Contains the full lesson along with a supporting toolkit, including teachers’ notes.
The beginning of radio
The first radio transmission was made in 1895. It did not carry audio signals (voice or music) but consisted of just short bursts of a fixed radio frequency using Morse code. Morse code had been in use from the 1840s to send messages via telegraph wires. The code consists of short and long pulses, commonly known as dots and dashes, and may be represented by electrical signals or flashes of light. For example, the code for ‘S’ is • • • and the code or ‘O’ is — — —. The code for SOS is therefore • • • — — — • • •.
Magnetic and electric fields
An electric current in a conductor (e.g. a copper wire) produces a magnetic field around the wire. If the current is reversed then the magnetic field is also reversed. This effect, which was discovered by Ørsted in 1831, is easy to demonstrate.
The strength of the magnetic field decreases as the cube of the distance from the wire increases (if the distances are large compared with the length of the wire). For example, the magnetic field strength at a distance of one kilometre is a millionth of its strength at a distance of ten metres.
In a similar way, opposite electric charges have an associated electric field, the direction of which is designated as ‘positive to negative’, as shown in the diagram. If the charges are reversed then the direction of the electric field is reversed.
But how quickly are these effects propagated? Do the changes occur instantly or do they take some time to travel? Can the effects travel faster than light?
How is a radio signal transmitted?
Consider an antenna (aerial) made of two vertical conductors attached to variable voltage source as shown in the diagram. When it is switched on, almost instantly the top part becomes positively charged and the bottom part becomes negatively charged. In that brief instant while the charges are moving a magnetic field is set up. Once the charges are established no further charge moves (i.e. there is no longer any electric current) and so the magnetic field strength drops to zero.
At the same time an electric field is set up between the positive and negative charges. This field is perpendicular to the magnetic field. If the aerial is vertical then the magnetic field is horizontal and the electric field is vertical; in this case the propagated wave is said to be vertically polarised.
If the voltage source is reversed then the magnetic and electric fields reverse. If this happens two million times per second then the aerial radiates alternating electric and magnetic fields, i.e. ‘radio waves’, at a frequency of one megahertz (1 MHz). These electromagnetic waves travel at the speed of light. (We now know that light itself is an electromagnetic wave.) Note that one full wave or oscillation involves two changes of direction.
From Morse code to analogue radio
Maconi’s original radio transmitters and receivers were intended to replace wired telegraphy with wireless telegraphy. His radio signals were detected by a relatively simple circuit which would activate an electromagnet or turn on a small light when a burst of radio frequency signal was picked up by a large receiving antenna. In 1897 he established a number of transmitters on the south coast of England for communication with ships.
In 1907 the first commercial transatlantic wireless telegraph service was set up by Marconi between stations at Clifden, Ireland, and Glace Bay in Newfoundland. The power of the Clifden transmitter was 150,000 watts.
Wireless voice communication was first demonstrated publicly in 1900 by Roberto Landell de Moura (in Brazil). In 1920 regular radio transmission for the public started; broadcasting had begun. Initially the audiences were small because relatively few people had radio receivers. Crystal radios could be bought or made but they required headphones and were little more than a curiosity. The BBC radio service began in 1922. Ireland’s national radio station, known as 2RN, began broadcasting from McKee Barracks in Dublin, on 1st January 1926.
Its remit covers all kinds of transmission networks including: