Wireless Broadcasting

Broadcasting means to send out in all directions. A radio broadcasting system is illustrated in Figure. The transmitter sends out electromagnetic radio waves radiated from its antenna. Receivers can pick up transmitted radio signal by means of receiving antenna or aerial. The receiver reproduces the desired signal transmitted by the broadcast station. There are many radio signals in space from different transmitters, but the receiver can be tuned to the frequency of the station we want.


In Figure, the electromagnetic wave shown is a radio-frequency (RF) carrier signal with amplitude modulation (AM). The Amplitude or strength of the RF carrier varies in step with variations in desired voice of music information, which is the audio signal. This technique of modulating a carrier wave is necessary because the audio signal itself cannot be used for wireless transmission. The variations are too slow for effective radiation from an antenna. A higher-frequency carrier wave is chosen for the best radio transmission. Its modulation provides the desired signal information.

In the method of frequency modulation (FM), the modulating signal varies the frequency of the RF carrier wave. Either AM or FM can be used for any type of modulating signal.

Frequency is an important characteristic of any varying voltage or current, to specify how fast the amplitudes change. A complete set of changes in one cycle. The number of cycles repeated in a second is the frequency. The unit for frequency is the hertz (Hz), equal to one cycle per second (cps). As an example, the 60 cycle ac power line has a frequency of 60 Hz.

Radio frequencies are generally considered to be about 30,000 Hz and above. Radio-frequency carrier frequencies for wireless transmission are usually specified in kilohertz (kHz), equal to 1000 Hz, and megahertz (MHz), where 1 MHz = 1,000,000 Hz.