Effect of antenna impedance mismatch on the signal-to-noise ratio of a radio receiving system

The noise factor of linear two-port networks are derived for several networks of interest, including those of a passive transmission line and of an active receiver, whose source impedance may be mismatched to the characteristic impedance of the line. It is shown that a large impedance mismatch at the antenna-transmission line interface of a radio receiving system can cause a significant increase in the system internal noise factor (more than a 50 dB increase for a voltage reflection coefficient of 09999). Numerical results are presented for a VHF-FM radio receiving system with an electrically-short monopole antenna. In the absence of a matching network, the limiting noise of the system is generated within the system (by the transmission line and receiver) and can be more than 20 dB larger than the external man-made noise. With a matching network, the limiting noise of the system is external man-made noise.