Gain and Noise Analysis of Non-Foster Matched Antennas

Non-Foster matching networks can overcome the gain-bandwidth limitations of passive small antennas, but at the cost of increased noise generated by the active matching circuit. Gain and noise measurements of a non-Foster matched small loop antenna were compared with those of the passive loop antenna to quantify the improvement in received signal-to-noise ratio (SNR) with non-Foster matching. The average gain improvement of 7 dB from 30-135 MHz was accompanied by an average added noise of 8.9 dB, indicating that the non-Foster matched antenna did not provide any improvement in the received SNR compared with the passive antenna. These results led to a study of the various noise sources in a non-Foster receiving system. Noise models were developed for the balanced and unbalanced Linvill negative impedance convertor circuits and verified with simulations. Simulations were then done to analyze the noise figure (NF) of a non-Foster receiving system for various small antennas, different biasing conditions, and receiver noise levels. These were then compared with NF simulations of small antennas attached to an amplifier and a noisy receiver. The results indicate that for receivers with low-noise floor levels, a receiving system consisting of a passive small antenna and an amplifier can provide better SNRs compared with a receiving system with a non-Foster matched small antenna.