Rubaiyat Islam

A planar metamaterial co-directional coupler that couples power backwards

A metamaterial coupler made of a regular microstrip line and a negative-refractive-index (NRI) line is examined. Peculiar coupling effects are observed in which co-directional traveling waves result in contra-directional Poynting vectors on the lines thus leading to power being coupled backwards. This coupler demonstrates better performance in terms of coupled power and port isolation, but at a slightly reduced bandwidth, when compared to a commensurate regular microstrip coupled-line coupler.

Phase-agile branch-line couplers using metamaterial lines

Two branch-line couplers are introduced that utilize a combination of regular microstrip (MS) and negative-refractive-index (NRI) lines. Interesting phase compensation (0/spl deg/ phase shift) at the output port and a choice of /spl plusmn/90/spl deg/ phase shift at the through port, with respect to the input, are possible. On the other hand, one of the dimensions of the coupler is significantly reduced, compared to a corresponding conventional branch-line coupler, but without any bandwidth degradation.

Printed high-directivity metamaterial MS/NRI coupled-line coupler for signal monitoring applications

Using the general coupled-mode formulation, it is shown that the interaction between a regular transmission line and a negative-refractive-index (NRI) line leads to complex coupled modes. Specifically the coupling between a regular microstrip (MS) and a NRI line is analyzed and the theory reveals conditions giving rise to high directivity. Experimental results are presented for a fully printed MS/NRI coupled-line coupler exhibiting a coupling level of -27dB, isolation of -72dB, and directivity of 45dB at the design frequency of 2.04GHz. Such low-coupling levels but high-directivity couplers are well suited for signal monitoring and instrumentation applications.

On the Independence of the Excitation of Complex Modes in Isotropic Structures

We investigate the independence of the excitation of complex modes in lossless and isotropic structures. We clarify some of the ideas in literature regarding the excitation of complex mode pairs. In particular, we show in this work that it is practically possible to independently control the excitation amplitude of an individual mode in a pair of conjugate complex modes. Theoretical clarification of the claim that such modes in a complex pair must be excited with equal amplitudes, as suggested by some authors, is also provided. The eigenmodes of a coupler comprising a regular forward-wave and a metamaterial backward-wave or negative-refractive-index transmission-line (NRI-TL) provides a representative platform to investigate our assertions which are verified using a commercial microwave circuit simulator.

An ultra-short contra-directional coupler utilizing surface plasmon-polaritons at optical frequencies.

A nano-scaled coupled-line coupler based on the guidance of surface plasmon-polaritons (SPPs) is proposed, designed and simulated at optical frequencies. The coupler comprises layered dielectric materials and silver, which serve as two stacked nano-transmission lines to achieve broadside coupling. The key property of this coupler is that it operates based on the principle of contra-directional coupling between a forward and a backward wave giving rise to supermodes that are characterized by complex-conjugate eigenvalues (even when the materials are assumed lossless). The resulting exponential attenuation along the coupler leads to dramatically reduced coupling lengths compared to previously reported co-directional SPP couplers (e.g. from millimeters to submicrons). The effect of material losses and finite coupler width are also analyzed.

Modal Analysis and Wave Propagation in Finite 2D Transmission-Line Metamaterials

In this paper we examine the propagation of plane waves and Gaussian beams in 2D periodic grids constructed with lumped reactive immitances. We demonstrate the equivalence between the Bloch/Floquet modal description and the multiple coupled-line modal analysis of finite-sized periodic grids. This establishes that the Floquet analysis of 2D metamaterial negative-refractive-index grids and the associated exotic wave propagation/refraction/focusing phenomena, can be deduced from traditional analysis of M coupled lines (and hence M eigenmodes), albeit with considerable algebraic complexity. We present simulation results of Gaussian beams through these grids to demonstrate that they can be analyzed using simple geometrical optics along with the index of refraction associated with Bloch/Floquet analysis.

Beam-forming by mutual coupling effects of parasitic elements in antenna arrays

An innovative method of beamforming is described using mutual coupling effects of parasitic elements in an antenna array. It is shown that null and mainbeam placement can be achieved by varying the reactances at the center of the dipoles.

Compact Corporate Power DividerUsing Metamaterial NRI-TLCoupled-Line Couplers

A low-loss, planar and compact metamaterial 1:4 corporate power divider design is presented. The power divider uses a cascade of three microstrip/negative-refractive-index transmission-line coupled-line couplers allowing it to be compact in the longitudinal direction. The fabricated prototype operates at 2.10 GHz and is 11.6 cm wide-designed for feeding a four-element patch antenna array. The 3 dB transmission bandwidth ranges from 0.73 GHz to 0.46 GHz for the various ports with a total insertion loss of 1.1 dB, good phase balance and better than 20 dB isolation among the output ports.

Anomalous Negative Group Velocity in Coupled Positive-Index/Negative-Index Guides Supporting Complex Modes

Anomalous negative group velocity (NGV; group velocity antiparallel to the power flow) is reported in a guided-wave structure supporting complex modes. This structure consists of a coupled-line system comprising a positive-index microstrip line edge coupled to a negative-index line. The NGV can be observed on the positive-index line under suitable excitation and termination conditions. What is remarkable about this structure is that the anomalous NGV neither requires any material losses nor any strong reflections on the observed line. This work verifies that absorption or reflection are not necessary conditions for observing NGV, rather NGV can be observed in specific lossless coupled-line structures as well. The general conditions for obtaining NGV in coupled positive-index/negative-index guides are analytically derived and corresponding NGV observations are experimentally reported at microwave frequencies. Specifically, we report the propagation of modulated pulses exhibiting a negative group delay as well as phase shifters maintaining a constant phase over a broad bandwidth. This opens up the possibility of utilizing such coupled-line guides for applications in pulse shaping, delay control, constant phase shifters (vs. frequency) and reducing beam squinting in series-fed antenna arrays.

Analysis of a finite length microstrip/negative-refractive-index coupled-line coupler

The scattering parameters of finite-length MS/NRI coupled-line couplers are derived analytically using coupled-mode theory. Harnessing this theory, a -25 dB high-directivity planar coupler is designed and tested experimentally having a directivity around 35 dB over a 100 MHz bandwidth centered at 1.75 GHz