Experimental Study of Electrically Compact Retrodirective Monopole Antenna Arrays

Auto-pointing and angular super-resolution properties of the radiation patterns generated by an electrically compact retrodirective monopole antenna array are demonstrated experimentally for the first time. The operation of electrically compact (element spacings less than one-fifth of the radiation wavelength) retrodirective antenna arrays that were theoretically considered in our previous work is confirmed by measurement. Particularly, it is shown that the direction-of-arrival information carried by an incident electromagnetic wave can be encoded into the evanescent near field of an electrically small resonant antenna array with a spatial rate higher than the spatial oscillation rate of the incident field in free space. This observation is supported by the near-field measurements, demonstrating that the magnitude of the scattered (evanescent) field in the array environment can exceed the magnitude of the incident field in free space by at least 6 dB. Retrodirective array antenna matching and the feasibility of a frequency-division full-duplex communication link based on the proposed antenna arrays are also discussed.

I. Introduction

Electrically small antennas and antenna arrays have been extensively studied for several decades due to their theoretical and practical importance [1]–[6]. As a result of their small electrical size, these antennas and antenna arrays find many applications, including wireless communications, electromagnetic (EM) sensors, small aperture radars, and compact microwave and millimeter-wave imaging systems [1], [6], [7]. Several interesting designs of electrically small antennas have been proposed recently. These include folded antenna geometries [1], [2], [6], [8], metamaterials [9]–[11], dielectric resonator antennas [1], [12], and biologically inspired resonantly loaded antennas [13], [14].