ABC of FM for the
Private Broadcaster

The new FM policy announced by the government opens FM broadcasting to private broadcasters. This article contributed by Vasant Dave of Gujarat Communications and Electronics looks at the technical aspects of FM broadcast and the setting up of FM broadcast stations. GCEL provides turnkey solutions for TV & sound terrestrial transmission systems.

by Vasant Dave

Its body from one hump to the next is a cycle, and the distance between them a wavelength. Frequency is the number of cycles occurring in one second. We measure it in Hertz (Hz) in memory of the scientist who discovered electric waves.

What is modulation?
   In order to radiate effectively, antenna should measure a quarter wavelength of the frequency used. Wavelength multiplied by frequency

is the speed of radio waves, that is 300,000 km per second. For audio frequency of 15,000 Hz, the wavelength = 300,000 km per sec divided by 15,000 Hz = 20 km. Quarter of it is 5 km. An antenna of that size is ludicrous. Hence we need to reduce the wavelength of the audio signal before sending it in space.
   We learnt at school that mines often transport ore through pipelines. They crush

the ore, mix it with water, and pump the slurry in a pipeline. At the destination they evaporate water and recover the ore. Modulation is similar to ‘making a slurry’ of two signals, ‘pumping’ them in space, and ‘evaporating’ the carrier to retrieve the main signal.
   A radio wave can convey useful information by changing either its amplitude, frequency or phase in tune with change in that information. Amplitude indicates size.

Aradio station basically records and transmits. It often shares studio facilities, but it owns the transmitter that is its heart. Let us first recall some electronic techniques, then see how an FM station broadcasts, and finally examine project execution.We create sound waves when we speak. We convert them to electrical waves in order to transmit them far. Both waves possess frequency. Suppose a string of sitar vibrates 440 times per second. A microphone turns these vibrations to an electrical signal that also changes 440 times a second. Let us imagine the signal as a moving caterpillar.