A new type of metallic electromagnetic structure has been developed that is characterized by having high surface impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas.

/pdf/high-impedance-electromagnetic-surfaces-with-a-forbidden-3vtu72a5an.pdf

High-impedance electromagnetic surfaces with a forbidden frequency band

Herschel was launched on 14 May 2009, and is now an operational ESA space observatory o ering unprecedented observational capabilities in the far-infrared and submillimetre spectral range 55 671 m. Herschel carries a 3.5 metre diameter passively cooled Cassegrain telescope, which is the largest of its kind and utilises a novel silicon carbide technology. The science payload comprises three instruments: two direct detection cameras/medium resolution spectrometers, PACS and SPIRE, and a very high-resolution heterodyne spectrometer, HIFI, whose focal plane units are housed inside a superfluid helium cryostat. Herschel is an observatory facility operated in partnership among ESA, the instrument consortia, and NASA. The mission lifetime is determined by the cryostat hold time. Nominally approximately 20,000 hours will be available for astronomy, 32% is guaranteed time and the remainder is open to the worldwide general astronomical community through a standard competitive proposal procedure.

/pdf/herschel-space-observatory-an-esa-facility-for-far-infrared-3pdcouv7zo.pdf

Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Transformation optics describes the capability to design the path of light waves almost at will through the use of metamaterials that control effective materials properties on a subwavelength scale. In this review, the physics and applications of transformation optics are discussed.

Transformation optics and metamaterials

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

Resonators are traditionally characterised by their quality factor Q, which is inversely proportional to the relative bandwidth. Small antennas are often resonant, so they can be characterised by a Q, except for the fact that the correct quality measure of an antenna is the inverse Q, that is the relative bandwidth, rather than Q. Still, it has become common to study fundamental size limitations of small antennas in terms of a so-called radiation Q (or antenna Q). This study explains how this intrinsic radiation Q relates to: (i) the bandwidth-efficiency product of small single-resonance-type antennas, (ii) the gradual cut-off of spherical waves for wideband gradual-transition-type small antennas and (iii) the number of resonances needed to cover a certain frequency band for wideband cascaded-resonance-type small antennas. The study also introduces one intrinsic radiation Q for basic single TE and TM spherical mode sources, and another for combined TE and TM sources.

Effects of intrinsic radiation q on mismatch factor of three types of small antennas: single-resonance, gradual-transition and cascaded-resonance types

The FOV of a radio telescope fed by a cluster of conventional horns in a one-horn-per-beam configuration is limited to a few beamwidths due to the field distortions which appear for off-axis directions of observation. To overcome this limitation, it has been suggested to use dense focal plane arrays (FPAs) of electrically small elements in which several of these elements contribute to each beam. The primary goal of the paper is to investigate the impact of reflection, coupling and dissipation losses in dense FPAs on the total aperture efficiency of the reflector antenna. We present and discuss results obtained for two dense array realizations: one reference realization of miniaturized open-ended hard waveguides, and another practical FPA of Vivaldi elements.

Control of reflection and mutual coupling losses in maximizing efficiency of dense focal plane arrays

A parabolic cylindrical reflector antenna is proposed in this paper for wireless multi-gigabit communications at 60 GHz. The line feed is realized by a power distribution in parallel plate waveguide (PPW). Inside the PPW waveguide there are metal pin rows forming the contours of an H-plane horn structure in front of a parabolic reflector, where we see possible mechanical advantages in not requiring metal contact of the pins to more than one metal plate. They will still work as a total reflecting wall by making use of the same stopband features as in gap waveguide technology. The result is a low loss, low cost and very versatile 60 GHz antenna with high gain. The electrical design for maximum aperture efficiency and minimum loss is explained.

http://publications.lib.chalmers.se/records/fulltext/177758/local_177758.pdf

Parabolic cylindrical reflector antenna at 60 GHz with line feed in gap waveguide technology

The main concern of the current communication is to elicit the influences of beam-forming upon different Multiple-Input Multiple-Output (MIMO) parameters in a rich scattering environment. A detailed study of desired parameters is presented with particular stress on total radiation efficiencies observed on Element ports and Beam ports. Measurements are performed in a Reverberation Chamber (RC). The results illustrate that performance as viewed through Element and Beam ports are, in general, equal. Hence, they confirm that in a rich and isotropic scattering environment, beam-forming technique does not prove beneficial. They also demonstrate that as long as received radiation power is preserved, the shapes of the elements' patterns are irrelevant.

Study of excitation on beam ports versus element ports in performance evaluation of diversity and MIMO Arrays

The paper considers the fundamental limitations on directivity and radiation efficiency of antennas with both single and multiple ports. The treatment is limited to antennas made of lossless materials. The maximum available directivity measured on a single port of both small antennas and large antennas is considered in terms of the diameter of the smallest sphere that can surround the antenna. The maximum available directivity of large antennas is given by the common aperture directivity formula, and that of small antennas is 4.67 dBi (same as for a Huygen source). These two expressions are combined in a heuristic manner to a continuous maximum directivity limitation. The theoretical limitation is plotted together with directivities of known antennas which are known to have large directivities. The paper discusses also fundamental limitations of dense arrays. Dense linear arrays can be used to provide superdirectivity, but their superdirectivity is lower than what can be obtained with normal antennas under the present definition of maximum available directivity. Dense arrays also suffer from severe gain reduction due to strong coupling to the ports of the neighbouring elements. This gain reduction can be characterized in terms of a coupling efficiency, also referred to as embedded element efficiency, and appears as a reduction of the total radiated power. The fundamental property of this coupling efficiency is known from classical papers, and it has recently been verified by measurements in a reverberation chamber. (6 pages)

Per-Simon Kildal

Papers

Effects of intrinsic radiation q on mismatch factor of three types of small antennas: single-resonance, gradual-transition and cascaded-resonance types

Control of reflection and mutual coupling losses in maximizing efficiency of dense focal plane arrays

Parabolic cylindrical reflector antenna at 60 GHz with line feed in gap waveguide technology

Study of excitation on beam ports versus element ports in performance evaluation of diversity and MIMO Arrays

Fundamental Directivity and Efficiency Limitations of Single- and Multi-Port Antennas