Q.Explain any two factors which justify the need for modulating a signal. OR A transmitting antenna at the top of a tower has a height 32m and the height of the receiving antenna is 50m. What is the maximum distance between them for satisfactory communication in LOS mode? (Given radius of earth 6.4x10⁶m.)

Baseband (audio) signals cannot be transmitted efficiently on their own because of antenna-size and radiated-power constraints, both of which are resolved by modulating them onto a high-frequency carrier.

1. Practical antenna height: For efficient radiation, an antenna needs a size comparable to the wavelength of the signal (typically of the order $\lambda/4$). Audio-frequency signals (20 Hz – 20 kHz) have wavelengths of the order of tens of kilometres, which would require an impossibly large antenna to transmit directly. Modulating the audio signal onto a much higher-frequency carrier (with a correspondingly short wavelength) allows antennas of a practical, manageable size to be used.

2. Effective power radiated: The power effectively radiated by a linear antenna of length $l$ for a wave of wavelength $\lambda$ is proportional to $(l/\lambda)^2$. For low audio frequencies (large $\lambda$) and a realistically sized antenna, this ratio — and hence the radiated power — would be negligibly small. Using a high-frequency carrier makes $l/\lambda$ large enough for a significant, useful amount of power to be radiated.

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