Moving forward in retrodirective antenna arrays - Potentials, IEEE

0278-6648/03/$17.00 © 2003 IEEE IEEE POTENTIALS Antenna arrays are individual radiating elements combined so that they function and perform like a single large antenna. Besides high radiating beam directionality, antenna arrays offer the capability to scan the main beam and control sidelobe radiation elect r o n i c a l l y . Retrodirective antenna arrays, as the name suggests, have a special feature. When receiving a signal from an unspecified direction, the array can automatically transmit a signal response to that same direction without any previous knowledge of the source direction. This function is performed automatically without the use of phase-shifters or digital circuitry. Compared to other array antennas that rely on digital signal processing (DSP) for beam direction control, this approach is much simpler and potentially faster because digital calculation is not needed. The automatic beam control nature of retrodirective arrays is well suited for RFID (Radio Frequency Identification) and microwave tracking beacon applications. As an example, for the RFID application, the electronic interrogator identifies the retrodirective RFID by first transmitting a control signal. The retrodirective array reacts to this signal by transmitting a response signal back to the interrogator’s direction automatically. Because the retrodirective array response is a directed beam rather than omni-directional, the communication link gain is increased by way of the power combining effect of the antenna array. This in turn implies that the burden on the transmitting and the receiving amplifiers is reduced while still maintaining a reasonable link gain. Another consequence of directional transmission of the retrodirective array is a reduction of the probability of signal detection by unwanted third-party receivers, increasing data security. A related application of this technology is use as a microwave-tracking beacon. In this application scenario, an interrogation signal is transmitted in all directions. The retrodirective array, upon receiving the signal, is able to automatically respond by transmitting a signal back to the interrogator. The interrogator is then able to determine the tracking beacon’s position based on this response. This article will discuss the basic functionality as well as the ongoing research efforts and developments in retrodirective array architectures. Self-phasing attributes of different arrays will be presented, as well as their applicability in practical communication systems.