An antenna system includes a first antenna, a second antenna, and a tuning stub, where the first antenna and the second antenna have a same first operating frequency band or similar first operating frequency bands. The tuning stub is electrically connected to the first antenna, and the tuning stub is configured to adjust an equivalent current path of the first antenna, so that a connection line between a projection point of a maximum equivalent current point of the first antenna and a projection point of a maximum equivalent current point of the second antenna on a first plane tends to be more perpendicular to a projection of the equivalent current path of the first antenna or of the second antenna on the first plane; or the equivalent current path of the first antenna and of the second antenna tend to be more perpendicular to each other.

A matching technique for receivers with ESA so that the receiver can achieve low noise performance over a broad bandwidth. An electrically small dipole is matched directly to a parametric amplifier, without using any inductors. The parametric amplifier operates in a time-varying fashion, driven by a higher RF frequency pump, and exhibits a high impedance, time-varying capacitive load to the ESA. Therefore, the low frequency voltage output from the ESA is detected at the input of the parametric amplifier, being amplified and upconverted to the proximity of the pump frequency at the output with minimum contamination of noise in this process.

A matching technique for receivers with ESA so that the receiver can achieve low noise performance over a broad bandwidth. An electrically small dipole is matched directly to a parametric amplifier, without using any inductors. The parametric amplifier operates in a time-varying fashion, driven by a higher RF frequency pump, and exhibits a high impedance, time-varying capacitive load to the ESA. Therefore, the low frequency voltage output from the ESA is detected at the input of the parametric amplifier, being amplified and upconverted to the proximity of the pump frequency at the output with minimum contamination of noise in this process.