ESPAR Antenna - For Commercial Adaptive Technology

Signal to Interference Noise Ratio (SINR) for Wireless Local Area Network (WLAN) systems and cellular networks can be improved by using adaptive antenna systems. Conventionally phased array antennas have been used for beam and null steering but the problem with them is that they are intentionally not made compact to avoid mutual coupling between elements. To overcome this limitation and due to many other advantages, ESPAR Antenna has become very popular recently. ESPAR stands for Electronically Steerable Parasitic Array Radiator. Conceptual overview of ESPAR antenna structure is shown in Fig. 1*. There is an active monopole in center surrounded by several parasitic elements in circular configuration. This active monopole induces voltage in the surrounding elements. By theory of arrays, if somehow the current through individual array elements can be controlled, the overall radiation pattern can be controlled. For this purpose, each parasitic element is loaded by a varactor, whose variable reactance (capacitance) is varied by changing the applied DC voltage.

Each phase shifter in the phased array is replaced by a cheap varactor. Moreover, since there is a single feed element, only one transmitter and receiver circuit is required*. Since a varactor diode operates under reverse bias, almost no current flows through it which accounts for low energy consumption. Due to it's low cost, compact size, uncomplicated structure and low power requirements, the antenna has immense potential to be massively embedded in handheld devices.

* Takashi Ohira and Kyouichi Iigusa, Electronically Steerable Parasitic Array Radiator Antenna, Electronics and Communications in Japan (Part II: Electronics), Volume 87, Issue 10, pages 25–45, October 2004.