N. Grant

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.

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.

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.

Phase control of reflectarray patches using liquid crystal substrate

In this proof of concept study we employ numerical and measured results at X- band to demonstrate that the dielectric anisotropy of nematic state liquid crystal can be exploited to produce electronically controlled phase shifters for printed reflectarray antennas. Phase agility is realized by inserting a layer of liquid crystal in the region between the resonant patch array and the conductive ground plane. Applying a low frequency biasing voltage produces a small change in the permittivity of the substrate and this is shown to create a large shift in the phase of the reflected signal. Ansoft HFSS version 10.0 is employed to study the scattering behaviour of the array elements in the range 9¿11 GHz using the dielectric properties of commercially available liquid crystals. The simulated phase range, bandwidth and reflection loss are shown to be in close agreement with measurements that were obtained from a waveguide simulator. The most significant impact of this new active control strategy is in the mm and sub-mm wave band and therefore a technique is proposed for characterising the electrical performance of liquid crystals at these frequencies. This paper summarises the progress that has been made in the first stage of a collaborative academic/industrial project to investigate the feasibility of creating high frequency beam scanning reflectarray antennas for future space science instruments.

Dual polarised Sub-mm wave frequency selective beamsplitter

In this paper we describe the design of a Frequency Selective Surface (FSS) which gives a polarisation independent spectral response when the filter is orientated at 45° incidence. The structure consists of one or more freestanding screens each containing a closely packed array of nested short circuited annular slots. A computer model is used to design a two layer FSS which can separate the bands 316.5¿325.5GHz and 349.5¿358.5GHz with >20dB isolation and <1dB insertion loss in both the TE and TM incident planes. A single periodic array, which is the basic building block of the cascaded screen structure, has been fabricated using a precision micromachining technique. Quasi-optical swept frequency transmission coefficients which were measured over the range 305¿345GHz are shown to be in good agreement with the numerical results. A concept for improving the robustness of the structure in order to provide space qualified beamsplitters for future polarimetric atmospheric sounders is presented.

Multilayer aperture ring frequency selective surface modelling

This paper describes the design of a low loss quasi-optical beam splitter, which is required to provide efficient diplexing of the bands 316.5-325.5 GHz and 349.5-358.5 GHz with at least 20 dB of isolation. To minimise the filter insertion loss, frequency selective surfaces (FSS) that consists of freestanding resonant shorted ring elements is proposed. In addition, to achieve the stringent isolation a two layer FSS structure is required. Two commercial electromagnetic modelling tools, a time domain finite integral method (FIT) and a finite element method (FEM) solver are used to obtain the specified transmission response. The FIT solver was used to generate fast design information at normal incidence while the FEM was only used to obtain the final design of the FSS at 45/spl deg/ due to its substantially increased run times.

Performance of reflectarray cells printed on liquid crystal film

In this paper we demonstrate that the anisotropic property of liquid crystal can be exploited to control the phase of signals that are reflected from a reflectarray cell. Numerical and measured results at X-band are used to compare the plane wave scattering from two reflectarray cells which are constructed on liquid crystal film of thickness 200 mum and 500 mum. The phase agility, bandwidth and reflection loss are shown to be dependent on both the thickness and the voltage controlled permittivity of the tunable substrate. A tunable phase range greater than 250deg is achieved over a 6.1% bandwidth.