Millimetre and sub-millimetre wave sounders employ Frequency Selective Surfaces (FSS) as beamsplitters in the feed train of the quasi-optical receiver. The function of these linearly polarised FSS is to separate spatially either TE or TM polarised components of the natural emissions that are received by the instrument's antenna. Polarimetric sounders which will enhance the impact of Earth observation studies are planned for future ESA missions. These advanced instruments require FSS beamsplitters, working at 45° incidence, exhibiting polarisation independent and very low loss spectral responses. The purpose of this paper is to describe the design, construction and electromagnetic performance of an innovative FSS which is currently being developed to satisfy these stringent signal control requirements.

The beamsplitter consists of two identical freestanding 12 micron thick metal screens, each of which is perforated with a close packed array of nested short circuited rectangular slot elements. A commercially available electromagnetic simulation tool has been used to optimise the FSS design to meet the requirement of separating the 316.5 - 325.5 GHz channel from the 349.5-358.5 GHz band. The filter is shown to give a maximum insertion loss of 0.6 dB, a channel isolation greater than 30 dB and crosspolar levels below 25 dB in both the TE and TM planes at 45° incidence. The filter is fabricated using precision micromachining techniques, including the use of Deep Reactive Ion Etching, to pattern the individual screens in the 30 mm diameter centre portions of silicon on insulator (SOI) wafers. The structures are then coated with a 1 micron thick layer of electrodeposited copper. To complete the FSS, precision spacers are employed to separate two optically flat screens of the aligned structures. The computed copolar and crosspolar spectral response of the individual and cascaded FSS are compared with swept frequency measurements in the range 300 - 370 GHz.

This paper describes some novel techniques for analysing quasi-optical systems in the sub-mm bands. This work is being carried out in the context of Centre for Earth Observation Instrumentation (CEOI) initiative in the UK and has focussed on the design and analysis the Swedish Space Agency’s Limbsounder STEAM-R in the 310 to 360 GHz band. This Limbsounder is proposed to operate as a multi-beam system, with the earth limb sampled from individual feed horns in an array feeding the main reflector via a quasi-optical system, rather than mechanical scanning of the main or sub-reflectors.

The analysis techniques to be described encompass computer code more usually used for Optical and Infrared applications, and by careful application may be used to analyse systems in the microwave and sub-mm bands. Recent advances in the capabilities of such optical codes as CODE-V ™ and Zemax™ make them far more applicable to the sub-mm bands, with the addition of non-sequential ray tracing and Beam Propagation analysis. By using theses techniques, and combining them with PO analysis for reflectors, and modal analysis for feed horns, it is proposed that a complete quasi-optical system, feeding a large (2m) reflector may be designed and analysed effectively.

Further to this the use of non-sequential ray tracing in the quasi-optical system allows the effects of reflections from adjacent components to be assessed in the quasi-optical design, as well as diffraction effects from the components themselves. Ray tracing techniques are commonly used to determine coupling and scattering on payloads at lower microwave frequencies, and the existing optical codes provide some of that capability.
In addition, the quasi-optical system model may be simply modified to provide the system performance for calibration functions view hot targets or cold space. This can be achieved by reverse tracing through the system between feed horn and target, with non-sequential ray tracing assuring the design.

An example of the CODE-V model, the “Straw Man” being developed in support of the STEAM-R program is shown in Fig. 1. An example diffracted beam within the quasi-optical system is also shown in the figure.

A Cassegrain dual-reflector antenna which employs a flat reflectarray as a subreflector was analysed in a recent paper [1]. It was shown that the antenna beam can be scanned by introducing an appropriate progressive phase distribution across the reflectarray surface. This configuration is very attractive for steerable beam applications, because it combines the high gain and broad bandwidth properties of the parabolic main reflector with the simplicity of manufacturing a small reconfigurable microstrip reflectarray.

