What fundamental property of antennas states that electrical characteristics are the same whether transmitting or receiving?

• A. Directivity
• B. Reciprocity
• C. Aperture
• D. Feedhorn

Answer: (B)

Reciprocity is the principle that makes an antenna behave the same way whether it’s transmitting or receiving. In a linear, passive environment, the paths and impedances are symmetric, so the radiation pattern, polarization, and input impedance you see when the antenna sends energy into space are the same as those you would observe if a plane wave from that direction illuminated the antenna and induced current. Because of this symmetry, the gain you derive from transmitting corresponds to the same effective aperture when receiving, via the relationship Aeff = G λ^2 / 4π.

This idea is distinct from directivity, which describes how concentrated radiation is in particular directions, not whether those characteristics swap between transmitting and receiving. It’s also separate from aperture, which is about the antenna’s ability to collect energy, and from feedhorn, which is a specific type of antenna structure rather than a fundamental property.