Experimental Study of Electrically Compact Retrodirective Monopole Antenna Arrays

Abstract

Abstract-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 are confirmed by measurement. Particularly it is shown that the Direction-of-Arrival (DoA) 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 6dB. Retrodirective array antenna matching and the feasibility of a frequency-division full-duplex communication link based on the proposed antenna arrays are also discussed.

Antenna terminals based on ultra‐compact retrodirective antenna arrays

“…In (3), ρ and ρ n are the observation point radius-vector in the array plane at z = h A and the radius-vector of the nth antenna element centre, respectively. In (3), the '+' sign should be chosen for z ≥ h A and the '−' sign should be taken for z ≤ h A , h A is the distance between the array plane and the reflector [20,24,25] at z = 0. It should be stressed that the wavenumber χ of the evanescent harmonics exceeds the wavenumber in free space χ > 2π/λ [20,30] and therefore when the evanescent components (reactive field) dominate over the incident field the DoA information Φ n is encoded into the scattered field with higher spatial rate ∼ 2π/ χ than the spatial rate of the incident wave in the free space.…”

“…The retrodirective properties of the compact ATs are enabled by the PC circuitry whose operation is considered in detail in [20,24,26]. It is important to note that due to a very close electrical proximity of the retrodirective antenna elements (with separation of order of λ/10), the realisation of full-duplex communication link at the same frequency/time is challenging due to a very low isolation between the receive and transmit (TX) chains and therefore time or frequency division between the downlink and uplink has to be used.…”

“…It should be also noted that in the simulation models used in this paper, the effect of complex PC circuitry in Fig. 1b is modelled in the TX (retrodirective) mode by numerical complex conjugation of the RX voltage v n RX across the nth antenna port [20,24,31] Fig. 3a shows the retrodirected beam in the yz plane generated by the resonant five-element antenna array with dimensions as shown in Fig.…”

“…These spacings enable mitigation of EM coupling effects degrading dense antenna array performance due to out-of-phase currents generation and lowering of the radiation resistance [13,22]. In dense arrays (with reduced inter-element spacings) the scattered EM has a dominant reactive part which in general leads to higher EM coupling and reduced antenna return loss bandwidth [13,[22][23][24].…”

“…Angular super-resolution is achieved by encoding/sampling the DoA information over the highly oscillatory evanescent harmonics generated in the dense antenna array environment [20]. As demonstrated in [20,24] the spatial sampling rate of the DoA information appears to be much higher than the characteristic spatial variation of the incident waveform (given by half-wavelength of radiation). Next, retrodirection of both the propagating and reactive parts of the EM field in the antenna array environment generates an array excitation vector [20] resulting in the radiated beam with auto-pointing properties.…”