One very useful application of the proposed dual-reflector concept is in radiometric remote-sensing instruments which operate in the millimetre and sub-millimetre wave bands. These currently use mechanically scanned antennas, however the proposed antenna design can potentially produce the required scan profile electronically by using a sub-reflectarray which is constructed on a thin film of liquid crystals (LC) [2].

In this paper we show that by replacing a conventional metal subreflector with a small reflectarray in a dual-reflector antenna, it is possible to steer the beam ± 5 degrees about boresight. This is a typical scene angle requirement for limbsounder instruments. A double reflector antenna has been designed at 94 GHz using an offset parabolic reflector of diameter 90-mm, and a flat subreflector of diameter 26-mm. The beam scanning capability of the antenna is demonstrated by replacing the solid metal subreflector, which produces a focussed beam in the axis of the paraboloid, with two passive microstrip patch reflectarrays which are printed on 100-micron quartz wafers. The phase distribution across the passive reflectarrays is designed to generate a focussed beam in either the +5 or -5 degrees direction. The validity of this novel beam scanning concept will be demonstrated in the final paper by comparing the simulated and measured radiation patterns at 94GHz. Preliminary work to create a phase agile sub reflectarray based on liquid crystals will be presented.

References

[1] Manuel Arrebola, Leandro de Haro, Jose A. Encinar, "Analysis of a cassegrain antenna with a reflectarray as subreflector", 29th ESA Antenna Workshop on Multiple Beams and Reconfigurable Antennas. Innovation and challenges, Noordwijk (The Netherlands), 18-20 April, 2007.

[2] W. Hu, M. Y. Ismail, R. Cahill, J. A. Encinar, V. F. Fusco, H. S. Gamble, D. Linton, R. Dickie, N. Grant and S. P. Rea, "Liquid-crystal-based reflectarray antenna with electronically switchable monopulse patterns" , Electron. Lett., vol. 43, no. 14, pp. 899-900, July 2007.

The well-known antenna program GRASP has recently been updated to handle design of quasi-optical networks. Since Gaussian beams must be used to describe the wave propagation in such networks, new tools are needed in GRASP to deal with this complication. At present the following components can be included in the designs: feeds, lenses, mirrors, reflectors, beam-splitters, interferometers and loads. All these components can be converted into elements known to GRASP, and can therefore be analyzed with full accuracy. To add design capabilities as well, it is necessary to include tools to achieve certain properties, important in quasi-optical designs.

The most important are: cross-polarization, phase slippage, radius of phase front and beam width. In standard, diffraction limited dual-reflector designs, where Geometrical Optics provide a satisfactory description of the field, the Mizuguchi condition is sufficient to suppress cross-polar radiation. In a quasi-optical network, cross-polar suppression is no longer possible in a dual-reflector design, at least three reflectors are required. For three or more reflectors another condition, which we shall refer to as the Furono condition, can be used to eliminate cross-polarization. This has been included in GRASP, but can only be used if no other components are involved. To allow the program to optimize on all combinations of components, and to optimize all the above properties, two LMS optimizers have been included as well. These will allow the user to optimize the focal distances of lenses and reflectors for various combinations of the properties at a particular point along the beam, simultaneously limiting the beam width at an arbitrary number of points. The optimization also permits upper and lower boundaries to be placed on the value of the focal distances. Examples will be presented at the Work Shop.

The development of new detectors for future astrophysical space research in the FIR/sub-mm regime (300 GHz - 5 THz) is driven by high sensitivity and the potential to make arrays with many elements (typically 10.000 pixels). Interestingly, this development also has an appealing application in security as it allows for stand-off detection of concealed objects at video rate.

