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Why can an antenna absorb radiation at the same frequency that it emits?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

An antenna can absorb and emit radiation at the same frequency because of the principle of reciprocity, which dictates that the properties of an antenna—such as its radiation pattern and impedance—are identical whether it is functioning as a transmitter or a receiver.12

At its core, an antenna is a transducer that converts between guided electrical energy and free-space electromagnetic radiation. When transmitting, an oscillating current in the antenna structure creates vibrating electric and magnetic fields that propagate as waves. Conversely, when an incoming electromagnetic wave strikes the antenna, the oscillating fields induce a corresponding alternating current within the conductor.231

The efficiency of this process depends on the antenna's physical dimensions relative to the wavelength of the radiation. Because the antenna's geometry is tuned to resonate at specific frequencies, it naturally facilitates the efficient transfer of energy in both directions: it effectively projects energy into space when driven by a transmitter and effectively extracts energy from passing waves when acting as a receiver.21