Joni Vehmas

Functional metamirrors using bianisotropic elements.

Conventional mirrors obey the simple reflection law that a plane wave is reflected as a plane wave, at the same angle. To engineer spatial distributions of fields reflected from a mirror, one can either shape the reflector or position some phase-correcting elements on top of a mirror surface. Here we show, both theoretically and experimentally, that full-power reflection with general control over the reflected wave phase is possible with a single-layer array of deeply subwavelength inclusions. These proposed artificial surfaces, metamirrors, provide various functions of shaped or nonuniform reflectors without utilizing any mirror. This can be achieved only if the forward and backward scattering of the inclusions in the array can be engineered independently, and we prove that it is possible using electrically and magnetically polarizable inclusions. The proposed subwavelength inclusions possess desired reflecting properties at the operational frequency band, while at other frequencies the array is practically transparent. The metamirror concept leads to a variety of applications over the entire electromagnetic spectrum, such as optically transparent focusing antennas for satellites, multifrequency reflector antennas for radio astronomy, low-profile conformal antennas for telecommunications, and nanoreflectarray antennas for integrated optics.

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.

Omega Transmission Lines with Applications to Effective Medium Models of Metamaterials

In this paper, we introduce the concept of transmission lines with inherent bi-anisotropy and establish an analogy between these lines and volumetric bi-anisotropic materials. In particular, we find under what conditions a periodically loaded transmission line can be treated as an effective omega medium. Two example circuits are introduced and analyzed. The results have two-fold implications: opening a route to emulate electromagnetic properties of bi-anisotropic omega media using transmission-line meshes and understanding and improving effective medium models of composite materials with the use of effective circuit models of unit cells.

Transmission lines emulating moving media

In this paper, we study how the electromagnetic phenomena in moving media can be emulated using artificial composite structures at rest. In particular, we introduce nonreciprocal periodically loaded transmission lines which support waves obeying the same rules as plane electromagnetic waves in moving media. Because the actual physical structure is at rest, in these transmission lines there are no fundamental limitations on the velocity values, which may take values larger than the speed of light or even complex values (considering complex amplitudes in the time-harmonic regime). An example circuit of a unit cell of a “moving” transmission line is presented and analyzed analytically. Besides the fundamental interest, the study is relevant for potential applications in realizing engineered materials for various transformations of electromagnetic fields.

Electromagnetic energy sink

The ideal black body fully absorbs all incident rays, that is, all propagating waves created by arbitrary sources. The known idealized realization of a black body is the perfectly matched layer (PML), widely used in numerical electromagnetics. However, ideal black bodies and PMLs do not interact with evanescent fields existing near any finite-size source, and the energy stored in these fields cannot be harvested. Here we introduce the concept of the ideal conjugate matched layer (CML), which fully absorbs energy of both propagating and evanescent fields of sources acting as an ideal sink for electromagnetic energy. Conjugate matched absorbers have exciting application potentials, as resonant attractors of electromagnetic energy into the absorber volume. We derive the conditions on the constitutive parameters of media which can serve as CML materials, numerically study the performance of planar and cylindrical CML and discuss possible realizations of such materials as metal-dielectric composites.

Millimeter-Wave Channel Characterization at Helsinki Airport in the 15, 28, and 60 GHz Bands

Airport terminal is one of the indoor scenarios envisioned for mm-wave 5G deployment. In this paper, we characterize the propagation channel in the Helsinki airport at 15, 28, and 60 GHz bands by means of directional wideband channel sounding. The radio environment was characterized by studying the specular propagation paths, specular and diffuse power contributions, polarization, and the delay and angular spreads. All the studied metrics in different bands were compared and the frequency dependency was analyzed.

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.

Transmission-Line Cloak as an Antenna

Transmission-line cloak provides a simple yet wideband solution for cloaking an object. In this letter, it is shown how the basic transmission-line cloak geometry can be modified so that the cloak structure itself acts as an antenna at a frequency well below the cloaking frequency. The structure retains its cloaking properties despite these modifications. Two types of cloak-antennas are presented: a dipole antenna and a monopole antenna. The operation of these antennas is verified using simulations and measurements. It is also shown how the antenna resonance frequency can be tuned by changing its geometry.

Spatio-temporal channel sounding in a street canyon at 15, 28 and 60 GHz

Spatio-temporal channel sounding was performed at frequency bands of 15,28 and 60 GHz in the backhaul-link street canyon scenario. Analysis of the measured channels from the sounding show 1) path loss exponents close to the free-space in the line-of-sight condition, with extra losses of 5 to 30 dB in the non-line-of-sight conditions; 2) the azimuth angular and delay spreads mostly less than 25° and 30 ns at the three frequency bands; 3) decreasing angular and delay spreads as the link distance is longer, 4) up to fourth-order specular reflection observed in the 100-m long street canyon when looking at the strongest 30-dB dynamic range of the channels, and finally, 5) no decisive frequency dependency of the angular and delay spreads and the reflection order, though the presence or absence of cars in the street canyon during the measurements seemed to make some impacts on the spread values given the antenna height of our channel sounding.

Inhomogeneous microwave lens based on periodically loaded transmission lines

A new design for a low-re∞ection inhomogeneous microwave lens based on periodically loaded one-dimensional transmission lines is proposed and experimentally tested. The inhomogeneous efiective refractive index of this ∞at-proflle lens is achieved by loading the transmission lines comprising the lens with difierent inductive elements.