Zvonimir Sipus

Waveguide miniaturization using uniaxial negative permeability metamaterial

A rectangular waveguide filled with anisotropic uniaxial metamaterial with transversal negative effective permeability is investigated both theoretically and experimentally. It is shown that such a waveguide supports propagation of the backward wave below the cutoff frequency, thus, it can be considered as a dual of the ordinary waveguide. The transversal dimension of this waveguide can be arbitrarily smaller than half of a wavelength in the filling material, provided that the transversal permeability is negative. This peculiar behavior may be used for fabrication of miniaturized rectangular waveguides. Several experimental miniaturized waveguides loaded with double ring resonators in 7 GHz frequency band have been designed, fabricated and tested. The measured results revealed backward-wave passband located below the cutoff frequency. Furthermore, it was experimentally shown that the increase of the physical length of the waveguide caused the decrease of the electrical length. This is a direct proof of the backward-wave propagation since the phase of the backward wave increases along the waveguide.

Non-Uniform Metasurface Luneburg Lens Antenna Design

A metasurfing concept is demonstrated and applied in the design of Luneburg lens antennas. Using an array of size-varying circular patches on a dielectric substrate inside a parallel-plate waveguide (PPW) structure variable surface impedance is obtained, which realizes an equivalent refraction index as that of a Luneburg lens. The obtained lens has good bandwidth characteristics and significant fabrication advantages with respect to conventional dielectric lenses. Based on this PPW lens, an H-plane antenna has been designed and simulated.

Analysis and measurements of conformal patch array antennas on multilayer circular cylinder

In this paper, we present a method of moments (MoM) program to analyze different configurations of arrays of cylindrical-rectangular patches. The patches can be located inside or on the surface of a multilayer circular-cylindrical structure and can be arbitrarily rotated. The antenna structure is rigorously taken into account by using proper Greens functions, and the array is analyzed by using an element-by-element approach. The elements of the MoM matrix are calculated in the spectral domain, and special attention is given to their numerical treatment when analyzing cylindrical antennas with large radii. A laboratory model is built to test different configurations of patches and to validate calculated results. The agreement between measurements and calculations is very good.

Analysis of Uniaxial Multilayer Cylinders Used for Invisible Cloak Realization

Uniaxial cylindrical cloaks have recently been proposed to prevent scattering of electromagnetic waves, i.e. to render objects invisible. In this paper frequency and time domain analysis of such a cloak has been carried out. Furthermore, influence of number of cloak layers on obtained invisibility has been studied, with special attention given to the cloak with just three layers that should be relatively easy to practically realize.

Study of Mutual Coupling Between Circular Stacked-Patch Antennas on a Sphere

A rigorous mathematical analysis is given of spherical stacked-patch arrays with emphasis on the physical interpretation of mutual coupling mechanisms present in doubly- curved convex structures. The analysis method is based on electromagnetic field representation in terms of spherical harmonics where each harmonic has the same angular variation as the spectral source component. To obtain the spectral representation the vector-Legendre transformation is applied to currents and fields. A novel approach to the mutual coupling calculation within the method of moments analysis of spherical arrays is applied. By expressing the patch current in terms of two suitable potential-like auxiliary functions, it is possible to avoid the use of Eulers formulas for coordinate system rotation and the related lengthy integrations. Instead, the rotation of antenna elements and corresponding current distributions can be done in closed form with the help of Vilenkins addition theorem for associated Legendre functions. It is shown that the new approach results in significant acceleration and improved accuracy of the analysis of spherical patch antenna arrays. The algorithm is successfully tested against a commercially available electromagnetic software and measurements performed on the developed laboratory model, confirming its accuracy for both input impedance and mutual coupling calculation and with only a small difference between the predicted and measured resonant frequencies, due to limitations in the experimental model. The influence of the structure parameters on mutual coupling level is extensively investigated, including all coupling mechanisms and leakage of energy due to curvature of the structure. It is shown that stacked-patch antennas can have reduced coupling level comparing to single patch antennas with possible deep nulls above the antenna resonant frequency.

