Electrically Small Antenna with Embedded Operational Amplifier Circuit Surpasses the Passive Upper Bound of the Gain-Bandwidth Product

A near-field resonant parasitic (NFRP) active electrically small transmitting antenna is reported in this paper. It consists of an electrically small NFRP antenna and an operational amplifier (OpAmp). In the design, the NFRP element serves as the main radiator and the feedback pathway of the OpAmp simultaneously, i.e., the electrically small antenna (ESA) and the OpAmp circuit are arranged within a shared ultra-compact space. This configuration significantly increases the effective gain of the ESA without consuming any additional real estate. The overall electrical size of its radiating element is extremely small with $ka=0.15$ at 414 MHz, i.e., $a\approx \lambda/42$. The measured results of this active ESA, in good agreement with their simulated values, demonstrate that its effective gain can be dynamically tuned within a 6.01 dB range. Compared with its corresponding passive counterpart, the measured maximum effective gain and hence, the effective isotropic radiated power (EIRP) of our developed active ESA witnesses a 9.152 dB (8.23 times) improvement. Thus, the measured gain-bandwidth product (GBWP) surpasses the corresponding passive Bode-Fano upper bound by approximately 15.2 times.