Invention of Radio - Physics of Wireless Signalling

Physics of Wireless Signalling

Various scientists proposed that electricity and magnetism were linked. In 1802 Gian Domenico Romagnosi suggested the relationship between electric current and magnetism but his reports went unnoticed. In 1820 Hans Christian Ørsted performed a simple and today widely known experiment on man-made electric current and magnetism. He demonstrated that a wire carrying a current could deflect a magnetized compass needle. Ørsted's work influenced André-Marie Ampère to produce a theory of electromagnetism.

Several different electrical, magnetic or electromagnetic physical phenomena can be used to transmit signals over a distance without intervening wires. The various methods for wireless signal transmissions include:

• Electrical conduction through the ground, or through water.
• Magnetic induction
• Capacitive coupling
• Electromagnetic waves

All these physical phenomena, as well as various other ideas such as conduction through air, were tested for the purpose of communication. Early researchers may not have understood or disclosed which physical effects were responsible for transmitting signals. Early experiments used the existing theories of the movement of charged particles through an electrical conductor. There was no theory of electromagnetic wave propagation to guide experiments before Maxwell's treatise and its verification by Hertz and others.

Capacitive and inductive coupling systems today are used only for short-range special purpose systems. The physical phenomenon used today for long-distance wireless communications involves the use of modulated electromagnetic waves, which is radio.

Radio antennas radiate electromagnetic waves that can reach the receiver either by ground wave propagation, by refraction from the ionosphere, known as sky wave propagation, and occasionally by refraction in lower layers of the atmosphere (tropospheric ducting). The ground wave component is the portion of the radiated electromagnetic wave that propagates close to the Earth's surface. It has both direct-wave and ground-reflected components. The direct-wave is limited only by the distance from the transmitter to the horizon plus a distance added by diffraction around the curvature of the earth. The ground-reflected portion of the radiated wave reaches the receiving antenna after being reflected from the Earth's surface. A portion of the ground wave energy radiated by the antenna may also be guided by the Earth's surface as a ground-hugging surface wave.

Any change in the electrical conditions of a circuit, whether internal, such as a change of load, starting and switching operations, short circuits, or external, such as due to lightning, involves a readjustment of the stored electromagnetic and electrostatic energy of the circuit; that is, a so-called transient. Such transient is of the general character of a condenser discharge through an inductive circuit. The phenomenon of the condenser discharge through an inductive circuit therefore is of the greatest importance to the engineer, as the foremost cause of high-voltage and high-frequency troubles in electric circuits.

With the development of radio communication—whether wireless or wired—the condenser discharge through an inductive circuit has assumed a great additional importance since, with the exception of a few of the highest power transoceanic stations, which use power-driven high-frequency alternators, the source of power in the radio communication up to 1922 was the condenser discharge through the inductive circuit, whether as a damped wave or as an undamped wave. In undamped wave radio communication, the condenser discharge circuit is coupled with a source of electric power—a battery—in such a manner, that, without interfering with the character of the oscillation, sufficient energy is fed into the circuit to maintain the oscillation, similarly as in the clock, the pendulum is coupled with a source of mechanical power—weight or spring— so as to maintain its oscillation undamped. The usual method of producing a condenser discharge through an inductive circuit is gradually to charge a condenser from a source of electric power, until the condenser voltage has risen sufficiently high to jump a spark gap (the rotary gap, or quenched gap of the damped wave wireless for instance) and thereby discharge through the inductive circuit.