Efficiency Analysis of Switched Resonant Antennas for Pulse Radiation

Abstract

A new pulse transmitting scheme based on switched resonant antenna is proposed. Instead of using antenna as the last component in the transmitter, the essential idea of the pulse transmitting scheme is to directly modulate the antenna by integrating switch devices on the antenna. Rather than using the antenna’s linear frequency response as the design basis, we focus on the resonant antenna’s nonlinear transient response in the new scheme. Different from conventional integrated or reconfigurable antennas the switching devices create varying boundary conditions in real time for the antenna. Specifically for a patch antenna, they turn on and off the antenna’s radiating slots directly to generate a fast turning on/off magnetic currents on them. The transmission of short pulses is realized as the radiation is simply associated with the magnetic currents flowing on the radiating slots. In this paper, the pulse transmitting scheme is based on a rectangular microstrip patch antenna with switches mounted on one of the two radiating slots and PEC on the other radiating slot. The physical structure of the pulse transmitting is presented and the fundamental working principles will be studied based on the cavity model and transmission line model of this particular structure. 2. Analysis The physical structure and the equivalent cavity model of the pulse transmitting scheme based on a patch antenna are illustrated in Fig.1 (a) and (b) respectively. Switch devices such as Schottky diodes or PIN diodes are integrated on one of the two radiation edges, to provide open or short circuit depending on the biasing voltage, while, a perfect conductor is put on the other radiating edge. The radiation behavior of such an antenna is rather complicated, which is related to both the antenna’s transient behavior and the device characteristics. However, a simple intuitive analysis is possible based upon the understanding of resonance behavior in the patch antenna. From energy point of view, the RF energy is initially injected into the antenna resonator when the switch is turned on. During this period, the antenna operates at a non-radiating resonant mode, which has a half-sinusoid electric field distribution with two zeros at the edges and maximum at the center. The electromagnetic energy accumulates in the antenna until the full capacity is reached. This happens when the dissipated power is equal to the injected power. When the switch is turned off, the previous resonant boundary condition is destroyed immediately and the operating mode is also destructed simultaneously. Therefore, the magnetic current e.g. the fringe electric field at the edge is turned on immediately and the previously stored energy starts to radiate into free space. The instantly radiated energy is neither directly from the source supply nor from the stored energy in the radiating mode as the radiated mode has not established at that moment. Instead, it is from the energy stored previously in the nonradiating mode. By controlling the switches on and off, the boundary condition changes and the resonance starts or stops or changes rapidly and the magnetic current e.g. the fringe electric field at the edges are turned on and off so that series of short pulses are generated accordingly. Therefore, the antenna behaves as a pulse modulator and radiator at the same time.