Raymond Dickie

Design and Measurement of Reconfigurable Millimeter Wave Reflectarray Cells With Nematic Liquid Crystal

Numerical simulations are used to study the electromagnetic scattering from phase agile microstrip reflectarray cells which exploit the voltage controlled dielectric anisotropy property of nematic state liquid crystals (LCs). In the computer model two arrays of equal size elements constructed on a 15 mum thick tuneable LC layer were designed to operate at center frequencies of 102 GHz and 130 GHz. Micromachining processes based on the metallization of quartz/silicon wafers and an industry compatible LCD packaging technique were employed to fabricate the grounded periodic structures. The loss and the phase of the reflected signals were measured using a quasi-optical test bench with the reflectarray inserted at the beam waist of the imaged Gaussian beam, thus eliminating some of the major problems associated with traditional free-space characterization at these frequencies. By applying a low frequency AC bias voltage of 10 V, a 165deg phase shift with a loss 4.5-6.4 dB at 102 GHz and 130deg phase shift with a loss variation between 4.3-7 dB at 130 GHz was obtained. The experimental results are shown to be in close agreement with the computer model.

325 GHz Single Layer Sub-Millimeter Wave FSS Based Split Slot Ring Linear to Circular Polarization Convertor

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 H2O. 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.

Liquid Crystal Tunable mm Wave Frequency Selective Surface

A frequency selective surface (FSS) which exploits the dielectric anisotropy of liquid crystals to generate an electronically tunable bandpass filter response at D Band (110-170 GHz) is presented. The device consists of two printed arrays of slot elements which are separated by a 130-mum thick layer of liquid crystals. A 3% shift in the filter passband occurs when the substrate permittivity is increased by applying a control signal of 10 V. Measured results show that the insertion loss increases from -3.7 dB to -10.4 dB at resonance (134 GHz), thus demonstrating the potential to create a FSS which can be switched between a transmitting and a reflecting structure.

THz Frequency Selective Surface Filters for Earth Observation Remote Sensing Instruments

The purpose of this paper is to review recent developments in the design and fabrication of Frequency Selective Surfaces (FSS) which operate above 300 GHz. These structures act as free space electromagnetic filters and as such provide passive remote sensing instruments with multispectral capability by separating the scene radiation into separate frequency channels. Significant advances in computational electromagnetics, precision micromachining technology and metrology have been employed to create state of the art FSS which enable high sensitivity receivers to detect weak molecular emissions at THz wavelengths. This new class of quasi-optical filter exhibits an insertion loss <;0.3 dB at 700 GHz and can be designed to operate independently of the polarization of the incident signals at oblique incidence. The paper concludes with a brief overview of two major technical advances which will greatly extend the potential applications of THz FSS.

Spatial demultiplexing in the submillimeter wave band using multilayer free-standing frequency selective surfaces

In this paper, we show that a multilayer freestanding slot array can be designed to give an insertion loss which is significantly lower than the value obtainable from a conventional dielectric backed printed frequency selective surface (FSS). This increase in filter efficiency is highlighted by comparing the performance of two structures designed to provide frequency selective beamsplitting in the quasioptical feed train of a submillimeter wave space borne radiometer. A two layer substrateless FSS providing more than 20 dB of isolation between the bands 316.5-325.5 GHz and 349.5-358.5 GHz, gives an insertion loss of 0.6 dB when the filter is orientated at 45/spl deg/ incidence in the TM plane, whereas the loss exhibited by a conventional printed FSS is in excess of 2 dB. A similar frequency response can be obtained in the TE plane, but here a triple screen structure is required and the conductor loss is shown to be comparable to the absorption loss of a dielectric backed FSS. Experimental devices have been fabricated using a precision micromachining technique. Transmission measurements performed in the range 250-360 GHz are in good agreement with the simulated spectral performance of the individual periodic screens and the two multilayer freestanding FSS structures.

Submillimeter Wave Frequency Selective Surface With Polarization Independent Spectral Responses

This paper reports the design, construction and electromagnetic performance of a new freestanding frequency selective surface (FSS) structure which generates coincident spectral responses for dual polarization excitation at oblique angles of incidence. The FSS is required to allow transmission of 316.5-325.5 GHz radiation with a loss les 0.6 dB and to achieve ges 30 dB rejection from 349.5-358.5 GHz. It should also exhibit crosspolarization levels below -25 dB, all criteria being satisfied simultaneously for TE and TM polarizations at 45 deg incidence. The filter consists of two identical, 30 mm diameter, 12.5 mum thick, optically flat, perforated metal screens separated by 450 mu m. Each of the ap 5000 unit cells contains two nested, short circuited, rectangular loop slots and a rectangular dipole slot. The nested elements provide a passband spectral response centered at 320 GHz in the TE and TM planes; the dipole slot increases the filter roll-off above resonance. The FSS was fabricated from silicon-on-insulator wafers using precision micromachining and plating processes including the use of deep reactive ion etching (DRIE) to pattern the individual slots and remove the substrate under the periodic arrays. Quasi-optical transmission measurements in the 250-360 GHz range yielded virtually identical copolarized spectral responses, with the performance meeting or exceeding the above specifications. Experimental results are in excellent agreement with numerical predictions.

Sub-mm Wet Etched Linear to Circular Polarization FSS Based Polarization Converters

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.

Accurate and Efficient Modeling to Calculate the Voltage Dependence of Liquid Crystal-Based Reflectarray Cells

Two models that can predict the voltage-dependent scattering from liquid crystal (LC)-based reflectarray cells are presented. The validity of both numerical techniques is demonstrated using measured results in the frequency range 94-110 GHz. The most rigorous approach models, for each voltage, the inhomogeneous and anisotropic permittivity of the LC as a stratified media in the direction of the biasing field. This accounts for the different tilt angles of the LC molecules inside the cell calculated from the solution of the elastic problem. The other model is based on an effective homogeneous permittivity tensor that corresponds to the average tilt angle along the longitudinal direction for each biasing voltage. In this model, convergence problems associated with the longitudinal inhomogeneity are avoided, and the computation efficiency is improved. Both models provide a correspondence between the reflection coefficient (losses and phase-shift) of the LC-based reflectarray cell and the value of biasing voltage, which can be used to design beam scanning reflectarrays. The accuracy and the efficiency of both models are also analyzed and discussed.

Design and Demonstration of an Electronically Scanned Reflectarray Antenna at 100 GHz Using Multiresonant Cells Based on Liquid Crystals

The design, fabrication, and measured results are presented for a reconligurable reflectarray antenna based on liquid crystals (LCs) which operates above 100 GHz. The antenna has been designed to provide beam scanning capabilities over a wide angular range, a large bandwidth, and reduced side-lobe level (SLL). Measured radiation patterns are in good agreement with simulations, and show that the antenna generates an electronically steerable beam in one plane over an angular range of 55° in the frequency band from 96 to 104 GHz. The SLL is lower than -13 dB for all the scan angles and -18 dB is obtained over 16% of the scan range. The measured performance is significantly better than previously published results for this class of electronically tunable antenna, and moreover, verifies the accuracy of the proposed procedure for LC modeling and antenna design.

Electronically Reconfigurable Liquid Crystal Based Mm-Wave Polarization Converter

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.