Question of 11
Q.(a) A receiver in a communication system must have
(i) pick-up antenna
(ii) demodulator
(iii) amplifier
(iv) all of these (Score : 1)
(b) Which of the following statements is wrong ?
(i) The attenuation of surface waves increases with increase in frequency.
(ii) The phenomenon involved in sky wave propagation is similar to total internal reflection.
(iii) Space wave mode of propagation is used in satellite communication.
(iv) Sky wave propagation is useful only in the range of frequencies 30 to 40 MHz. (Score : 1)
Kerala DhseKerala DHSE Plus Two Board 2016Subjective· 2mImportance★★★★★
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Start your 14-day free trial to unlock the full solution →A practical receiver needs all three stages (antenna + amplifier + demodulator); and of the four statements about wave propagation, the one that restricts sky-wave use to "only 30 to 40 MHz" is the incorrect one.
- A communication receiver has to (1) pick up the transmitted electromagnetic wave — this is the job of the pick-up antenna; (2) strengthen the very weak received signal — the amplifier; and (3) recover the original message signal from the modulated carrier — the demodulator. A working receiver needs all three stages together, so the correct choice is (iv) all of these.
- Check each statement against known propagation facts:
- True — a surface (ground) wave loses energy to the ground by induction, and this attenuation increases sharply with frequency; that is exactly why ground-wave propagation is only practical at lower (MF/LF) frequencies.
- True — sky-wave propagation relies on the ionosphere bending the wave back to Earth. As the wave travels upward through layers of increasing free-electron density (decreasing refractive index for radio waves), it bends more and more; beyond a critical angle it turns back down instead of escaping — the same geometric idea as total internal reflection at a boundary of decreasing refractive index. …
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