Amir Khurrum Rashid

A Novel Band-Reject Frequency Selective Surface With Pseudo-Elliptic Response

A novel frequency selective surface (FSS) exhibiting pseudo-elliptic band-reject response is presented. The proposed FSS consists of a two-dimensional periodic array of microstrip lines. For an incident wave of linear polarization perpendicular to the printed lines, virtual magnetic walls are formed between the microstrip lines. Since a microstrip line shielded with magnetic side walls supports two quasi-TEM modes, the two-dimensional array effectively forms a dual-mode resonator. The elliptic response is thus realized by coupling the incident wave to both modes of the resonator. A wideband pseudo-elliptic response is achieved by including higher-order modes of the array. To analyze this FSS, we treat it as a cascaded junction of air-to-microstrip line discontinuities, and investigate the problem using the full-wave mode-matching method. We provide useful design guidelines for both narrow-band and wide-band FSSs, while explaining the effect of various geometrical parameters on the performance of this FSS. Narrow-band and wideband FSS prototypes are fabricated and measured results validate our design concept. This new class of FSS is easy-to-assemble, exhibits a superior performance, and is thus potentially useful for many practical applications.

An Elliptical Bandpass Frequency Selective Structure Based on Microstrip Lines

A high-performance bandpass frequency selective structure (FSS) is presented, which consists of a 2-D periodic array of shielded microstrip lines. It exhibits stable elliptical filtering response under large variation of the angle of incidence. Its operating principle and design guidelines are discussed through equivalent circuit models. An efficient full-wave mode-matching method has been developed to analyze the proposed FSS. The proposed structure provides a simple and practically feasible solution to high-performance FSS since it can be fabricated using readily available microstrip line technology.

Wideband Microwave Absorber Based on a Two-Dimensional Periodic Array of Microstrip Lines

The modeling and design is presented of a new kind of circuit analogue absorber, which intercepts the electromagnetic waves through a two-dimensional periodic array of microstrip lines loaded with lumped circuit elements. For a plane wave incidence of polarization perpendicular to the strips, virtual magnetic walls are formed between the strips, and the geometry can be divided into many identical unit-cells. We first study the propagation characteristics of the unit-cell using the singular integral equation method. An RC network is then proposed to match this array with the free space and to dissipate the intercepted energy over a wide frequency range. The complete design procedure is explained through a design example that exhibits a bandwidth of 113% while the absorber thickness is less than 10% of the free space wavelength at the lowest operating frequency.

Three-dimensional frequency selective surfaces

A large number of different frequency selective surface (FSS) designs have been proposed over the past four decades. Most of these designs are actually derived from the well-known traditional configuration that consists of a cascaded combination of two-dimensional (2-D) FSS and dielectric layers. Recently, an interesting concept of three-dimensional (3-D) FSS has been demonstrated to obtain superior filtering response. This 3-D FSS comprises a 2-D periodic array of multimode cavities whose coupling with air can be controlled to obtain a desired frequency response. The new approach results in an elliptical filtering performance, which was rarely found in traditional FSSs. Based on their superior features and distinct configuration, we classify them as a separate class of high performance FSS, and highlight their notable characteristics in this paper. We also present a review of the current state of the art of research work related to 3-D FSSs, and suggest a few topics for future explorations.

Scattering by a Two-Dimensional Periodic Array of Vertically Placed Microstrip Lines

Scattering by a two-dimensional (2-D) array of vertically placed microstrip lines is important for a number of recently proposed interesting applications. We investigate this scattering problem using an efficient full-wave mode-matching method. Our analysis comprises two parts. First, the propagation characteristics of the 2-D array are studied by solving a unit-cell of this structure, which is a microstrip line sandwiched between two periodic boundaries. In the second part, an air-to-array discontinuity is solved using Floquet modes in the air-region. Based on our full-wave analysis, we then present two frequency selective surfaces using this array, which exhibit stable quasi-elliptic filtering response under a large variation of the angle of incidence. Results of our full-wave analysis are in excellent agreement with those obtained from measurements and CST Microwave Studio.

Bandpass frequency selective surface based on a two-dimensional periodic array of shielded microstrip lines

Frequency selective surface (FSS) is widely used in antenna sub-reflctors, radomes, and related applications [1]. Traditional two-dimensional (2-D) FSS is known to suffer from poor filtering selectivity. A cascade of multiple 2-D layers results in an improved filtering response. However, practical restriction on the thickness of this cascaded structure limits its performance. Furthermore, in most of such cases, the filtering response follows a Butterworth or Chebyshev function where it is difficult to obtain sharp rejection skirts.

On the inverse relationship between quality factor and bandwidth of small antennas

Relationship between the reflection coefficient bandwidth and quality factor (Q) of a small antenna is revisited using a simple frequency independent model. A tuned small antenna is considered in this case, which is modeled as either a series or a shunt RLC network. When the antenna resistance is matched with the characteristic impedance of the feed-line, both configurations lead to an identical relation, which is also interestingly identical to the well-known inverse bandwidth-Q formula for small antennas. However, this well-known formula is only applicable when the antenna is perfectly matched to its feed line. In this paper, a more general and yet simple form of this expression is derived, which can encompass the effect of mismatch between the antenna and its feed-line. Furthermore, we derive separate relations for the bandwidth-Q product, for both the loaded and unloaded cases. As an illustrative example, a microstrip patch antenna is considered to validate the results.

Design of Dual-Polarized Frequency Selective Structure With Quasi-Elliptic Bandpass Response

This letter is concerned with the design of a dual-polarized frequency selective structure (FSS) consisting of a two-dimensional periodic array of dielectric-filled concentric waveguides. The operating principle and design guidelines of this dual-polarized structure are explained. A design example is provided, which shows a good agreement between simulated and measured results. It is demonstrated that the proposed FSS exhibits quasi-elliptical frequency response under dual polarizations, and its filtering performance is insensitive to the incident angle of incoming plane waves.

On the optimum design of a single-layer thin wideband radar absorber

This paper considers the four most common equivalent network models of a single-layer circuit-analog absorber, and demonstrates that the series RLC configuration performs superior to the others. With proper selection of the component values, this configuration can realize two resonances, which lead to an overall wideband absorption. Based on this finding, we derive closed-form design expressions for an optimally thin and wideband single-layer absorber. To demonstrate the superiority of the series RLC absorber, a design example has been provided where 10-dB bandwidth of 128% is obtained through a single-layer absorber whose thickness is less than λ/11 at the lowest frequency of absorption.

Dual-polarized bandpass frequency selective structure with quasi-elliptic response

This paper presents a new design of frequency selective structure based on a two-dimensional periodic array of hollow square coaxial waveguides, which exhibits a narrow bandpass response under dual polarizations. The design is insensitive to different polarizations due to its structural symmetry and produces a stable quasi-elliptic filtering response under different incident angles. Extensive simulation results are given to examine the effects of various structural parameters on its reflection and transmission coefficients.