Comparison of frequency-selective screen-based linear to circular split-ring polarisation convertors

This study presents the use of periodic arrays of freestanding slot frequency-selective screens (FSS) as a means for generating circularly polarised signals from an incident linearly polarised signal at normal incidence to the structure. Measured and simulated results for crossed, linear and various ring slot element shapes in single and double-layer polarisation convertor structures are presented for 10 GHz operation. It is shown that 3 dB axial ratio (AR) bandwidths of 21% can be achieved with the one-layer perforated screen design and that the rate of change is lower than the double-layer structures. An insertion loss of 0.34 dB can be achieved for the split circular ring double-layer periodic array, and of the three topologies presented the hexagonal split-ring polarisation convertor gives the lowest variation of AR with angle of incidence 1.8 dB/45° and 3.6 dB/45° for the single and double-screen FSS, respectively. In addition, their tolerance to angle of incidence variation is presented. The capability of the surfaces reported here as twist polariser or spatial isolator components has been demonstrated with up to -30 dB isolation between incident and re-reflected signals for the double-layer designs being measured. © 2010 The Institution of Engineering and Technology.

Design and experimental validation of liquid crystal-based reconfigurable reflectarray elements with improved bandwith in F-band

A reconfigurable reflectarray which exploits the dielectric anisotropy of liquid crystals (LC) has been designed to operate in the frequency range from 96 to 104 GHz. The unit cells are composed of three unequal length parallel dipoles placed above an LC substrate. The reflectarray has been designed using an accurate model which includes the effects of anisotropy and inhomogeneity. An effective permittivity that accounts for the real effects of the LC has also been used to simplify the analysis and design of the unit cells. The geometrical parameters of the cells have been adjusted to simultaneously improve the bandwidth, maximize the tunable phase-range and reduce the sensitivity to the angle of incidence. The performance of the LC based unit cells has been experimentally evaluated by measuring the reflection amplitude and phase of a reflectarray consisting of 52x54 identical cells. The good agreement between measurements and simulations validate the analysis and design techniques and demonstrate the capabilities of the proposed reflectarray to provide beam scanning in F band.

Design of engineered reflectors for radar cross section modification

A technique is proposed for the design of engineered reflectors consisting of doubly periodic arrays printed on thin grounded dielectric substrates that reflect an incoming wave from a given incoming direction to a predetermined outgoing direction. The proposed technique is based on a combination of Floquet theory for propagation in periodic structures and reflect-array principles. A flat surface designed to reflect a TE polarized wave incident at 45 back in the direction of the impinging signal at 14.7 GHz is employed as an example. By means of full-wave simulations, it is demonstrated that the monostatic RCS of a finite reflector is comparable with the specular RCS of a metallic mirror of the same dimensions. It is further shown that comparably high monostatic RCS values are obtained for angles of incidence in the 30-60 range, which are frequency dependent and thus open opportunities for target localization. A prototype array is fabricated and experimentally tested for validation. The proposed solution can be used to modify the radar cross section of a target. Other potential applications are also discussed. © 1963-2012 IEEE.

Effect of environmental multipath on line of sight body to body communication at 2.45 GHz

Body to body links are the most scenario dependent form of body centric communications with performance highly dependent on the users' movements, relative positioning and the local operating environment. This paper focuses on line of sight cases which although they should be the most dependent, still have considerable variability depending on local propagation conditions. The results presented in the paper also raise important questions about the statistical characterisation of such links and the effect of different approaches to local mean averaging on fading characteristics. © 2012 IEEE.

A double U-slot patch antenna with dual wideband characteristics

Microstrip patch antennas are strong candidates for use in many wireless communications applications. This paper proposes the use of a patch antenna with two U-shaped slots to achieve dual band operation. A thick substrate helps broaden the individual bandwidths. The antenna is designed based on extensive IE3D simulation studies. A prototype antenna is fabricated and experimentally verified for the required performance.

Artificial surfaces of intertwined square spirals:A CPW model

Intertwining planar spirals arranged in doubly periodic arrays enables a substantially subwavelength response of the unit cell smaller than 1/40 of wavelength with large fractional bandwidths. These properties are important for application at low frequencies, conformal curved surfaces, or with compact radiators. It is shown that interleaving counter-wound spiral arms extended into adjacent unit cells dramatically increase the array equivalent capacitance while reducing the inductance. A coplanar waveguide (CPW) model has been developed to analytically estimate the equivalent capacitance and inductance of intertwined spiral array elements in terms of their geometrical parameters. The proposed CPW model is shown to provide an accurate prediction of the fundamental resonance frequency and can be instrumental in the design of the arrays for a specified frequency response. © 2012 IEEE.

ARTIFICIAL SURFACES WITH INTERWOVEN AND TESSELLATED PATTERNED CONDUCTORS: PROPERTIES AND PHENOMENOLOGY

The features of artificial surfaces composed of doubly periodic patterns of interwoven planar conductors are discussed. The free-standing intertwined quadrifilar spirals and modified Brigid's crosses are presented as illustrative examples to demonstrate the highly stable angular reflection and transmittance response with low cross-polarisation and a broad fractional bandwidth. The main mechanisms contributing to the substantially sub-wavelength response of these arrays are discussed showing that interweaving their conductor patterns provides concurrent control of both the equivalent capacitance and inductance of the unit cell. The effects of dielectric substrate and conductor thickness on the properties of intertwined spiral and modified Brigid's cross arrays are discussed to provide insight in the effect of the structure parameters on array performance.

