Alexandros P. Feresidis

Artificial magnetic conductor surfaces and their application to low-profile high-gain planar antennas

Planar periodic metallic arrays behave as artificial magnetic conductor (AMC) surfaces when placed on a grounded dielectric substrate and they introduce a zero degrees reflection phase shift to incident waves. In this paper the AMC operation of single-layer arrays without vias is studied using a resonant cavity model and a new application to high-gain printed antennas is presented. A ray analysis is employed in order to give physical insight into the performance of AMCs and derive design guidelines. The bandwidth and center frequency of AMC surfaces are investigated using full-wave analysis and the qualitative predictions of the ray model are validated. Planar AMC surfaces are used for the first time as the ground plane in a high-gain microstrip patch antenna with a partially reflective surface as superstrate. A significant reduction of the antenna profile is achieved. A ray theory approach is employed in order to describe the functioning of the antenna and to predict the existence of quarter wavelength resonant cavities.

Tailoring the AMC and EBG characteristics of periodic metallic arrays printed on grounded dielectric substrate

The artificial magnetic conductor (AMC) and electromagnetic band gap (EBG) characteristics of planar periodic metallic arrays printed on grounded dielectric substrate are investigated. The currents induced on the arrays are presented for the first time and their study reveals two distinct resonance phenomena associated with these surfaces. A new technique is presented to tailor the spectral position of the AMC operation and the EBG. Square patch arrays with fixed element size and variable periodicities are employed as working examples to demonstrate the dependence of the spectral AMC and EBG characteristics on array parameters. It is revealed that as the array periodicity is increased, the AMC frequency is increased, while the EBG frequency is reduced. This is shown to occur due to the different nature of the resonance phenomena and the associated underlying physical mechanisms that produce the two effects. The effect of substrate thickness is also investigated. Full wave method of moments (MoM) has been employed for the derivation of the reflection characteristics, the currents and the dispersion relations. A uniplanar array with simultaneous AMC and EBG operation is demonstrated theoretically and experimentally.

A broadband high-gain resonant cavity antenna with single feed

A high gain resonant cavity antenna with broadband performance is presented. A single waveguide-fed slot is used to feed the cavity. The bandwidth enhancement is achieved by virtue of an optimised design of a double-layer periodic metallo-dielectric array and is demonstrated by means of simulations and measurements of a fabricated prototype.

Analysis and Design of Sub-Wavelength Resonant Cavity Type 2-D Leaky-Wave Antennas

A rigorous analysis is presented together with novel designs of high-gain sub-wavelength resonant cavity antennas. The antennas under investigation are formed by partially reflective surfaces placed in close proximity over artificial periodic metamaterial ground planes with specific reflection phase responses. A full-wave periodic leaky-wave analysis is applied to this class of antennas for the first time and the results are reported and discussed. Finite-size antenna designs are studied at wireless LAN frequencies (3.7 GHz) using 3-D full-wave software and the results are compared to the predictions of the leaky-wave analysis. Furthermore, the field distribution and the effect of losses are investigated. Finally, practical designs of sub-wavelength 2-D leaky-wave antennas with high-gain performance are presented.

Closely coupled metallodielectric electromagnetic band-gap structures formed by double-layer dipole and tripole arrays

The concept of closely coupled metallodielectric electromagnetic band-gap (CCMEBG) structures is introduced and investigated using two-dimensional (2-D) double-layer dipole and tripole arrays. An efficient numerical method based on a set of coupled integral equations is used to simulate the double-layer array response. The arrays are placed in close proximity to each other and shifted appropriately in order to produce maximum element coupling. Measurements are presented for oblique plane wave and surface wave incidences. A substantial decrease of the stopband center frequency is observed with the CCMEBG design for both element geometries. Furthermore, wider bandwidth and improved angular stability as compared to single-layer MEBG is obtained. The tripole arrays arranged on a hexagonal lattice exhibit common stopband for any polarization of the incident field due to the symmetry of the element in conjunction with the lattice. The lowering of the resonance for up to 4 to 1 in simulation results emerges as the layers are separated by less than /spl lambda//1200 (0.1 mm at 2.5 GHz).

