Oleksandr Malyuskin

Active Phase Conjugating Lens With Sub-Wavelength Resolution Capability

Experimental results are presented for the focusing capability of an active phase conjugating lens for a single and a dipole source pair and these are compared with predictions. In addition for a single source we illustrate the ability of the lens to project a null at the lens focus instead of a peak. A scheme is also presented such that when a source or pair of sources is imaged through an identical pair of passive scatterers located symmetrically about the lens that imaging with sub-wavelength resolution is possible. The rationale for the operation of the lens and aberrations observed due to its finite array size is discussed and is supported throughout by means of numerical simulation.

Phase conjugating wire FSS lens

Analysis of a microwave lens formed using a pair of frequency selective surfaces comprised of back-to-back double-periodic wire arrays interconnected through phase-conjugating microwave circuitry is given in this paper. It is shown that such a structure when illuminated with a plane wave generates a phase-conjugated forward-transmitted plane wave which experiences negative refraction. Negative refraction makes it possible to produce focusing of wave packets incident on such structures both in the far- and near-field regions. We provide analytical formulation of the problem and develop a rigorous calculation of the EM field transmitted through the structure: i) when excited by a plane wave; and ii) due to an electric line source suspended above it. Numerical simulations illustrate the focusing properties of the proposed lens. These properties are shown to include half wavelength compression of the focused line source image in the transverse direction in the far-field zone. A physical explanation of why this effect occurs is also provided

Far Field Subwavelength Source Resolution Using Phase Conjugating Lens Assisted With Evanescent-to-Propagating Spectrum Conversion

The imaging properties of a phase conjugating lens operating in the far field zone of the imaged source and augmented with scatterers positioned in the source near field region are theoretically studied in this paper. The phase conjugating lens consists of a double sided 2D assembly of straight wire elements, individually interconnected through phase conjugation operators. The scattering elements are straight wire segments which are loaded with lumped impedance loads at their centers. We analytically and numerically analyze all stages of the imaging process; i) evanescent-to-propagating spectrum conversion; ii) focusing properties of infinite or finite sized phase conjugating lens; iii) source reconstruction upon propagating-to-evanescent spectrum conversion. We show that the resolution that can be achieved depends critically on the separation distance between the imaged source and scattering arrangement, as well as on the topology of the scatterers used. Imaged focal widths of up to one-seventh wavelength are demonstrated. The results obtained indicate the possibility of such an arrangement as a potential practical means for realising using conventional materials devices for fine feature extraction by electromagnetic lensing at distances remotely located from the source objects under investigation.

Near Field Focusing Using Phase Conjugating Impedance Loaded Wire Lens

The properties of a lumped loaded wire array as a means for near field focusing using phase conjugation is studied. The constructive role of phase conjugation in the near field image formation is justified both analytically and numerically. The generation of phase conjugated energy and how this is influenced by lumped impedance loading of the wires constituting the lens is discussed. In particular it is shown that inductive loading of the constituent lens wire elements is essential for subwavelength focusing since this leads to the creation of a phase conjugated near field predominantly determined by a convolution of the array current distribution with the real part of the Greens function which oscillates at a subwavelength scale. The characteristic resolution of the lens in terms of the full width at half maximum is shown to be ~ λ/7 for a single source and better than λ/4 for two dipole sources at λ/10 source-lens separation distance.

Tunable Frequency Selective Surfaces Characterisation

In this paper a new means for accurate characterisation of active varactor loaded frequency selective surfaces (FSS) is demonstrated. The varactor loaded active doubly periodic wire FSS acts as a variable impedance surface, in which its surface reflectivity and transmitivity can be modified by adjusting by d.c. bias voltage control. The properties of the active FSS are characterised using a specially designed parallel plate waveguide (PPW) simulator. This permits normal incidence excitation of the FSS under test over the frequency range of 2.6 GHz to 3.95 GHz. In addition we present a novel hybrid de-embedding technique to remove the influence the PPW simulator and reveal the true property of active FSS. Typical results are presented and validated against a new 3D EM modelling led de-embedding method for the composite FSS and PPW structure.

