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IETE TECHNICAL REVIEW, Vol 23, No 5, 2006

 

The recording and playback facilities will Centre on a hard disk based system assisted by compact disc and cassette players etc.

TRANSMISSION CONFIGURATION

Major components in a transmitting chain consist
of Transmitter, Antenna, Feeder Cable and Tower.
Various combinations can, therefore, be worked out
to suit individual requirements and keeping in view
the availability of funds. These combinations will,
however, revolve around the following parameters.

  • Power of the transmitter: 50 W
  • Configuration: Single or 1 +1
  • Antenna gain: Not more than 3 dB
  • Polarization: Vertical, Horizontal, circular
  • Tower: Not more than 30 M

Why FM for Community Radio?

Each AM channel is assigned a range of frequencies, typically about 10 kHz wide. The common frequency identification represents the midpoint of this operating channel range. Actually, the station uses only about 2.5 kHz on either side of this midpoint because of the interference problems outside. Therefore, the total bandwidth of an AM channel is typically about 5 kHz.

 

The musical range of the human ear is about 20 kHz, and that is the reason why AM sounds imperfect.


Then there is the FM band of frequencies. These correspond to frequencies between 88 MHz and 108 MHz. Each FM channel is assigned a bandwidth of 200 kHz. The FM bandwidth can easily cover the musical range of the human ear of about 20 kHz, and that is why FM radio sounds better than AM radio. In fact, the bandwidth allows FM to be broadcast in stereo.

The scatter plot (Fig 1) contains a 45 degree diagonal line. Any radio program type that is below this diagonal line is preferred more in FM, and any program type above this diagonal line is preferred more in AM. Clearly, the consumers prefer to listen to music in FM than to spoken words in AM.

There are two interpretations of this phenomenon. First of all, this reflects the strengths of each transmission method — larger coverage with theoretically poorer quality for AM, and smaller coverage with theoretically better quality for FM. Alternately, this simply reflects the fact that AM stations carry mostly spoken words and FM stations carry most by music, each playing to its own perceived strengths. Thus, the most and the best music programs are found on the FM spectrum, and the most and the best news programs are found on the AM spectrum.


Fig 1 Scatter plot of preference of AM & FM