Dr. Oscar Quevedo-Teruel

Assistant Professor

Electromagnetic Engineering

School of Electrical Engineering (EES)

KTH Royal Institute of Technology

Research

Higher Symmetries

Periodic structures possess twist symmetry when their lattice is created by a translation and an angular rotation. If the angular translation is 2-fold, the angular movement becomes a mirroring, and the higher symmetry is named glide. Both glide and twist symmetries are higher-symmetry periodic structures.

Higher symmetries were intensively studied in the ‘60s and ‘70s, especially for the case of propagation in 1D periodicities, which were employed to produce leaky wave antennas.

Recently, the study of higher symmetries is again an active research topic in the fields of microwave technology, antennas and metamaterials.

At KTH, we have been recently demonstrated that 2D glide-symmetric structures can be employed to produce low dispersive equivalent refractive indexes. These periodic unit cells can be used to produce ultra-wide band lens antennas.

Additionally, glide-symmetric structures have demonstrated to be able to produce low-cost gap waveguide technology. Glide-symmetric configurations have a similar bandgap as the conventional bed of nails, but they require holes that are typically thicker, and with a high tolerance in variations of their height dimensions.

Metasurfaces

Metasurfaces are extremely thin layers of sub-wavelength unit cells that can be employed for directing or preventing the propagation of electromagnetic waves.

The concept of metasurfaces can be applied to waves from radio-frequencies to optics, including microwaves and TeraHertz (THz) regimes.

Their applications are numerous, and may be used to improve the performance of already known devices such as antennas, fibre optics, circuits, radio-telescopes and satellites.

At KTH, we have studied the possibilities of using glide symmetries to achieve ultra wide band solutions.

Transformation Optics

According to the Transformation Optics theory, any given electromagnetic device can be transformed into an infinite number of new ones with same electromagnetic responses.

For instance, this tool was proposed to redesign traditional devices such as polarization rotators, wave collimators and beam benders; or to reduce the size of conventional lenses, to enhance transmission through sub-wavelength aperture and generally to simplify the complex geometry of certain antennas.

At KTH, we have developed the quasi-conformal transformations optics theory to develop bespoke lenses to a given specific feeding. Therefore, we can produce lenses which are quasi-perfect for a specific type of antenna. These solutions are ultra wide band if dielectric materials are employed.

Surface plasmons Antennas

Traditionally, the existence of surface waves was originally demonstrated for dielectric materials placed over a metal surface. However, these surface waves can be also excited without the presence of a dielectric material by tailoring the metallic surface with a periodic repetition of obstacles or holes in the direction of propagation

Recently, we have proposed antennas which can be excited directly from transmission waveguides that use the concept of domino plasmons. The operation of this antenna consists of the transformation of surface waves into leaky waves, which are radiated in a certain direction.

Microstrip Patch Antennas

Patch antennas are associated with a family of antennas which come from microstrip technology. A microstrip patch antenna consists of a metallic strip (or patch) printed on a dielectric that is placed above a ground plane. Typically, the substrate must be thin when compared to the wavelength, and its dielectric constant must be low since they provide good radiation efficiencies. The most desirable are substrates with low dielectric constants, since they provide lower losses (and therefore the radiation efficiency is higher), however, substrates with high dielectric constants allow more compact designs. Relative to the thickness, patch antennas with thin substrates present narrow bands of operation while thick ones excite surface waves and can complicate the antenna feeding.

We have investigated how to make these antennas more compact, and how to achieve different radiation patterns by the use of different modes of radiation.

Reconfigurable devices

Some investigators have proposed making the devices reconfigurable, i.e., to develop antennas whose bands are modified by using an external parameter, typically a dc voltage.

Since some of compact antennas are based on resonances created by a capacitance and an inductance, one of the most common techniques described in the literature is based on using varactor diodes that provide a changeable capacitance as a function of the inverse voltage introduced across their terminals, thus allowing the operation bands to be tuned.

In this way, even if the instantaneous bandwidth of a nontunable antenna is statically narrow, the combined operational range produced by superposing the dynamic bands of a reconfigurable version will be much larger.

