Bandwidth and SNR of Small Receiving Antennas: To Match or Not to Match

In this article, the merits of impedance matching a small antenna to the receiver’s impedance are investigated. The goal is to demonstrate that a small antenna with low radiation resistance and high input reactance, when directly connected to a high-impedance (high-impd) receiver, can provide significantly better signal-to-noise ratio (SNR) than when it is matched to a standard 50Ωinput-impedance receiver. Theoretical analysis is first carried out to compare the noise performance of the two receivers by deriving analytical expressions for the SNRs of the receivers. It is shown that the SNR of a high-impd receiver is superior to that of an impedance-matched receiver. Subsequently, two test receivers are constructed and characterized to verify the theoretical calculations. The measured SNR of the high-impd receiver is 11.4 dB higher than that of the matched receiver, even though the first-stage amplifier used in the high-impd receiver has a much worse noise performance compared to that used in the matched receiver. This study demonstrates that matching a receiving antenna to the load impedance is not always the optimal solution as far as the SNR is concerned. Furthermore, unlike a small antenna with a narrow-band matching circuit, a small antenna connected to a high-impd receiver without any matching network provides a very wide bandwidth and overcomes the gain-bandwidth limitation of small antennas. This discovery breaks new ground in the realization of compact wideband systems for low-frequency communications.