Q.Give the basic function of antenna.

For a linear (dipole) antenna, the power radiated is inversely proportional to the square of the wavelength used.

For an antenna of length $l$ radiating at wavelength $\lambda$, the radiated power scales as $P \propto \left(\dfrac{l}{\lambda}\right)^2$. So for a fixed antenna length, as wavelength increases, radiated power decreases as $1/\lambda^2$ — this is why practical antennas are made comparable in size to the wavelength they are meant to radiate/receive efficiently.

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Direct transmission of an audio-frequency electrical signal would need an antenna many kilometres long, which is impractical — so the signal is first modulated onto a high-frequency carrier wave.

When sound waves are converted into electrical signals (e.g. by a microphone), the resulting signal has audio frequencies, roughly 20 Hz to 20 kHz. For efficient radiation, the length of a transmitting antenna must be comparable to the wavelength of the wave being transmitted (typically at least λ/4). For a frequency of, say, 20 kHz, the wavelength is

$\lambda = \dfrac{c}{f} = \dfrac{3\times10^{8}}{20\times10^{3}} = 1.5\times10^{4}\ \text{m} = 15\ \text{km}$ …

An antenna is a transducer between guided electrical signals and free-space electromagnetic waves — it radiates a signal as EM waves when transmitting, and picks up EM waves and converts them back to an electrical signal when receiving.

An antenna (aerial) is essentially a specially shaped conductor. When an oscillating/alternating current of the same frequency as the signal to be sent flows through it, the accelerating charges radiate energy as electromagnetic waves that propagate outward — this is transmission. Conversely, when an electromagnetic wave passes over a receiving antenna, its oscillating electric field induces a small alternating current in the conductor, which is then amplified and processed as the received signal — this is reception. So the antenna's basic job, in either direction, is to couple electrical energy in a circuit to electromagnetic radiation in free space.

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