and has resulted in the devisement of several successful systems for
confining the propagation of the electric waves to certain directions.
[Illustration: FIG. 96.—Braun's method for directing wireless telegraph
signals.]
A general diffusion of waves is often very undesirable for the reasons
that the message may be received by an unfriendly neighbor or enemy and
also because it is wasteful of energy. By so directing the waves that
they may be sent over the earth to any desired point of the compass and
only in that direction, it is possible to communicate without disturbing
another station and also for a vessel at sea to secure its bearings and
position by tuning its apparatus to respond to electric waves from two
different known stations.
The manner in which the problem has been solved varies considerably
according to the inventor. All are interesting and ingenious.
It will be remembered that electric waves possess all the
characteristics and properties of light waves, etc., and may be
reflected, refracted and polarized.
Ferdinand Braun has devised a system consisting of a number of metallic
strips arranged to compose a parabolic surface. Another similar set of
strips below the first set completes the arrangement. The two sets are
connected to the terminals of a spark gap and induction coil. This
apparatus acts as a huge reflector and sends out waves in one direction
only, but however interesting and ingenious it may be is not entirely
practical.
[Illustration: FIG. 97.—Bellini-Tosi radio-goniometer for directive
wireless telegraphy.]
Another method devised by Braun employs two or more aerials at certain
distances apart. The alternating currents used to excite the
oscillations differ in phase, i. e. are so arranged that they have
different comparative values at the same moment. It is possible to send
very strong signals in a direction lying in the same plane as the
aerials. By the use of three or more antennae suitably differing in
their phase of excitation and situated at the vertices of a triangle it
is possible to send strong signals in certain directions only.
[Illustration: FIG. 98.—Arrangement of Bellini and Tosi for directive
wireless telegraphy.]
Messrs. Bellini and Tosi have devised a very ingenious method of
directively transmitting and receiving electric waves as shown in the
accompanying diagrams. The antenna consists of two closed or nearly
closed circuits of triangular shape arranged in two perpendicular
planes. The two aerials each contain a circular coil of wire
perpendicular to each other with their windings in the planes of the
antenna circuits respectively. A third coil is connected to the
receiving apparatus when the messages are incoming and to the condenser,
spark gap and coil when the signals are to be transmitted.
Waves coming in from any particular direction produce oscillations in
the two aerial circuits whose intensity varies according to the
direction in which the waves These currents passing through the coils
generate a magnetic field having a direction perpendicular to that from
which the waves come. The strength of the currents in the movable coil
will depend upon its position in the resultant magnetic field and will
be at a maximum when the coil embraces as many as possible of the lines
of magnetic force.
[Illustration: FIG. 99.—Complete receiving and transmitting outfit.]
By providing the movable coil with a pointer it is possible to thereby
determine the plane in which the station producing the signals lies. Any
ambiguity regarding the final position of the station, whether it is
located in the same direction indicated by the pointer or in the
opposite one, is only removed by general knowledge of the location of
existing stations.
The processes involved in sending messages are the reverse of those
entering into the receiving apparatus. The movable coil being connected
with the condenser, gap and transformer or induction coil creates a
magnetic field which induces oscillating currents in the other two coils
and consequent waves in the aerial whose strongest exertions will lie in
a plane determined by the third coil. Changing the position of the
latter will send the messages in any direction desired.