Three-wave Nonlinear Scattering by Quasiperiodic Dielectric Structure

The combinatorial frequency generation by a Fibonacci type quasi-periodic dielectric multilayered structure illuminated by two plane waves has been analysed. The effects of the layer parameters and Fibonacci sequence order on the properties of the combinatorial frequency waves emitted from the stacked nonlinear layers are discussed.

Analysing admittance of obstacles in evanescent waveguide using the Imaginary Smith Chart

We present a technique, using the Imaginary Smith Chart, for determining the admittance of obstacles introduced into evanescent waveguide. The admittance of an inductive iris, capacitive iris, capacitive post, variable width strip and length of evanescent waveuide are investigated. © 2012 IEEE.

Metasurfaces with interwoven conductor patterns on dielectric substrates

The properties of metasurfaces comprised of interwoven conductor patterns on dielectric substrates have been examined. The significant reduction of the fundamental resonance frequency fr and expanded fractional bandwidths (FBWs) offered by the intertwined spirals and Brigid’s crosses extended beyond a single unit cell has been achieved with the aid of thin dielectric substrates. A qualitative model has been proposed and proved to adequately predict the main properties of entwined spiral arrays on dielectric substrates.

Novel Generic Bounds on the Sum Rate of MIMO ZF Receivers

This paper introduces some novel upper and lower bounds on the achievable sum rate of multiple-input multiple-output (MIMO) systems with zero-forcing (ZF) receivers. The presented bounds are not only tractable but also generic since they apply for different fading models of interest, such as uncorrelated/ correlated Rayleigh fading and Ricean fading. We further formulate a new relationship between the sum rate and the first negative moment of the unordered eigenvalue of the instantaneous correlation matrix. The derived expressions are explicitly compared with some existing results on MIMO systems operating with optimal and minimum mean-squared error (MMSE) receivers. Based on our analytical results, we gain valuable insights into the implications of the model parameters, such as the number of antennas, spatial correlation and Ricean-K factor, on the sum rate of MIMO ZF receivers. © 2011 IEEE.

Amplitude modulated signal transmission through a phase conjugating lens

Free space transmission of an on-off modulated sinusoidal signal through a phase conjugating lens (PCL) is theoretically examined using a combined time/frequency domain approach. The on-off keyed (OOK) signal is generated by a dipole antenna located in the far-field zone of the lens. The PCL consists of a dual layer of antenna elements interconnected via phase conjugating circuitry. We demonstrate that electromagnetic interference between antenna elements creates spatially localised areas of good-quality reception and zones where the signal is significantly denigrated by interference. Next, it is shown that destructive interference and packet desynchronisation effects critically depend on bit rate. It is also shown that a circular concave lens can be used to produce high-quality signal reception in a given direction while suppressing signal reception in all other directions. The effect that the bandwidth of the phase conjugating unit has on the transmitted signal properties for the cases of high and low bit rate OOK modulation are studied and a signal quality characterisation scheme is proposed which uses cross-correlation. The results of the study yields understanding of the performance of phase conjugating arrays under OOK modulation. The work suggests a novel approach for realising a secure communication wireless system.

Increased Bandwidth Evanescent Open-Ended Waveguide Antenna Design Using the Imaginary Smith Chart

We present a novel antenna matching technique that uses the Imaginary Smith Chart to permit wideband matching of an evanescent open-ended waveguide antenna using a dielectric sheet air-spaced from the aperture. The fabricated antenna design is demonstrated to have a measured bandwidth of 24%, from 2.13–2.7 GHz, for reflection coefficient $qquad{<} -!!10~{rm dB}$ , with 2.725 GHz waveguide cutoff. The antenna's maximum aperture dimension is ${< 0.5}lambda_{0}$ at the upper frequency in the bandwidth and so it is suitable for use in a wide angle scanning phased array

Reflection and Transmission at a Phase Discontinuous Interface

In this paper we extend the derivation of the modified form Snells's law that occurs when an additional phase profile is introduced at the material interface. We show that this permits electromagnetic (EM) beam steering, negative refraction and retrodirective action opportunities for such engineered surfaces even if they are immersed in a uniform dielectric. Simple expressions for the retrodirected and negatively refracted beams are derived along with the propagation conditions that occur at the boundary interface inside the critical angle range. It is also demonstrated how the transmission and reflected power levels are affected by the additional phase taper introduced at the surface.

Developments in retrodirective array technology

There has been significant interest in retrodirective antennas, especially considering the wealth of applications that could be significantly enhanced, or created, by the use of such technology. There is enormous potential for retrodirective antennas where complicated automatic tracking systems would benefit from being replaced by much simpler systems. Retrodirective array technology offers one solution pathway since it can offer extremely fast tracking with relatively simple circuitry. Retrodirective or self-steering arrays are suited for low radio frequency (RF) power mobile terminal use particularly on or between un-stabilised vehicles. In this type of operational scenario, high degrees of relative movement are expected, and power consumption and weight of the antenna must be kept to a minimum. In this study, the authors give a brief historical review of basic retrodirective technology and elaborate on some recent developments at Queens University of Belfast associated with retrodirective antenna technology in relation to, two-way communications, ultrafast RADAR, microwave imaging, spatial power transmission, mitigation of multipath effects and spatial encryption.