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An analysis of the noise performance of an active receiving antenna system consisting of an antenna, active coupling network, and high-gain preamplifier is described. It is shown how this noise performance can be described by circular loci in the output-impedance plane of the coupling network. These loci are not restricted to the right-half plane. A coupling network having an output impedance in the left-half plane was built using a negative-impedance converter (NIC), and experimental results obtained by using this circuit in conjunction with a short monopole are presented.