Realized Noise Figure of the General Receiving Antenna

In discussions of antenna noise figure, the effect of antenna impedance mismatch is often neglected or not correctly accounted for. Antenna impedance mismatch can degrade signal-to-noise ratio, even with a lossless antenna, particularly when the received signal and/or the received external noise powers approach the receiver's noise floor. This letter presents a formula for the realized noise figure of the general receiving antenna expressed as a function of the antenna's radiation efficiency and impedance mismatch loss. This realized noise figure is defined for the situations where the antenna is connected to a receiving system at a noise temperature of as well as a general noise temperature. This formula for realized antenna noise figure allows the antenna engineer to understand how the antenna design, particularly the tradeoffs between impedance matching and radiation efficiency, affect the signal-to-noise ratio performance in the general receiving system.