This paper describes a new quasi-optical switching method for more efficient calibration of millimetre and submillimetre wave Earth Observation (EO) radiometers. Passive remote sensing instruments must be calibrated regularly to ensure high measurement accuracy and current systems require switching between known hot and cold reference target loads using large and heavy power hungry motor driven mechanical systems. The proposed switching method is based on a reconfigurable Frequency Selective Surface (FSS) which is composed of a thin dielectric slab placed above a periodic array of polarisation independent slot elements. The spacing between the FSS and the high permittivity dielectric is adjusted to provide a good impedance match to free-space and hence high transmission for the 'ON' state, whereas for the 'OFF' state the separation distance is chosen to degrade the match and hence suppress the signal transmission by -30 dB mid band. Numerical predictions and experimental results in the frequency range 316 - 334 GHz are presented to demonstrate the operation of the FSS switch.

This paper reports on recent developments in the manufacture and measured electromagnetic (EM) performance of a new class of thin microwave absorbers based on metal backed resistively loaded Frequency Selective Surfaces (FSS). Ink jet printing technology is shown to be a viable method for creating patterned FSS elements very close to the desired resistance values. This is demonstrated by comparing simulated and measured reflectivity plots for an ink jet and a stencil printed polarisation independent absorber that is designed to work in the frequency range 7.1–24 GHz. Numerical simulations show that thin FSS absorbers provide radar backscatter suppression, not only in the far field of the illuminating antenna, but also when the EM source is placed very close to the surface of the periodic array. This technology can therefore provide a solution for enhancing the performance of wireless sensors placed close to lossy loads such as the human body.

The purpose of this paper is to provide an overview of the innovative design and manufacturing strategies that have led to the creation of a new class of freestanding Frequency Selective Surface (FSS) with unrivalled electromagnetic, thermal and structural performances. These structures, when deployed as free space electromagnetic filters, provide passive remote sensing instruments with multispectral capability by separating the scene radiation into separate frequency channels. Ultra-low loss spatial beam splitting enables high sensitivity receivers to detect weak molecular emissions at mm to sub-mm wavelengths, independent of the polarisation and angular direction of the propagating waves. This new generation of FSS is shown to satisfy the technically challenging performance and functionality that is required for next generation Earth observation radiometers. Moreover, building on this work we present a new concept for creating electronically tunable quasi optical switches based on a reconfigurable FSS structure that is suitable for deployment in future radiometer calibration arrangements.

This paper presents recent developments in Liquid Crystal-based reflectarray antennas for mm-wave applications, future perspectives for this technology and its particular use in SATCOM applications.

This paper describes the design of a frequency selective surface (FSS) which provides transmission of 228 - 230 GHz radiation and rejection from 164 - 191.3 GHz with insertion losses under 0.25 dB for TE wave polarization at 45° incidence. This state-of-the art filter topology consists of two air spaced freestanding perforated screens, comprising unit cell elements of resonant slots folded for the purpose of miniaturisation to enhance angular stability. The reported geometry enhances the angular stability (45° ± 10°) of the FSS beyond what is possible with canonical linear slots and satisfies the stringent electromagnetic performance requirements for signal demultiplexing in the quasi-optical feed train of the Microwave Sounder (MWS) instrument.

Two Liquid crystal-based reflectarrays that operate at 100 GHz and 125 GHz are presented. The first pro totype (100 GHz) is used to validate the modeling and the design procedure proposed for this class of antenna. Experimental validation of the beam scanning is carried out by measuring the received power in a quasi-optical test bench, which is able to rotate the receiver in the horizontal plane. These results are used to des ign a second prototype antenna (125 GHz) which exhibits 2D beam scanning capabilities with a large bandwidth and scanning range that is sufficient for radar and communications applications.

An electronically tunable reflection polarizer which exploits the dielectric anisotropy of nematic liquid crystals (LC) has been designed, fabricated and measured in a frequency band centered at 130 GHz. The phase agile polarizing mirror converts an incident slant 45° signal upon reflection to right hand circular (RHCP), orthogonal linear (-45 °) or left hand circular (LHCP) polarization depending on the value of the voltage biasing the LC mixture. In the experimental set-up this is achieved by applying a low frequency bias voltage of 0 V, 40 V and 89 V respectively, across the cavity containing the LC material.

In this paper we describe the development of an electromagnetic modelling technique to investigate edge illumination effects on finite size FSS performance. The work extends the commonly used unit cell approach and models the FSS as a linear array with Gaussian beam excitation. Bistatic scattering from the FSS is calculated at 23.8 GHz for a 45 incident beam. The results presented relate the beam size, edge illumination and scattering performance.