Key components for such systems are a detector and an antenna. We have combined a very efficient and novel antenna geometry, the flat lens antenna, with the very sensitive Kinetic Inductance Detector (KID). So far, these were developed independently. The flat lens antenna has been developed at TNO Defense Security and Safety and has been tested at low frequencies (~ 10 GHz), but is now implemented at 675 GHz. The KID is a cryogenic radiation detector currently being developed at SRON Netherlands Institute for Space Research. It is a superconducting pair breaking detector that measures the amount of quasiparticle excitations in a 1/4 λ superconducting coplanar waveguide (CPW) resonator. The resonator is coupled via a coupler capacitor to a CPW line that runs over the entire chip.

We have combined a twin slot antenna feed with a KID resonator as shown in the figure. The entire structure has been fabricated on bare Si wafers and on a flat lens consisting of three di-electric layers of Si, SiO₂ and Si. A standard elliptical Si lens with the antenna in its second focus is used to allow efficient coupling to free space. We present measurements of the antenna beam pattern, frequency response and sensitivity of a single pixel detector.

Reflectivity investigations have been made of representative samples of satellite antenna reflectors. The investigations have been carried out at ambient and at cryogenic temperatures. It has been shown that thin metal coating of composite carbon- fibre antennas can have high reflective losses, which can be inadmissible for certain applications. For ultra-low noise observations of the Cosmic Background radiation it would lead to very essential increase in noise temperature of the high-sensitive cryogenically cooled receivers.

The report refers to results of measurements at ambient and liquid Nitrogen temperatures of metal and metallised composite samples with metal thickness several skin-depths as well as the much lower skin-depth.

The result obtained at the coldest temperature not far from ~80 K, shows well the reduction of the reflection loss, but it is not directly as anticipated from calculations.

Some conclusions about reflectivity measurements of metalized carbon fibre reflectors at cryogenic temperatures are discussed.

MADRAS (Microwave Analysis & Detection of Rain & Atmospheric Structures) is a conical scan radiometer providing imaging capability in the 18-157 GHz range. MADRAS is one of the three instruments of the MEGHA-TROPIQUES Satellite to be launched next year (2009). The MEGHA-TROPIQUES Satellite will study the water cycle and energy exchanges in the tropical belt. The Satellite is jointly developed by the Indian Space Research organisation (ISRO) and the French Space Agency (CNES).

EADS ASTRIUM provides the main part of the MADRAS Radiometer called MARFEQ, including the Antenna & Calibration System and the Receiver equipments. It is a nine channels self-calibrating passive microwave radiometer.

This instrument places stringent requirements on antenna electrical performance due to the low-noise signal levels and the high beam efficiency involved. The present paper focuses on the electromagnetic design and development of the 65 cm diameter rotating antenna operating at 18, 24, 36, 89 & 157 GHz. The primary Feed Cluster is an essential part of the overall antenna. It has to provide not only the low-loss frequency separation of the five bands but also the proper antenna illumination in amplitude and phase in order to satisfy the required resolution and high beam efficiency.

After a presentation of the antenna concept, the Feed Cluster design and development is presented. The novelty of this equipment is the KBFA (K & Ka-band Feed Assembly) an 18/24/36 GHz feed based on a common corrugated horn associated with a five channels multiplexer in waveguide. The Reflector technology includes an innovative coating which participates also to the high level performance of the instrument.

The self-calibrating system is based on a secondary reflector redirecting feed radiation to the cold space at each antenna rotation through a dedicated hole performed in the main structure. An on-board calibration target provides the hot calibration point to the radiometer.

The antenna radiation pattern measurements at instrument level performed on the flight model are used to derive the overall antenna electrical performance.

The instrument is currently in a proto-flight model assembly, integration & test phase at EADS ASTRIUM in Toulouse premises (France).

The driving reason for this effort is the desire to diminish the impact of the reflections at the dielectric air interface when these structures are proposed as focal plane imagers.

The number of elements that can be located in the focal plane of a dielectric lens is limited because the performances of the elements located at a large distance from the central focus are significantly degraded with respect to the elements in focus

[1]. This effect can be interpreted as low spill-over efficiency of the lens since a significant portion of the power radiated by the feeds is intercepted by the dielectric-to-air interface at locations that do not directly contribute to a focused beam. To increase the number of radiators on the ground plane, elements of larger directivity should be used so that they all excite efficiently the central portion of the lens is larger.

