Buffer amplifiers for an electrically small broadband antenna

Abstract

Professional direction finding applications cover broad bandwidth of one to two decades and performance of the radiative element is of fundamental importance for such receiver oriented applications. It is important to optimize radiative element, i.e, antenna, for the bandwidth of interest. However, portability and weight concerns for the system limits radiator dimensions and eventually restricts the antenna design. Such antennas with their limitations are referred as electrically small antennas. For the application, utilization of a single electrically small monopole antenna that is capable of recovering the electromagnetic signal within a bandwith more than a decade is considered. Due to electrically small radiator, the antenna has a large reactance and small resistance as its input impedance, making it strictly narrowband. For wideband applications, presence of highly reactive impedance of the antenna implies significant reflection loss. This work is based on buffering an electrically small monopole receiver antenna that is to be used by a compact direction finding system. In the bandwidth of operation (20MHz–1GHz), the antenna is not suitable for passive matching, therefore a buffer amplifier is considered. Transferring the power directly to the receiver is not possible due to input impedance of the antenna. Alternatively, the approach is to probe the antenna output voltage by the active circuit with a high input impedance. This implies antenna voltage is taken without any reduction, then reference voltage is buffered to the output while providing a constant output impedance. Two types of buffer amplifiers for the monopole are designed, simulated and tested combined with the antenna.