# Explain any two factors which justify the need for modulating a low frequency base-band signal. - Physics

Explain any two factors which justify the need for modulating a low frequency base-band signal.

#### Solution

1.For efficient transmission and reception, the transmitting and receiving antennas must have a length equal to quarter wavelength
of the audio signal. For a frequency of 15 kHz of audio signal, the length of the antenna of this size is practically impossible.
"wavelength"="wave velocity"/"frequecny" =(3×10^8)/" frequecny"

so to transmit a signal of frequency 20 kHz , we need an antenna of "height" =(3×10^8)/(20×10^3)=15 km

Antenna of such a height cannot be constructed.

2.Effective power radiated by an antenna - For linear antenna of length l, it is seen that power radiated ∝(1/λ^2)`
thus for the same antenna length, the power radiated by short wavelength or high frequency signals would be large.
If the audio signals(of longer wavelength) are directly radiated into space, they die out out after covering some distance due to the low power radiated by the antenna.

Is there an error in this question or solution?
2016-2017 (March) Delhi Set 1

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