Efficient modeling of novel uniplanar left-handed metamaterials

This paper presents an efficient modeling technique for the derivation of the dispersion characteristics of novel uniplanar metallodielectric periodic structures. The analysis is based on the method of moments and an interpolation scheme, which significantly accelerates the computations. Triangular basis functions are used that allow for modeling of arbitrary shaped metallic elements. Based on this method, novel uniplanar left-handed (LH) metamaterials are proposed. Variations of the split rectangular-loop element printed on grounded dielectric substrate are demonstrated to possess LH propagation properties. Full-wave dispersion curves are presented. Based on the dual transmission-line concept, we study the distribution of the modal fields and the variation of series capacitance and shunt inductance for all the proposed elements. A verification of the left-handedness is presented by means of full-wave simulation of finite uniplanar arrays using commercial software (HFSS). The cell dimensions are a small fraction of the wavelength (approximately /spl lambda//24) so that the structures can be considered as a homogeneous effective medium. The structures are simple, readily scalable to higher frequencies, and compatible with low-cost fabrication techniques.

Sub-Wavelength Profile 2-D Leaky-Wave Antennas With Two Periodic Layers

A fast and accurate analysis and synthesis technique for high-gain sub-wavelength 2-D Fabry-Perot leaky-wave antennas (LWA) consisting of two periodic metallodielectric arrays over a ground plane is presented. Full-wave method of moments (MoM) together with reciprocity is employed for the estimation of the near fields upon plane wave illumination and the extraction of the radiation patterns of the LWA. This yields a fast and rigorous tool for the characterisation of this type of antennas. A thorough convergence study for different antenna designs is presented and the operation principles of these antennas as well as the radiation characteristics are discussed. Moreover, design guidelines to tailor the antenna profile, the dimensions of the arrays as well as the antenna directivity and bandwidth are provided. A study on the radiation efficiency for antennas with different profiles is also presented and the trade off between directivity and radiation bandwidth is discussed. Numerical examples are given throughout to demonstrate the technique. A finite size antenna model is simulated using commercial software (CST Microstripes 2009) which validates the technique.

Low-Profile Folded Monopoles With Embedded Planar Metamaterial Phase-Shifting Lines

This paper presents the analysis, design and measurement of novel, low-profile, small-footprint folded monopoles employing planar metamaterial phase-shifting lines. These lines are composed of fully-printed spiral elements, that are inductively coupled and hence exhibit an effective high- mu property. An equivalent circuit for the proposed structure is presented, validating the operating principles of the antenna and the metamaterial line. The impact of the antenna profile and the ground plane size on the antenna performance is investigated using accurate full-wave simulations. A lambda/9 antenna prototype, designed to operate at 2.36 GHz, is fabricated and tested on both electrically large and small ground planes, achieving on average 80% radiation efficiency, 5% (110 MHz) and 2.5% (55 MHz) -10 dB measured bandwidths, respectively, and fully omnidirectional, vertically polarized, monopole-type radiation patterns.

Uniplanar left-handed artificial metamaterials

Planar periodic arrays of metallic elements printed on grounded dielectric substrates are presented to exhibit left-handed properties for surface wave propagation. The proposed structures dispense with the need for grounding vias and ease the implementation of uniplanar left-handed metamaterials at higher frequencies. A transmission line description is used for the initial design and interpretation of the left-handed property. A thorough study based on full wave simulations is carried out with regards to the effect of the element geometrical characteristics and the array periodicity to the properties of the artificial material. Dispersion curves are presented and studied. The distribution of the modal fields in the unit cell is also studied in order to provide an explanation of the material properties. The scalability of the proposed structures to infrared frequencies is demonstrated.

Resonant Effects and Near-Field Enhancement in Perturbed Arrays of Metal Dipoles

The equivalent self impedance of a perturbed array of metal dipoles is derived and the resonant effects upon plane wave illumination are studied. It is revealed that as a result of the perturbation, the scattering within a frequency range is dominated by the excitation of the odd mode. This corresponds to significant deviation compared to the unperturbed case. It is demonstrated that within this frequency range, very strong near- fields are excited in the vicinity of the array. Following a careful calculation of the near-fields using the periodic method of moments, the near-fields for a number of perturbed array designs are calculated and an increase in the near-field strength of more than 70 times compared with the incidence is demonstrated. The results are corroborated with HFSS.