Olli Luukkonen

A Stepwise Nicolson–Ross–Weir-Based Material Parameter Extraction Method

Approaches of automated evaluation of electromagnetic material parameters have received a lot of attention in the literature. Among others, one method is to retrieve the material parameters from the reflection and transmission measurements of the sample material. Compared to other methods, this is a rather wideband method, but suffers from an intrinsic limitation related to the electrical thickness of the measured material. In this letter, we propose a novel way to overcome this limitation. Although being based on the classical Nicolson-Ross-Weir (NRW) technique, the proposed extraction technique does not involve any branch seeking and is therefore capable of extracting material parameters from samples thicker than λ/2, a measure that would otherwise cause problems in the NRW extraction technique. The proposed derivative of the NRW extraction technique is then used to study the effect of thermal noise on the extracted material parameters.

TE Surface Wave Resonances on High-Impedance Surface Based Antennas: Analysis and Modeling

Low-profile antennas comprising a horizontal dipole above a high-impedance surface are analyzed. The emphasis of this paper is on the additional resonances of the radiating structure caused by surface waves propagating on the high-impedance surface. It is shown that such resonances can be favorably used for broadening the bandwidth of the antenna. The phenomenon is thoroughly modeled by exploiting a parallel between the HIS structure and a waveguide resonator. In the second part of the paper we discuss homogenized approaches for modeling the radiating properties of the antenna with emphasis to the phenomenon discussed in the first part. As it turns out, it is necessary to take into account the spatially dispersive properties of high-impedance surfaces, and most of the simplified models commonly used for analyzing high-impedance surface based antennas fail in predicting the discussed resonance mode.

Continuous wave radar based vital sign estimation: Modeling and experiments

This paper studies the use of continuous wave (CW) radar for non-invasive estimation of vital signs, in particular respiratory and heart rate. We derive a realistic reference model for a stationary human subject. We propose methods for estimating the vital signs, including nonlinear and linear signal demodulation, as well as time-frequency methods utilizing the micro-Doppler structure of the signal. We show that real-world measurements with 24 GHz CW-radar taken from different positions close to human are in line with the simulation results. The results indicate that reliable estimation of the heart rate can be obtained with the nonlinear demodulation if the radar is suitably positioned w.r.t. the subject.

One-Way Waveguides Connected to One-Way Loads

Study of nonreciprocal devices has been the subject of research interest over the past several decades. In particular, gyrotropic waveguides have received considerable attention, owing to the fact that, under certain conditions, these waveguides may possess modes that propagate only in one direction, effectively acting as one-way conduits. We numerically investigate some of the consequences of connecting such one-way waveguides to “closed” terminal loads, such as cavities, or “open” terminal loads such as aperture antennas. We investigate how the energy is exchanged between this waveguide and its terminal loads, and how the conventional impedance mismatch may be less important in such waveguide-load interaction.

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.

Experimental verification of the suppression of spatial dispersion in artificial plasma

An isotropic plasma is composed of free charge carriers. Wire medium is an artificial plasma that is constructed by aligning conducive wires in one-, two-, or three dimensions (see e.g. [1]). In the simplest form, that is the one-dimensional case, the wires are aligned in parallel with a certain lattice constant. In more complicated two- and three-dimensional wire media the wires are aligned in a similar way in parallel in two or three orthogonal directions, respectively. Unlike in the gas-like plasma, the electromagnetic properties of this artificial plasma are constrained by its geometrical structure. Such constrains are, for instance, anisotropy and spatial dispersion.

An approach to finding the correct branch from the forest of possible solutions for extracted effective material parameters

In the classical Nicolson-Ross-Weir electromagnetic material parameter extraction technique the effective material parameters are obtained through reflection and transmission measurements of a planar material sample. One of the advantages of this technique is that it provides the result over a broad frequency band with just one measurement. This technique, however, does not provide us unambiguous results of the effective permittivity and permeability but the correct solution needs to be found through an additional deduction process. Here we present a derivative of the Nicolson-Ross-Weir extraction technique that can overcome this problem related to the infinitely many branches of the solution.

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.

Experimental validation of the suppression of spatial dispersion in artificial plasma

An isotropic plasma is composed of free charge carriers. Wire medium is an artificial plasma that is constructed by aligning conducive wires in one-, two-, or three dimensions (see e.g. [1]). In the simplest form, that is the one-dimensional case, the wires are aligned in parallel with a certain lattice constant. In more complicated two- and three-dimensional wire media the wires are aligned in a similar way in parallel in two or three orthogonal directions, respectively. Unlike in the gas-like plasma, the electromagnetic properties of this artificial plasma are constrained by its geometrical structure. Such constrains are, for instance, anisotropy and spatial dispersion.

One-Way Waveguides and Impedance Matching of Loads

We theoretically investigate interaction of propagating waves in one-way waveguides with terminal loads and cavities, showing that the impedance mismatch between the load and the waveguide has no effect on energy propagation through the structure.