Active Conjugated Matching Circuit for Bandwidth Enhancement of Electrically-Small Microstrip Antenna

In this letter, an electrically small microstrip antenna based on active matching is designed for ultra-wideband applications. The passive antenna is printed on a ceramic substrate with a dielectric constant of 10.2. A non-Foster circuit is added to produce negative inductance that can counteract the inductive reactance of the passive antenna. Balun is added to shift the real part of the input impedance to 50 Ω. A prototype of the active microstrip antenna is fabricated with a compact volume of λ_L/20 × λ_L/20 × λ_L/600 (λ_L is the wavelength at the lowest frequency point). The measured −10 dB bandwidth covers the 200–800 MHz range. Compared to the passive microstrip antenna, the proposed active antenna enhances the gain by about 20 dB. The combination of a non-Foster circuit and balun provides great flexibility to achieve wideband performance for electrically small antenna.