Optimization of Uniaxial Multilayer Cylinders Used for Invisible Cloak Realization

Uniaxial cylindrical cloaks have recently been proposed to prevent scattering of electromagnetic waves, i.e., to render objects invisible. While it is possible to achieve that the rays follow the proper path inside the cloak, the proposed cloaks with reduced variation of constitutive parameters suffer from nonzero reflectance. The purpose of this paper is to improve invisibility performance of the original cloak structure in terms of total scattering width reduction and the operating frequency bandwidth. Therefore, the program for analyzing uniaxial cylindrical structures is merged with the global optimization program based on the particle swarm optimization algorithm. The results show that it is possible either to obtain the “perfect” cloak at the expense of bandwidth or to improve the operating frequency bandwidth at the expense of limited reduction of total scattering width.

Modified rectangular patches for self-oscillating active-antenna applications

Two modifications to a rectangular-patch antenna, suitable for the integration of active devices, are presented. In the first modification, the impedance inverter was placed in the rectangular opening formed by removing the central part of the patch. This modification allows optimal matching of the active device by changing the position, width, and length of the impedance-line transformer. An oscillating antenna, using this modified patch and a Gunn diode, showed a high EIRP, higher spectral purity, and substantially lower cross-polarization levels, in comparison with the reference active antenna with an unmodified patch. Because of the higher overall Q, this modification is recommended for active-antenna applications with active devices that have a narrower negative-resistance bandwidth. In the second modification, an active device (Gunn diode) was integrated directly into a rectangular opening inside the patch, without the use of a line transformer. This reduced the overall Q of the antenna, thus allowing wide-band frequency tuning by changing the bias voltage. A clear spectrum, with no spurious components of the free-running oscillating antenna, was observed. Radiation patterns in the E and H planes were measured. Low levels of cross-polarization, as for the first modification, were obtained. Injection-locking properties were investigated throughout the tuning range. A relatively wide locking range, with a good locking gain, was achieved. Such a miniaturized wide-band VCO antenna is applicable for integration in injection-locked active arrays, and spatial power combiners.

Layered Circular-Cylindrical Dielectric Lens Antennas—Synthesis and Height Reduction Technique

An accurate computer-aided-design tool has been developed for the synthesis of broadband layered circular-cylindrical dielectric lens antennas. It is based on the combination of a fast method of analysis formulated in the spectral domain with a global optimization algorithm based on the particle swarm optimization technique. Using this tool, the influence of the number of layers is studied for lenses of moderate size. We show that single-layer lenses with an optimum permittivity value have nearly the same radiation characteristics as optimized multi-layer configurations. Finally we introduce a new height reduction technique that allows reducing the lens size by almost a factor of two while retaining the fan-beam pattern capabilities of cylindrical lenses in the elevation plane as well as their axial-symmetry needed for beam scanning in the horizontal plane. Numerical results have been compared successfully with experimental ones from 26 to 40 GHz, to validate the analysis.

Numerical and Experimental Investigation of Field Distribution in Waveguide Filled with Anisotropic Single Negative Metamaterial

A field distribution in a rectangular waveguide filled with anisotropic single-negative (SNG) metamaterial is analyzed theoretically, numerically and experimentally. A highly simplified scenario with ideal lossless, continuous (either mu-negative (MNG) or epsilon-negative (ENG)) filling material is used in theoretical analysis. The results show that distributions of magnitudes of E and H fields are similar to the associated distributions in ordinary waveguide (filled with double-positive (DPS) material) that operates above cut-off frequency. However, the crucial difference, responsible for recently demonstrated peculiar phenomenon of backward wave propagation below cut-off is a pattern of field lines of either H field (a case with MNG filling) or E field (a case with ENG filling). Numerical approach that dealt with realistic structures (double-ring-resonator-based MNG metamaterial and thin-wire-based ENG metamaterial) confirmed existence of backward-wave propagation. Two experimental waveguides filled with double-ring resonators (an MNG filling) and thin wires (an ENG filling) were successfully designed, manufactured and tested in 8 GHz band. The backward-wave propagation was verified by measurement of a phase distribution along a waveguide with the help of a small E field probe

The cylindrical omnidirectional patch antenna

The need for omnidirectional antennas for wireless applications has increased considerably. The antennas are used in a variety of bands anywhere from 1.7 to 2.5 GHz, in different configurations which mainly differ in gain. The omnidirectionality is mostly obtained using back-to-back elements or simply using dipoles in different collinear-array configurations. The antenna proposed in this paper is a patch which was built in a cylindrical geometry rather than a planar one, and which generates an omnidirectional pattern in the H-plane.