Recent investigations indicate that Electromagnetic Band Gap (EBG) super-layers can be used to enhance the directivity of planar and waveguide antennas. In particular, in [2] and [3], the efficient excitation of reflector antennas by means of EBG enhanced printed waveguide feeds has been discussed in depth. The main result is that the efficiency of single-feed reflectors and imaging arrays can be increased significantly with respect to radiators operating in free space, at low to moderate manufacturing cost. Here a similar design strategy has been applied, In the frame of cooperation activity between TNO and SRON, to increase the directivity of the focal plane feeds typically used to excite dielectric lenses. While the use of EBG super-layers at microwave frequencies was justified mainly by the electric performances of the systems, the manufacturing advantages of the EBG solutions based on dielectric may end up being the most important criteria in the sub-mm wave regimes. This particular difference can be understood from the intrinsic difficulty in realizing corrugated waveguide horns with micrometric accuracies and the integration of these horns with the receivers could be insurmountable.

References
1. D.F. Filippovic, S.S. Gearhart, and G. M. Rebeiz: "Double Slot on Extended Hemispherical and Elliptical Silicon Dielectric Lenses" IEEE Trans. Microwave Theory Tech., 1993, Vol. 41, Issue 10.
2. Nuria Llombart, Andrea Neto, Giampiero Gerini, Magnus Bonnedal, Peter De Maagt, "Leaky Wave Enhanced Feed Arrays for the Improvement of the Edge of Coverage Gain in Multi Beam Reflector Arrays", accepted for publication on IEEE Transactions on Antennas and Propagation,
3. A. Neto, N. Llombart, G. Gerini, M. Bonnedal, P. de Maagt, "EBG Enhanced Feeds for High Aperture Efficiency Reflector Antennas", on the IEEE Transactions of Antennas and Propagation, Vol. 55, no.7, August 2007

In preparation of the EUMETSAT post-EPS user consultation process the European Space Agency has studied future operational meteorological microwave missions including the EGPM (European contribution to the Global Precipitation Measurement mission) type requirements at Pre-Phase A level. These requirements include a dual 54/118 GHz channel capability.

EADS Astrium has been selected by ESA to carry out the Multi-Frequency Feeds for Earth Observation Applications Study. The main objective of this activity is to design and test a defocused dual-frequency horn, operating at frequencies 54 and 118 GHz, to be combined with an offset reflector. To derive the required precipitation products from the measurements, both frequency bands (54 and 118 GHz) have to be optimised for a footprint overlap of at least 90% of the largest footprint and for high beam efficiency (> 95%).

The Dual-Frequency Feed (DFF) includes the dual-frequency horn and the diplexer for 54/118 GHz channels separation. EADS Astrium, as Study responsible, is in charge of the overall antenna engineering, including the reflector, the diplexer design and the DFF manufacturing and test. IEEA is responsible for the horn design. The horn shall exhibits high level performance in both 54 and 118 GHz frequency bands (VSWR, cross-polar, sidelobes) whilst the diplexer shall discriminate both channels with high level isolation and with minimum insertion loss.

This document presents the design activities performed by IEEA on the dual-frequency horn and the "waveguide diplexer with integrated FSS (*)", an original diplexer design proposed and studied by EADS Astrium. For final performance assessment, the antenna radiation patterns at reflector level are also derived with DFF predicted performance as input.
(*) Frequency Selective Surface

One of the future trends in numerical modelling for antenna design consists in the development of user-friendly electromagnetic tools for the fast and accurate synthesis of complex three-dimensional (3-D) devices. Generally speaking, electrically-small antenna configurations can be synthesized using full wave solvers (the required computational resources are acceptable, which makes feasible the combination of 3-D analysis tools with general purpose optimisation algorithms), whereas high frequency techniques can be implemented for the synthesis of large antenna systems. Nevertheless, the optimisation and synthesis of antennas whose size is in the order of several wavelengths remain challenging since the above techniques cannot be applied directly.

