Research

• Lens antennas
• Geodesic lenses
• Glide symmetries
• Ray tracing and physical optics
• Multimodal analysis for periodic structures
• Metasurfaces
• Transformation Optics
• Twist Symmetries
• Leaky-wave antennas

Lens antennas

Lens antennas are considered as a type of aperture antennas. They make use of a lens to modify the field distribution at the aperture of the antenna, which is typically fed by a single source.

The lens is employed to transform the waves arriving from the source into a desired radiation pattern. Commonly, the desired radiation pattern is a directive beam in a given direction.

At KTH, we investigate new lens antenna solutions. These new solutions are aimed to reduce the cost, complexity and losses in the feeding network. Our lens antennas make use of glide symmetries and/or transformation optics. We have conducted research on geodesic lenses and arrays combined with lenses/radomes.

Geodesic lens antennas

Geodesic lenses have attracted the attention of the antenna community due to two of their advantageous properties: low cost and high efficiency.

These lenses make use of a physical path that mimics an equivalent graded refractive index. Therefore, differently to conventional lenses, they can be implemented in a fully metallic configuration using parallel plate waveguides. This makes these lenses ideal for high-frequency applications, where dielectric materials have prohibitive losses.

Moreover, since geodesic lenses can mimic the properties of graded-index lenses, they can provide wide-angle scanning capabilities with reduced scan losses.

Therefore, these lenses are an excellent candidate for new applications in the millimeter frequency regime. For example, they are being considered for antenna solutions in 5G/6G, satellite communications in Low-Earth Orbit constellations and automotive radars.

Glide Symmetries

A periodic structure is glide symmetric if it can be repeated after both a translation and a mirroring operation.

The study of glide symmetries is 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 broadband lens and leaky-wave antennas.

Additionally, glide-symmetric structures have demonstrated to be able to produce low-cost gap-waveguide technology. We have demonstrated that glide-symmetric configurations have a similar bandgap as the conventional bed-of-nails.

Ray tracing and physical optics

Under the assumption that the wavelength of the electromagnetic wave is small enough, the study of the wave propagation in inhomogeneous isotropic medium can be approached by means of geometrical optics. This means that we can use geometrical considerations to determine the paths of its associated rays as well as to compute its corresponding intensity.

We have implemented a general code that combines a ray tracing with ray tube and diffraction theories; and the results are extremely accurate for geodesic lenses, as well as for dielectric radome/lenses.

Our code permits the calculation of radiation patterns, scan losses, gain, directivity and radiation efficiency; and it is not limited to rotationally-symmetric shapes.

Multimodal analysis for periodic structures

The first step in the design of devices containing periodic structures is the dispersive analysis of the structure, i.e., computing the wavenumber of modes propagating.

This can be done using the eigenmode analysis tool of general electromagnetic solvers. However, higher precision and reduced computational time can be obtained with developed specialized programs for narrower class of electromagnetic structures of interest.

Some commonly used techniques for these specialized programs are circuit models, multimodal analysis and the mode-matching method. At KTH, we have developed a number of these programs, specialized in solving periodic structures possessing higher symmetries. In addition, these methods permit the calculation of the losses in bandgaps, radiation and disipation.

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-symmetric metasurfaces to achieve wide band lenses, such as Luneburg lenses that find application in 5G/6G communication systems and high-speed vehicles.

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.

Twist Symmetries

Twist symmetry is one type of higher symmetry where the additional geometrical operation is a rotation around a twist axis.

More specifically, a structure possesses an m-fold twist symmetry if the compound geometrical operation consists of a translation of p/m along a given axis (the periodicity axis) and a rotation of 2π/m around the same axis, where p is the periodicity of the unit cell, and m is an integer number.

At KTH, we have recently proposed the use of twist symmetries to reduce the dispersion in transmissions lines and to produce stop-bands for filtering purposes.

Leaky-wave antennas

Leaky-wave antennas are realized by a gradual and controlled radiation of the waves that are confined in waveguides or transmission lines.

Leaky-wave antennas can produce high radiation efficiency due to their simplified feeding network. They are commercially used in airplane radars or train communications inside of tunnels.

One of their common drawbacks is that they are dispersive. This means that their radiation pattern changes with the frequency. At KTH, we have recently demonstrated that dispersive prisms can be employed to compensate the inherent dispersion of leaky-wave antennas.