Single-Transistor Impedance Matching Circuit for Over-Hundred-Octave Active Antennas

In this article, a novel active impedance matching circuit with a single transistor is studied for antennas to cover over a hundred-octave bandwidth. In the proposed matching circuit, only one field effect transistor (FET) is utilized and biased in the ohmic region. The FET is integrated with a dipole antenna at the gate, and connected with a $50~\Omega $ transmission line at the drain. With small dimensions, this active antenna performs as a receiver with a wide impedance bandwidth of 10 MHz–1.9 GHz, in which the reflection coefficient is lower than −10 dB, covering 190 octave bandwidth. An equivalent circuit model of the ohmic-biased FET is built to analyze this unusual wideband performance, which exhibits promising applications in ultrawideband (UWB) electromagnetic sensing systems with extremely restricted volume.

I. Introduction

Bandwidth of impedance, i.e., the operating frequency range of impedance matching with low reflection loss, is one of the most critical criteria in evaluating the performance of an antenna [1], [2], [3], [4], [5]. Wideband antennas possess significant benefits, such as multiple mode sensing, high multipath resolution, high spectrum efficiency, and noise immunity, to name a few [6], [7], [8], [9]. Therefore, expanding the bandwidth of antennas has been a constant pursuit in antenna engineering [5], [10]. Over the last decades, numerous studies have validated a limit on the antenna’s achievable bandwidth over certain dimensions, which is known as Chu’s limit [11], [12], [13]. On the other hand, impedance matching is a critical method to enhance an antenna’s bandwidth without enlarging its dimensions. But there is still a bandwidth limit for certain antenna impedance cooperated with a passive matching circuit, known as the Bode-Fano limit [14], [15]. As the Bode-Fano limit indicates, the antennas with electrically small dimensions possess a large capacitive impedance, while their achievable bandwidth is still narrower even with a passive matching network [16]. Under these restrictions, wideband antennas are not realized with electrically small dimensions, restricting their applications in volume-limited systems, especially for multiantenna scenarios [17], [18], [19], [20].