Unit 5. Radio
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Radio is a form of communication in which intelligence is transmitted without wires from one point to another by means of electromagnetic waves. Early forms of communication over great distances were the telephone and the telegraph. They required wires between the sender and receiver. Radio, on the other hand, requires no such physical connection. It relies on the radiation of energy from a transmitting antenna in the form of radio waves. These radio waves, travelling at the speed of light (300000 km/sec.), carry the information. When the waves arrive at a receiving antenna, a small electrical voltage is produced. After this voltage has been suitably amplified, the original information contained in the radio waves is retrieved and presented in an understandable form. This form may be sound from a loudspeaker, a picture on television, or a printed page from a teletype machine. The principles of radio have been demonstrated in the early 1800s by such scientists as Michael Faraday and Joseph Henry. They had individually developed the theory that a current flowing in one wire could induce (produce) a current in another wire that was not physically connected to the first.
In fact, the radio waves travel in a straight line, but they are reflected from the back to the earths surface. This is how radio waves travel around the world. You have heard on the radio of long-wave transmissions and short-wave transmissions. In fact, there are two ceiling in the sky, not solid ceiling like the ceiling in the room, but more like nets which allow some radio waves to pass through them and reflect others. The lower ceiling is called Heaviside layer. This reflects long waves. Short waves are reflected by the higher ceiling, called Appleton layer. Radio transmitters can also send out ultra-short waves. These waves pass through both layers in the sky but are useful for communication with spaceships and can even be used to send sound to very long distances when they are reflected from satellites in space.
Hans Christian Oersted had shown in 1820 that a current flowing in a wire sets up magnetic field around the wire. If the current is made to change and, in particular, made to alternate (flow back and forth), the building up and collapsing of the associated magnetic field induces a current in another conductor placed in this changing magnetic field. This principle of electromagnetic induction is well known in the application of the transformer, where an iron core is used to link the magnetic field of the first wire or coil with a secondary coil. By this means voltages can be stepped up or down in value. This process is usually carried out at low frequencies of 50 or 60 Hz (Hertz, or cycles per second). Radio wave, on the other hand, consists of frequencies between 30 kHz and 300 GHz (1 GHz=1 billion Hz).
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In 1864, James Clerk Maxwell published his first paper that showed by theoretical reasoning that an electrical disturbance that results from a change in an electrical quantity such as voltage or current should propagate (travel) through space at a speed of light. He postulated that light waves were electromagnetic waves consisting of electric and magnetic fields. In fact, scientists now know that visible light is just a small portion of what is called the electromagnetic spectrum, which includes radio waves, X-rays and gamma rays.
Heinrich Hertz, in the late 1880s, actually produced electromagnetic waves. He used oscillating circuits (combinations of capacitors and inductors) to transmit and receive radio waves. By measuring the wavelength (designated by the Greek lower-case letter lambda) of the waves and knowing the frequency of oscillation (f), he was able to calculate the velocity (v) of the waves using the equation v=f (lambda). He thus verified Maxwell theoretical prediction that electromagnetic waves travel at the speed of light.
It apparently did not occur to Hertz, however, to use electromagnetic waves for long-distance communication. This application was pursued by Marconi in 1885, he produced the first practical wireless. In 1896 he received from the British government the first wireless patent. In part, it was based on the theory that the communication range, increases substantially as the height of the aerial (antenna) is increased. A. S. Popov, a Russian inventor, constructed a coherer detector for the study of lighting dischargers and his receiver was described in 1895.
The first wireless telegraph message across the English Channel was sent by Marconi in March 1899. The use of radio for emergencies at sea was demonstrated soon after by Marconis wireless company. (Wireless sets had been installed in lighthouses along the English coast, permitting communication radios aboard nearly ships). The first transatlantic communication which involved sending the Morse-code signal for the letters was sent on Dec. 12, 1901 from Cornwall England, to Saint Johns, Newfoundland, where Marconi had set up receiving equipment.
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Questions
What is radio? ?
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Radio is a form of communication in which intelligence is transmitted without wires from one point to another by means of electromagnetic waves. , .
2) At what speed do radio waves travel? ?
Radio waves travel at the speed of light (300000 km/sec). (300000 /)
3) Are the ultra-short eaves reflected by any of the atmospheres layers? - ?
No, the pass through both layers. , .
4) Who predicted that the electromagnetic waves travel at the speed of light? , ?
James Maxwell did it. .
5) And who proved it practically by measuring the wavelength and counting the speed? , ?
Heinrich Hertz did it. .
