The reciprocity theorem for antennas states which of the following?

• A. Transmit and receive characteristics are the same for a given frequency and pattern.
• B. The antenna can only operate in one mode (transmit or receive) at a time.
• C. Reciprocity applies only to isotropic antennas.
• D. Reciprocity theorem states that transmit and receive characteristics are always different.

Answer: (C)

Reciprocity in antennas means that the way an antenna radiates when you drive it with a signal is the same as the way it responds when a plane wave from the same direction and frequency hits it as a receiver. Because the system is linear and reciprocal, the radiation pattern, impedance, and gain you observe in transmission are identical to what you would measure when using the same antenna to receive. So, for a given frequency and pattern, the transmit and receive characteristics match.

This isn’t limited to isotropic antennas. Reciprocity applies to any reciprocal antenna, regardless of whether its pattern is directional or omnidirectional. It also doesn’t imply a mode restriction—antennas are designed to function in both transmitting and receiving roles, and reciprocity describes their identical behavior in those two modes. Conversely, transmit and receive characteristics being always different contradicts the reciprocity principle.