Nematic liquid crystals (LC) offer the capability to electronically reconfigure their dielectric permittivity tensor by applying a low AC voltage. This feature can find pertinent applications in submm-wave antenna technology in developing reconfigurable devices. In this contribution we review recent developments for the dielectric characterization of nematic liquid crystal samples at mm wavelengths and describe their applications in reconfigurable reflectarray antenna and tunable linear to circular polarization reflectors. The paper describes the theoretical aspects which underpin the technology that is employed for the realization, packaging and testing.

The design, predicted and measured performance of an ultra-broadband frequency selective surface, developed to separate spatially the spectral signals in the antenna feed network of the Microwave Sounder instrument, is reported. The low pass filter operates at 45° incidence in 7 discrete bands over the frequency range 23 - 230 GHz. Maximum measured in-band loss is 0.4 dB within the transmission and reflection bands.

A 50.22 × 48.6 mm beam-scanning reflectarray based on liquid crystals (LCs) has been designed to operate over a bandwidth of 5.5% in the low THz frequency range. The structure consists of a single-layer multi-resonant periodic array with unit-cells composed of three parallel unequal length dipoles. The gap between the array and ground plane is filled with electronically tunable LC which exhibits a permittivity value that is controlled by a low frequency bias signal. The simulations demonstrate that the beam can be scanned over an angular range of ±10° yielding a gain greater than 35 dBi and maximum sidelobe levels of 25 dB from 335 to 354 GHz.

The accurate measurement of the permittivity, loss tangent and dielectric anisotropy DC bias dependence for two different liquid crystal (LC) materials in the frequency range 140-165 GHz is described. The electrical characteristics are obtained by curve fitting computed transmission coefficients to the experimental spectral response of a new class of electronically reconfigurable frequency selective surface. The periodic structure is designed to yield bandpass filter characteristics with and without an applied bias control voltage in order to measure the tunability of the LC material which is inserted in a 705 mu m-thick cavity.

A reconfigurable reflectarray-cell has been designed to provide a large tunable phase range in the frequency band 117-130 GHz. The unit-cell comprises three parallel dipoles printed on a quartz-wafer and a tunable liquid crystal (LC) placed on a cavity between the dipoles and a ground plane. The simulation results show a tunable phase-shift in a range larger than 300 degree for a 10% bandwidth and low sensitivity to the angle of incidence.

This communication investigates the potential for fabrication of micromachined silicon sub-millimeter wave periodic arrays of freestanding slot frequency selective surfaces (FSS) using wet etch KOH technology. The vehicle for this is an FSS for generating circularly polarized signals from an incident linearly polarized signal at normal incidence to the structure. Principal issues and fabrication processes involved from the initial design of the core FSS structures to be made and tested through to their final testing are addressed. Measured and simulated results for crossed and ring slot element shapes in single and double layer polarization convertor structures are presented for sub-mm wave operation. It is shown that 3 dB axial ratio (AR) bandwidths of 21% can be achieved with the one layer perforated screen design and that the rate of change is lower than the double layer structures. An insertion loss of 1.1 dB can be achieved for the split circular ring double layer periodic array. These results are shown to be compatible with the more specialized fabrication equipment dry reactive ion etching approach previously used for the construction of this type of structure.

In this paper we report on two methods of fabricating Frequency Selective Surfaces (FSS) which are designed to give a polarisation independent band pass filter response centred at 321 GHz. One of the designs is based on metal encapsulated polymer while a later version uses silicon on insulator (SOI) as the structural material. The fabrication of the devices was carried out in a clean room laboratory using precision micromachining and plating processes. These include the use of reactive ion etching (RIE) to pattern the individual slots and deep RIE to remove the substrate underneath. This process yields a Freestanding FSS which can be duplicated to form multilayer structures with enhanced filtering performance. Quasi-optical transmission measurements in the 290 GHz – 360 GHz range yield spectral transmission coefficients which are in close agreement with the numerical predictions.

The design, manufacture and performance of a frequency selective surface (FSS) which is required to operate simultaneously in the TE and TM planes at 45° incidence over the frequency range 173 - 671 GHz is presented. The FSS was designed to allow transmission of radiation over a 2% bandwidth centred at 664 GHz and reflect four channels centred at 448, 325, 243 and 183 GHz with a loss < 0.5 dB. The...

A single layer, frequency selective surface based, sub-millimeter wave transmission polarizer is presented that converts incident slant linear 45° polarization into circular polarization upon transmission. The polarization convertor consists of a 30 mm diameter 10 thick silicon reinforced metalized screen containing 2700 resonator cells and perforated with nested split ring slot apertures. The screen was designed and optimized using CST Microwave Studio and predictions were validated experimentally by transmission measurements over the 250-365 GHz frequency range. This frequency range is used for remote environmental monitoring and 325 GHz represents a molecular emission line for H 2 O. The results obtained show good agreement between measured and modeled predictions. The measured 3 dB axial ratio bandwidth was 11.75%, measured minimum Axial Ratio was 0.19 dB and the measured insertion loss of the single layer screen was 3.38 dB.