Antti O. Karilainen

Synthesis of Polarization Transformers

We study the possibility of analytically synthesizing different polarization-transforming devices realized as arrays of small particles. The proposed method is based on general relations between the reflection and transmission coefficients and the polarizabilities of arbitrary bi-anisotropic particles. As an example, we reveal all possible types of inclusions which can be used to realize twist polarizers, select one of them and synthesize a novel twist polarizer. The synthesized twist polarizer is then fitted on a standard printed circuit board, optimized numerically, and finally manufactured and measured. The experimental results for the twist polarizer show good correspondence with the simulations and the new method is found to be a useful tool for developing any polarization-transforming devices. As one more example, a novel circular polarization selective surface is synthesized.

Experimental studies on antenna miniaturisation using magneto-dielectric and dielectric materials

Dielectric or magneto-dielectric materials can be used to miniaturise antennas, but there are many important parameters that must be considered when selecting which material to use. The authors discuss these figures of merit and a rigorous method to compare antennas with different material fillings. A meandered planar inverted-F antenna (PIFA) loaded with magneto-dielectric and dielectric materials is presented as an example antenna for testing. The magneto-dielectric material used is composed of mylar substrate and Fe–SiO2 sheets. Measurement results for the permeability of the material are presented. The radiation mechanism of the meandered PIFA is studied, and the proper position for dielectric and magneto-dielectric filling is discussed and identified. Miniaturisation by commercial dielectric and the presented magneto-dielectric fillings is measured and compared at the same resonance frequency using the radiation quality factor as the figure of merit. It is seen, that the antenna can benefit from the magneto-dielectric filling material in terms of the radiation quality factor only if the placement of the antenna filling is carefully selected.

Eliminating Electromagnetic Scattering From Small Particles

This paper presents and discusses the conditions for zero electromagnetic scattering by electrically small particles. We consider the most general bi-anisotropic particles, characterized by four dyadic polarizabilities and study the case of uniaxially symmetric objects. Conditions for zero backward and forward scattering are found for a general uniaxial bi-anisotropic particle and specialized for all fundamental classes of bi-anisotropic particles: omega, “moving”, chiral, and Tellegen particles. Possibility for zero total scattering is also discussed for aforementioned cases. The scattering pattern and polarization of the scattered wave are also determined for each particle class. In particular, we analyze the interplay between different scattering mechanisms and show that in some cases it is possible to compensate scattering from a polarizable particle by appropriate magneto-electric coupling. Examples of particles providing zero backscattering and zero forward scattering are presented and studied numerically.

Isotropic Chiral Objects With Zero Backscattering

We study electrically small chiral objects with isotropic response and zero backscattering. A bi-isotropic sphere is used as a simple example and its zero-backscattering conditions are studied. A theoretical model of an object composed of three orthogonal chiral particles made of conducting wire is presented as an analog of the zero-backscattering bi-isotropic sphere. A potential application of the object as a receiving antenna or a sensor with the ability to receive power from an arbitrary direction without backscattering is discussed.

Choosing Dielectric or Magnetic Material to Optimize the Bandwidth of Miniaturized Resonant Antennas

We address the question of the optimal choice of loading material for antenna miniaturization. A new approach to identify the optimal loading material (dielectric or magnetic) for maximization of bandwidth of resonant antennas is introduced. Instead of equivalent resonant circuits or transmission-line resonators, we use the analysis of radiation mechanism to identify the fields contributing mostly to the stored energy and determine the beneficial material type. The formulated rule is qualitatively illustrated using a dipole and a patch antenna, as well as a planar inverted-L antenna where the conventional analysis of circuit or a transmission-line resonator leads to incorrect conclusions. Guidelines are presented for miniaturizing different antenna types.

Circularly Polarized Receiving Antenna Incorporating Two Helices to Achieve Low Backscattering

We propose to use an antenna composed of two orthogonal helices as a low-scattering sensor. The vector effective length is derived for the antenna using the small dipole approximation for the helices. The antenna can transmit and receive circular polarization in all directions with the Huygens pattern. We observe that the antenna geometry does not backscatter, regardless of the polarization, when the incidence direction is normal to the plane of the helices. Scattered fields, scattered axial ratio, and the scattering cross section are presented. We show that the zero-backscattering property holds also for the antenna when it is capable to receive all the available power with conjugate loading. The approximate analytical model is validated with full-wave simulations.

Synthesizing a twist polarizer

A systematic approach to synthesize devices for different polarization-related operations is developed. The method can be used to find the required polarizabilities of a bianisotropic scatterer so that an array of such particles would perform any wanted polarization transformation of plane waves. The method is applied to develop a novel twist polarizer. The resulting design is verified with numerical simulations.

High-Impedance-Surface-Based Antenna With Two Orthogonal Radiating Modes

High-impedance surfaces (HISs) have been used as artificial magnetic conductors for low-profile dipole antennas. Usually, the desired operation has been designed using the phase-reflection simulations for normal incidence. Here, we study the properties of a mushroom-type HIS using reflection-phase calculations for oblique incidence and find two orthogonal resonant modes. An antenna based on a finite-sized HIS is designed to utilize both of these modes. Measurement results are presented for the antenna, and we report two separate modes with asymmetric radiation patterns. The first mode provides a dipole-like radiation pattern, and the second one a broadside pattern. Furthermore, the second mode can be coupled to the antenna with a proper coupling element in order to obtain a wide bandwidth. Both of the modes can be matched to 50-Ω coaxial cables, and good isolation levels between the ports are seen due to the orthogonality of the modes in the HIS.

A tri-band low-profile antenna based on a high-impedance surface

The constant need to miniaturize antennas for electronic devices without compromising antenna performance in modern telecommunication systems drives the research on innovative antenna solutions. As an example, high-impedance surfaces are used to allow bringing a horizontal wire antenna close to the ground plane, decreasing the thickness of the antenna substrate (see e.g. [1]). Commonly, the operation of antennas with artificial impedance surfaces is based on the use of the resonance of the surface: At the resonance the surface impedance of the structure becomes high in the absolute value and the reflection phase for plane waves is equal to 0°. At this frequency a plane-wave source can be brought very close to the ground plane and the reflected plane-wave fields interfere constructively with the fields created by the primary source [1, 2]. Recently, we have proposed to utilize a different radiation mechanism of mushroom high-impedance surfaces, exciting a plasmonic resonance mode in which the vertical pins of the surface are excited in phase, realizing a low-profile distributed vertical electric dipole [3]. This allowed us to design a dual-band antenna utilizing two different modes of the high-impedance surface.

Circularly polarized receiving antenna systems with zero backscattering

This paper discusses small helical antennas, and their uses in creating zero-backscattering Huygens antennas. By using two orthogonal helical antennas, a Huygens antenna for circular polarization can be formed. This kind of an antenna does not backscatter but directs the scattered power away from the incidence direction, when that direction is perpendicular to the axes of the helices. The antenna is complemented with a third helix, thus creating an isotropic antenna system with the same properties.