EBGs

EBG (Electromagnetic Band Gap) are periodical structures of a given unit cell which produce a forbiden band of the propagation of electromagnetic fields at a desired operational frequency.

We worked in a planar EBG for obtaining the reduction of the mutual coupling between microstrip patch antennas. In order to reduce the size of these structures (whose lattice can be in the order of or even larger than the wavelength), we made use of a multi-layer substrate (combining high and low permitivity materials).

Metamaterials

Metamaterials are those electromagnetic materials which exhibit two essential properties:

-Not observed in the constituent materials.

-Not observed in Nature.

The extraordinary operation of the metamaterials is due to their periodicity and their unit cells. The main attractive property which can be achieved with metamaterials is the known as backward propagation. This propagation can be used to cloack objects.

We studied how periodic repetitions of SRR (Split Ring Resonators) can be used to produce propagating modes below the cut-off frequency of conventional waveguides.

Soft Surfaces

Soft surfaces were defined by Prof. Kildal at the end of the 1980s. Soft surfaces have the ability of eliminating the electromagnetic wave propagation in one direction for the two main polarizations. Recently, the planar version of these surfaces has become of great industrial interest since they can be easily integrated in microstrip technology and to eliminate the surface waves. This is beneficial since it allows a reduction mutual coupling between antennas.

Other applications of soft surfaces include elimination of back radiation, increase of the gain of antennas, and ability to tailor the radiation pattern of microstrip patch antennas.

image
image

Extracted from:

M. Ebrahimpouri, O. Quevedo-Teruel, E. Rajo-Iglesias, "Design Guidelines for Gap Waveguide Technology Based on Glide-Symmetric Holey Structures," IEEE Microwave and Wireless Components Letters, vol. 27, no. 6, pp. 542-544, June 2017.

image

Extracted from:

O. Quevedo-Teruel, M. Ebrahimpouri, M. Ng Mou Kehn, "Ultra Wide Band Metasurface Lenses Based on Off-Shifted Opposite Layers," IEEE Antennas and Wireless Propagation Letters, vol.15, pp. 484-487, 2016.

image

Extracted from:

O. Quevedo-Teruel, W. Tang, R. C. Mitchell-Thomas, A. Dyke, H. Dyke, L. Zhang, S. Haq, Y. Hao, "Transformation Optics for Antennas: Why limit the bandwidth with Metamaterials?," Scientific Reports (Nature Publishing Group), vol. 3, article number 1903, 2013.

image

Extracted from:

O. Quevedo-Teruel, "Controlled radiation from dielectric slabs over spoof surface plasmon waveguides," Progress In Electromagnetics Research, vol. 140, pp: 169-179, 2013.

image

Extracted from:

E. Rajo-Iglesias, O. Quevedo-Teruel, M. Sanchez-Fernandez, "Compact Multimode Patch Antennas for MIMO Applications ," IEEE Antennas and Propagation Magazine, vol.50, no.2, pp.197-205, April 2008.

image

Extracted from:

M. Ng Mou Kehn, O. Quevedo-Teruel, E. Rajo-Iglesias, "Reconfigurable Loaded Planar Inverted-F Antenna Using Varactor Diodes," IEEE Antennas and Wireless Propagation Letters, vol.10, pp.466-468, 2011

image

Extracted from:

E. Rajo-Iglesias, O. Quevedo-Teruel, L. Inclán-Sánchez, "Mutual Coupling Reduction in Patch Antenna Arrays by Using a Planar EBG Structure and a Multilayer Dielectric Substrate," IEEE Transactions on Antennas and Propagation, vol.56, no.6, pp.1648-1655, June 2008.

image

Extracted from:

O. Quevedo-Teruel, M. Ng Mou Khen, E. Rajo-Iglesias, "Numerical and experimental studies of split ring resonators loaded on the sidewalls of rectangular waveguides," IET Microwaves, Antennas & Propagation, vol.3, no.8, pp.1262-1270, December 2009.

image

Extracted from:

E. Rajo-Iglesias, J.L. Vázquez-Roy, O. Quevedo-Teruel, L. Inclán-Sánchez, "Dual band planar soft surfaces," IET Microwaves, Antennas & Propagation, vol.3, no.5, pp.742-748, August 2009.