Near Field Enhancement and Subwavelength Imaging Using Resonantly Loaded Apertures

It is demonstrated that the electromagnetic (EM) transmission through a subwavelength or non-resonant aperture in a conductive screen can be dramatically enhanced by loading it with folded metallic strips exhibiting resonant properties. When illuminated by an EM plane wave these loaded apertures enable very tight, subwavelength, collimation of the EM power in the near field zone. We propose planar and quasi-planar resonant insertion geometries that should allow, for the first time, two-dimensional dual-polarization subwavelength field confinement along with ability to focus both electric and magnetic fields. The proposed technique for resonance transmission enhancement and near field confinement forms a basis for a new class of microwave near field imaging probe with subwavelength resolution capable of operating over a wide range of imaging distances (0.05-0.25λ). Measurement results demonstrate the possibility of high contrast (more than 3 dB in amplitude and 40 degrees in phase) near field subwavelength imaging of 2D and 3D resonant and non-resonant metallic and dielectric targets in free space and in moderately lossy layered media.

High-Resolution Microwave Near-Field Surface Imaging Using Resonance Probes

A novel microwave high-resolution near-field imaging technique is proposed and experimentally evaluated in reflectometry imaging scenarios involving planar metal-dielectric structures. Two types of resonance near field probes-a small helix antenna and a loaded subwavelength slot aperture are studied in this paper. These probes enable very tight spatial field localization with the full width at half maximum around one tenth of a wavelength, λ, at λ/100-λ/10 standoff distance. Importantly, the proposed probes permit resonance electromagnetic coupling to dielectric or printed conductive patterns, which leads to the possibility of very high raw image resolution with imaged feature-to-background contrast greater than 10-dB amplitude and 50° phase. In addition, high-resolution characterization of target geometries based on the cross correlation image processing technique is proposed and assessed using experimental data. It is shown that printed elements features with subwavelength size ~λ/15 or smaller can be characterized with at least 10-dB resolution contrast.

Parallel plate waveguide simulator design, characterisation and DUT de-embedding

In this paper, a new hybrid measurement/3D EM simulation technique to obtain reflection and transmission properties of an FSS under normal incidence has been investigated. The measurement fixture, a specially designed PPW simulator operating with normal incidence excitation, can be very accurately computed by commercial 3D EM simulation. To reveal the de-embedded characteristics of the DUT, (in this paper a wire FSS), a de-embedding method employing half structure models has been performed. This results in good agreement with theoretical prediction.

Spatial Data Encryption Using Phase Conjugating Lenses

Spatial encryption of a BPSK modulated signal radiated by a dipole-like antenna and transmitted through a phase conjugating lens (PCL) or through a system of PC lenses in free space is theoretically studied using a time domain electromagnetic (EM) formulation. The PC lenses are composed of antenna elements interconnected via phase conjugating circuitry. It is shown that when the PCLs are modulated with a phase encoded pump sequence at the same rate as the BPSK information signal, spatial encryption in the space between the two PCLs is possible, while the signal can only be decoded in well defined spatial regions after passing a second PCL. The effect that phase offset between PCL modulation signals has on viable spatial demodulation regions is also discussed. We show also that the signal detection area can be further confined using a third phase encoded pumped PCL located behind the second lens and modulated by an arbitrary BPSK pump sequence. The considered spatial encryption mechanism can find applications in secure communications.

Ultracompact Retrodirective Antenna Arrays With Superdirective Radiation Patterns

It is shown that the direction-of-arrival (DoA) information carried by an incident electromagnetic (EM) wave can be encoded into the evanescent near field of an electrically small resonance antenna array with a spatial rate higher than that of the incident field oscillation rate in free space. Phase conjugation of the received signal leads to the retrodirection of the near field in the antenna array environment, which in turn generates a retrodirected far-field beam toward the original DoA. This EM phenomenon enables electrically small retrodirective antenna arrays with superdirective, angular superresolution, auto-pointing properties for an arbitrary DoA. A theoretical explanation of the phenomenon based on first principal observations is given and full-wave simulations demonstrate a realizability route for the proposed retrodirective terminal that is comprised of resonance dipole antenna elements. Specifically, it is shown that a three-element disk-loaded retrodirective dipole array with 0.15λ spacings can achieve a 3.4-dBi maximal gain, 3-dBi front-to-back ratio, and 13% return loss fractional bandwidth (at the 10-dB level). Then, it is demonstrated that the radiation gain of a three-element array can be improved to approximately 6 dBi at the expense of the return loss fractional bandwidth reduction (2%).