To contribute to fill in this gap, we have developed a fast FDTD solver for the synthesis of arbitrary metallo-dielectric Bodies of Revolution (BoR). Due to the azimuthal field dependence, the Fourier series expansions are used and, instead of considering a 3-D volume discretisation, a cylindricallybased system lattice can be projected onto a two-dimensional (2-D) plane that contains solely ñ- and z-directions. Compared to full 3-D analyses, this allows important savings in terms of computational memory and time. As a consequence, this makes realistic the full-wave optimisation of microwave and (sub-)millimetre wave devices by combining BoR-FDTD with Genetic Algorithms (GAs) on a grid-enabled client/server model.

We will first describe the main features and performance of the proposed GA-based BoR-FDTD CAD tool. For numerical validations, this tool will then be applied to synthesize two kinds of 3-D antennas in V-band, namely waveguide-fed integrated lens antennas (ILAs), and dielectric-loaded horn antennas. In the first case, we will show that shaping the profile of medium-size ILAs enables to increase the maximum beam directivity compared to conventional designs based on extended hemispherical lenses. In the second case, we will demonstrate that loading a smooth-walled horn with optimised dielectric loads leads to a significant gain enhancement, which might be attractive for applications (e.g. focal arrays) where compact feeds with high aperture efficiencies are needed.

This paper presents a new integrated double-shell lens antenna to be used as a single feed for an imaging reflector system. The main challenge is to design a double shaped high permittivity integrated lens antenna with good scanning characteristics and low reflection losses that can produce multiple virtual foci compatible with a wide reflector focal arc. Use of the double-shell configuration favors power transmission across the lens. In this application, the reflector focal points for consecutive 3 dB overlapping beams are about 2λ apart. This separation is not enough for using multiple feeds to illuminate the reflector with proper field taper at its edge. Therefore, an array of printed feeds is integrated at the shaped lens base, with much smaller separation between consecutive elements, but each one producing a virtual focus far behind the lens base and coincident with the corresponding reflector focal points.

The lens is fixed at 90º off-set position with respect to the parabolic reflector, which is free to rotate about an axis passing through the lens axis. This enables mechanical azimuth beam scan while the lens feeding is responsible for electronic elevation scan. Experimental results for a fabricated scaled lens antenna prototype at 62.5 GHz, using materials with 5.5/2.53 permittivity, confirm the viability of this concept for application in a quasi-optical imaging reflector system at 500 GHz. A remarkable agreement between measurement and simulations was obtained for the lens radiation patterns for the different feed positions, both in terms of amplitude and phase. The amplitude and phase variation of the lens radiation pattern as a function of the lens feed offset position produce the adequate illumination of the reflector for ±2 beamwidth elevation scanning (less than 1.5 dB gain scan loss). The system performance can be improved by using higher permittivity materials like alumina/quartz. These results will be presented in more detail.

The proposed lens antenna was designed and evaluated with the ILASH software tool, which was developed at Instituto de Telecomunicações (IT), in the framework of an European Space Agency (ESA/ESTEC) funded project. ILASH is a versatile tool that combines Geometrical Optics (GO) design modules with Geometrical Optics/Physical Optics (GO/PO) analysis modules, with Genetic Algorithm (GA) optimization modules and a number of auxiliary graphical tools to design and evaluate both stand-alone lens radiation performance as well as when it is feeding a parabolic reflector. ILASH kernel and all the available functions and graphical output tools (including 3D visualization of a few lens diagnostic and performance indicators) are managed through a Windows-based user-friendly interface. A description of the ILASH tool and of its functionalities will be also addressed.