Freestanding Frequency Selective Surface Research Articles

Dyadic Green’s function for the graphene–dielectric stack with arbitrary field and source points

In this paper, the dyadic Green’s function for a graphene–dielectric stack is formulated based on the scattering superposition method. To this end, the scattering Green’s function in each layer is expanded in terms of cylindrical vector wave functions with unknown coefficients. Using the Kronecker delta function in the field expansion, it is considered that the field and source points lie in the arbitrary layers. Afterward, recurrence relations to calculate the unknown expansion coefficients are derived by applying the impedance boundary condition at the interface of a graphene sheet surrounded by two adjacent dielectric layers. The verification of the calculated coefficients is conducted by using them in the analysis of graphene-based structures with different numbers of layers, including (1) free-standing frequency-selective surfaces and (2) parallel plates with graphene walls. A potential application of our proposed structure is investigating the interaction of donor–acceptor pairs residing in the arbitrary layers of the graphene–dielectric stack with a desired number of layers.

Transmission properties of metal mesh filters at 90 GHz

Freestanding frequency-selective surfaces with cross-shaped aperture elements are investigated numerically and experimentally. Transmission characteristics of such two-dimensional periodic structures as functions of geometrical sizes are analyzed using the 3D finite element method. These structures find application in modern microwave and terahertz engineering as band-pass metal mesh filters. Utilizing the conjugate gradient method narrowband, 90 GHz filters have been developed and optimized numerically. The results of measurements carried out in the present study agree well with the obtained theoretical data. It is shown that some parameters of such metal mesh filters can be improved by the proper selection of structure periodicity. A single-layer filter with a fractional bandwidth of 5.3% and an insertion loss of − 0.5 dB at the central frequency is demonstrated.

Frequency Selective Surfaces With Multiple-Strip Group Elements

In this letter, a new group of elements of double and triple strips are proposed. Instead of a single perfectly conducting strip, a group of multiple strips constitutes a unit cell. In this case, the group is periodic, not the individual elements. The fraction of electromagnetic power reflected (reflectance) and transmitted (transmittance) of a new type of freestanding frequency selective surface (FSS), made of the group of elements per periodic cell, is examined theoretically for transverse magnetic (TM) polarization. The electric field integral equation (EFIE) technique with the subdomain overlapping piecewise sinusoidal (PWS) basis functions are employed to compute the unknown current coefficients induced on the metallic elements. It is observed that the spectral characteristic of single-strip FSS is enhanced by using periodic group cells. There exist transmission bands that allow full energy transfer for unequal lengths of double and triple strips. Double and triple resonances appeared in the event of unequal strip lengths. Since it is observed multiple resonances, the structure can be used in multiband antenna applications and subreflector systems. The computed results of strip FSS are compatible with the measured and calculated results in literature.

Analysis of a Freestanding Frequency Selective Surface Loaded with a Nonlinear Element

In this paper, the response of a frequency selective surface (FSS) loaded with a nonlinear element is analyzed with the help of the harmonic balance method. The linear section of this configuration is composed of a freestanding FSS while its nonlinear section is a p-n diode biased with a DC voltage source. The method of moments is used to determine the impedance of the linear section at the fundamental frequency and its harmonics. The nonlinear section is modeled using the voltage-current relation of the diode. No simplifying assumption is made such that the analysis can be used to determine the steady state response of the nonlinearly loaded FSS for both large-signal and small-signal excitation.

Perturbed frequency selective surfaces for multiband high impedance surfaces

Multiresonant high impedance surfaces (HIS) without grounding vias that perform as artificial magnetic conductors (AMC) in multiple frequency bands and furthermore exhibit electromagnetic band gaps (EBG) in the same bands are presented. This is achieved by introducing perturbed frequency selective surface (FSS) arrays printed on grounded dielectric substrates. Arrays of linear dipoles are employed as an example. Perturbations are introduced by means of reducing the length of every other array element. Starting from the characteristics of a perturbed free-standing FSS, the authors present the effect of the perturbation on the excited currents and on the reflection properties of a corresponding AMC. Conclusions about the performance limitations are derived. Subsequently, a parametric study on practical HIS is presented and an optimised design with dual-band AMC and EBG response is demonstrated. Method of moments-based software has been developed and utilised for the fast and accurate analysis of such arrays. Experimental results validate the performance of the optimised structure.

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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> 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.

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.

Ohmic loss in frequency-selective surfaces

The present study was undertaken in order to quantify absorption effects due to ohmic loss in frequency-selective surfaces (FSS) at infrared frequencies. The structures considered in this work act as electromagnetic filters, and as such, are of interest for use as thermophotovoltaic spectral control devices. For this application, absorption is of primary concern since it leads to reduced filter efficiency. This work focuses on the behavior of single-layer, free-standing FSS arrays comprised of circular apertures (holes) and circular loop apertures (rings). Numerical calculations of the transmission, reflection, and absorption characteristics of various arrays were carried out for wavelengths between 1 and 15 μm using a commercial finite-element software package. Absorption effects were included using measured optical properties as input parameters to a surface impedance boundary condition. Analytical techniques were then employed to determine the absorption behavior in the static limit. An interesting result of this analysis is the fact that absorption at 15 μm is predicted by the finite-element calculations to increase versus aperture radius (for fixed aperture separation), while in the static limit absorption was found (through the use of analytical techniques) to decrease. The results of the finite-element calculations were confirmed using another numerical method based on the periodic moment method. We are thus led to the conclusion that the qualitative relationship for absorption versus aperture radius must reverse for some frequency regime yet to be investigated.