Wireless Telegraphy and Telephony/Chapter 2
Chapter II
Principle and Theory of Wireless Telegraphy
3. Principle
The art of wireless telegraphy is based upon wave motion, and an analogy is found in the wave motion of water, as the following explanation should make clear. Picture a small pond of still water, with a chip or twig floating upon its surface, mm full view of the observer. Now if a stone be thrown into the water, the sudden impact of the stone would cause ripples, or small waves, to radiate from the point of impact of the stone with the surface of the water, the waves becoming weaker as the circles become larger, i.e., as the distance from the point of impact becomes greater. As the waves arrive at the point where the chip is floating, they will impart motion to the chip; hence the observer will be aware that there has been some disturbance in the water. (See Fig. 1.) After the waves have ceased, the chip will again lie motionless upon the surface of the water. It is obvious that the distance over which the signals may be sent by this means will depend (a) upon the force employed to start the waves, and (b) upon the lightness of the chip, or its sensitiveness to the motion of the waves. Moreover, if there were grass or other obstructions in the pond between the point where the waves are started and the pot where the chip is located, some of the energy would be absorbed in swaying the grasses: hence the effect upon the chip would not be so great, and the signalling distance would be lessened. Or if there were an
Fig 1.—Wave Motion in Water.
obstruction in the path of the waves, as, for instance, a protruding rock, the waves would be distorted by this obstruction; hence less energy would reach the chip.
It is also obvious that any number of chips might be placed at any number of points within the affected radius of wave motion, and all would be moved by the waves.
When it is considered that these water waves cover an ever-increasing area as the circles expand, and that the actual energy which disturbs the chip is an extremely small part of the total energy in the entire circular wave, it should be clear why so great an amount of energy is required at a wireless sending station in order to operate a very sensitive receiver many miles away.
If we consider that the light chip resting upon the surface of the water has practically no inertia, it will respond to almost any wave length, and, therefore, if the water were disturbed from some other source than the stone referred to, and while signals were being sent by means of the stone, confusion would result, as the chip would respond to the waves from both sources, and, for this reason, no accurate signals could be made out by watching the motions of the chip. However, this difficulty may be overcome by employing a transmitting device which will send out waves of a certain length, and a receiving apparatus which shall respond only to the wave length of the transmitter.
4. Synchronous Wave Motion and Tuning
If a weight suspended by a spiral spring, or a rubber band, be given a blow so as to cause it to move up and down, the weight will oscillate uniformly; that is, a definite number of times per minute, the frequency depending upon the elasticity of the spring, or rubber band, and the weight of the suspended mass. Now assume this device placed over the pond of still water, as depicted at the left in Fig. 2, and set in motion as described above. On each downward movement of the weight it will touch and disturb the water, and, since
Fig 2.—Principle of Synchronous Wave Motion
it oscillates uniformly, it will create, or generate, a definite number of waves per minute, all being of uniform length and size.
If now we substitute a similar spring and weight for the chip as a receiving device (shown at the right in Fig. 2), and place this within the radius of the transmitted waves, these waves in passing will set it in motion, as it oscillates at exactly the same frequency as the transmitting weight. If the receiving device did not oscillate at the same rate as the transmitter, and, therefore, was not in harmony with the transmitted waves, these would tend to counteract any motion imparted to the receiving spring-suspended weight, as the following example should make clear. Assume the receiving weight to be of such dimensions that it will oscillate once per second. Now if the sending weight be generating waves at the rate of two per second, the first wave will give the receiving weight an upward motion at its own frequency; but just as it starts on its downward stroke, the second wave will strike it, thus preventing any further motion of the weight.
It is, therefore, evident that the oscillations of the receiving device would be destroyed if the frequency did not harmonize with that of the sending device. Tuning is absolutely necessary for the successful operation of wireless telegraphy, and it should be thoroughly understood before continuing.
5. Theory
Wireless signals are a wave motion in, or disturbance of, the magnetic forces of the earth, and are propagated through this magnetic field, following the curvature of the earth, just as a tidal wave would follow the surface of the ocean. Practice indicates that the nodel points of the waves are at, or near, the earth’s surface.
As explained in Chapter I, ether waves do not traverse all substances with like velocities; this explains why wireless signals are propagated many times farther over water than over land, as the waves traverse air and water at practically the same velocities. In land the waves travel at a much slower rate.
Now to produce, electrically, the results described by the analogy of water, we must employ means for creating waves in the earth’s magnetic field, and use an electrical spring and weight, so to speak. The electrical spring effect is obtained by the electrical phenomena of capacity. Any surface of metal possesses capacity, which is the power to retain a charge of electricity. When this is disturbed it has the same elastic principle as the spring.
The inertia of the weight is represented, electrically, by the term inductance, which effect is produced when a constantly changing current is passed through a coil of wire. This causes the continually changing current to react upon itself, and, consequently, produces a retarding effect.
Referring to Fig. 3, let C represent a capacity connected to the ground MN through the adjustable inductance I. If means be employed to cause the residual charge of this capacity to oscillate, it will, in turn, cause a wave-like motion of the electromagnetic forces of the earth similar to the wave motion in water.
If either the capacity or inductance is increased, the vibrations will be slower, and the wave length will be
Fig. 3.—Theory of Wireless Telegraphy.
greater. The waves thus generated are propagated through the earth’s forces in ever-increasing circles, exactly as in the case of the water waves,
C' in Fig. 3 represents the receiving capacity connected to the ground through the inductance I', in the same manner as at the sending station. This capacity, of course, also contains a residual charge which is dormant under normal conditions, but as the wave front glides by the station, the rising and falling of the waves will impart a slight oscillatory motion to the residual charge.
Means for manifesting these oscillations permit us to correctly read all signals sent out from the transmitting station.
To clearly receive all signals transmitted from the sending station, it can be readily understood that the capacity and inductance must be adjusted to give exactly the same frequency: otherwise the natural frequency of the circuit would counteract the forced oscillations set up by the received waves, thus causing an interference or prevention of oscillations in this particular receiving station. Of course this would have no detrimental effect on other receiving stations which might be adjusted to harmonize with the waves.