Sensitivity and bandwidth of low-gain active antennas below 100 MHz

Below 100 MHz, external noise from natural and man-made origins is typically strong enough to be the limiting factor in the sensitivity of active antennas. As a result, even antennas with very narrow impedance bandwidth may have large effective bandwidth in a signal-to-noise ratio sense. Although this principle has been known and exploited for many years, the theoretical limits of performance are not well established. In this paper, this problem is studied in the context of a simple but realistic system model which is applicable to low-gain antennas and which yields deterministic predictions of sensitivity and effective bandwidth. This has implications for the design of elements for future low-frequency radiotelescope arrays as well as frequency-agile and ultrawideband radios operating at these frequencies. The analysis is demonstrated using a simple dipole-like antenna.