Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

Fig. 4. Simple Wireless Telegraph Transmitter and Receptor.

The wireless telegraph outfit illustrated in Fig. 1 would not serve for more than short distances of a few feet, and so a somewhat similar but more efficient apparatus is employed in practice. Fig. 4 shows such a system in its simplest form. In this case the secondary or high potential leads of the induction coil are connected, one to an earth and the other to an aerial or antenna composed of a number of bare copper wires insulated and suspended from a mast.

All electrically charged bodies are surrounded by an electrostatic field of force, the nature of which in theory is a state of strain.

The action of an induction coil connected as in Fig. 4 is to charge the upper part of the aerial above the spark gap, say with negative electricity and establish a field of force in its vicinity varying in area from a few feet to several miles. When the charge reaches a certain potential it is sufficient to puncture the layer of air in the gap and a spark takes place, setting up electrical oscillations.

Fig. 5. Electric Waves and Lines of Strain.

Previous to the rupture of the spark gap, lines of electric strain or force stretch from the aerial to the earth on all sides as in the center of Fig. 5. A line of force may be defined as a curve drawn in the electric field so that the direction of the curve is the same as that of the electric intensity at that point.

The aerial and the earth act like the two metal plates in Fig. 1 or like the opposite plates of a condenser. As soon as the air gap is punctured it becomes conductive and the aerial charge rushes down into the earth. With the discharge, the strain in the electrostatic field is released and the aerial charge rushes down into the earth, but in so relaxing produces a new current and builds up a strain around the antenna opposite in direction to the first. This process repeats itself very rapidly and electrical oscillations are thus set up in the antenna. Every oscillation changes the direction of the magnetic flux or dielectric strain and causes the imaginary lines which originally stretched from the aerial to the earth to be displaced and the ends terminating at the aerial to run down it and form semi-loops or inverted "U's" standing with their ends on the earth in a circular ripple around the aerial and moving away from it with the speed of light. In Fig. 5 three oscillations are supposed to have taken place. The shortest distance between two adjacent points at which the electric strain is at a maximum in the same direction and period of time is the wave length emitted by the aerial. The separate standing groups of dielectric strain moving away from the antenna are electromagnetic waves. In the figure, the adjacent groups are separated by half a wave length. These waves are emitted at right angles to the transmitting aerial, whence they pass through the ether to the other station. When they reach the receiving aerial they set up electrical oscillations therein which are too weak to be perceptible in the shape of sparks as in the original Hertz oscillator and resonator because of the great distance separating the stations, so they are made to flow through a detector, which in Fig. 4 is represented as being a crystal of a mineral called silicon. When the high frequency currents strike the silicon, they set up a weak pulsating direct current. This action is due to a peculiar rectifying property of the mineral. The direct current flows through the telephone receiver and produces an audible sound. If the aerial and ground were connected directly to the terminals of the telephone receiver, without the silicon, the oscillations would not pass because of the impeding or choking action of the electro-magnets in the telephone receivers.

Tuning.—It is sometimes desirable that messages should be made selective or secretive. It is obvious that if there were several large stations in the same neighborhood they could not all operate at the same time unless some means of preventing the stations from receiving more than one message at a time were possible. This is the object in view of the so-called "tuning" of wireless telegraphy. It also accomplishes a second purpose which is perhaps considered more important than the first. The length of the aerial may be too great or too short for the amount of energy and the length of the waves which it emits or receives. When this is the case, the oscillations are quickly damped out and do not generate very powerful waves or produce strong signals at the receiving station and thus by properly adjusting the circuit all undesirable messages may be cut out as well as the signaling range greatly increased. Every electrical circuit has a definite period or electrical length, determined by its inductance and capacity. A circuit emits waves of only one length for given values of inductance and capacity, and must also be of a certain length before it will respond to waves sent out by another transmitter. The careful adjustment of a circuit to emit or receive a given wave constitutes tuning.