Radio waves are electromagnetic (EM) waves generated by a radiating element (called Transmitting (Tx) antenna) and absorbed by Receiving (Rx) antenna.
An antenna is a conductor with a specified shape and geometry. Every free electron in the conductor is surrounded by an associated electric field. Whenever an alternating current flows through these passive devices i.e. whenever there is a motion of electrons in the conductor a varying magnetic field around the conductor is produced (Faraday’s Law) and also the displaced electrons’ shifts/varies the associated electric field.
On the whole, due to the flow of alternating current an EM field is created around the Antenna. This field of disturbance propagates through the media. The direction of propagation depends on the physical layout of the Tx antenna. For broadcasting purposes Omni-directional antennas are most suitable.
The field strength of this EM wave (expressed in dBm / dBW) depends on the amplitude of the current flow and varies inversely with the square of the distance between Tx and Rx antenna. Antennas are reciprocal passive devices i.e. their operation as a receiving antenna is exactly the reverse of transmitting antennas operation. In receiving mode the EM wave disturbance constitutes the flow of an electric current in the receiving antenna and this current contains the information sent by the Transmitter.
In radio broadcasting two entities are involved. One is the transmitting Base station the other is the receiving Transistor (colloquially a radio).In the Base station the audio information are converted into electric current using transducers (eg. Microphone) and this electrical signal, after modulating onto a high frequency carrier signal, is passed through (excites) the antenna which converts this electric energy into EM waves which propagates in free space and causes an EM disturbance in the Receiving antenna. As a result of this an electric current flows through the receiving antenna and this electrical signal, after demodulation (i.e., after removing the high frequency carrier), is converted into an audio information using transducers such as a loudspeaker inside the radio.
Published in The Hindu on Sep 27, 2012.