An audio signal of 15 kHz frequency cannot be transmitted over long distance without modulation because.

Options

A. the size of the required antenna would be at least `5 km` which is not convenient
B. the audio signal can not 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

A, B, D

Explanation

Here , `v_(m) = 15 kHz = 15 xx 10^(3) Hz`
`lambda_(m) = (c )/( v_(m)) = ( 3 xx 10^(8))/( 15 xx 10^(3)) = (1)/(5) xx 10^(5) m`
Size of the antenna required , ` l = ( lambda)/( 4) = (1)/(4) ((1)/(5) xx 10^(5)) = 5 xx 10^(3) m = 5 km`
The audio signals are of low frequency waves . They cannot be transmitted through sky waves as they are absorbed by atmosphere.
If the size of the antenna is less than ` 5 km` , the effective power transmission would be very low because of deviation from resonance wavelength of wave and antenna length.