Q. 105.0( 1 Vote )

An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because
A. the size of the required antenna would be at least 5 km which is not convenient.

B. the audio signal cannot be transmitted through sky waves.

C. the size of the required antenna would be at least 20 km, which is not convenient.

D. effective power transmitted would be very low, if the size of the antenna is less than 5 km.

Answer :

It is given that the frequency of the signal is .

Now, the wavelength is given by:





Now, the antenna has to be at least λ/4 i.e. 5 km.


It is not convenient to have such a huge antenna !


Hence, option (a) is correct and option (c) is incorrect.


The power radiated by the transmitter is proportional to


where l is the length of the antenna. The power radiated decreases with increasing wavelength and increasing length. If the antenna is less than 5km, then the power radiated will be less causing a weak and highly attenuated signal to reach at the receiver. Hence, option (d) is correct.


Sky waves are mode of transmission for signal with frequencies from a few MHz to 30-40 MHz. Now, the given signal is 15KHz or 0.015MHz. Thus, transmission via sky waves is not possible. Thus, option (b) is correct.

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