Omar Siddiqui

Periodically loaded transmission line with effective negative refractive index and negative group velocity

We present the design and implementation of a periodically loaded transmission line, which simultaneously exhibits negative refractive index (NRI) and negative group delay (and, hence, negative group velocity). This is achieved by loading the transmission line in series with capacitors and RLC resonators and in shunt with inductors. We discuss the dispersion characteristics of such a medium and identify the frequency bands of NRI and negative group delay. The structures are theoretically studied using S-parameters simulations on truncated loaded transmission lines of different lengths, and the predicted results are compared to the measured scattering parameters of such lines printed on circuit boards using coplanar waveguide technology.

Transmission line models for negative refractive index media and associated implementations without excess resonators

Recently, three-dimensional composite periodic media comprising split-ring resonators (SRR) and thin wires have been shown to exhibit a negative refractive index in the frequency range around the SRR resonance. In this letter, we propose transmission line models for studying and interpreting the electromagnetic propagation behavior of such materials. Based on these equivalent transmission line models, we show that by periodically loading a network of transmission lines with series capacitors and shunt inductors, a negative refractive index medium can be synthesized without excess resonators, thus leading to wideband behavior. These proposed media have tailorable properties over a broad frequency range. Moreover, they are completely planar, frequency scalable, more compact, and easier to implement for RF/microwave circuit applications than their SRR/wire counterparts.

Mutual Coupling Reduction Between Microstrip Patch Antennas Using Slotted-Complementary Split-Ring Resonators

A novel structure based on complementary split-ring resonators (SRRs) is introduced to reduce the mutual coupling between two coplanar microstrip antennas that radiate in the same frequency band. The new unit cell consists of two complementary SRR inclusions connected by an additional slot. This modification improves the rejection response in terms of bandwidth and suppression. The filtering characteristics of the band-gap structure are investigated using dispersion analysis. Using the new structure, it was possible to achieve a 10-dB reduction in the mutual coupling between two patch antennas with a separation of only 1/4 free-space wavelength between them. Since the proposed structures are broadband, they can be used to minimize coupling and co-channel interference in multiband antennas.

Time-domain measurement of negative group delay in negative-refractive-index transmission-line metamaterials

We have simulated and constructed a one-dimensional metamaterial composed of a periodically loaded transmission line that exhibits both negative and positive group velocities in a band of effective negative index of refraction. The negative group velocity or, equivalently, the negative group delay, is demonstrated theoretically and experimentally in the time domain using modulated Gaussian pulses. Due to this negative delay, we can show an output pulse peak emerging from the loaded transmission line prior to the input peak entering the line, i.e., the output pulse precedes the input pulse. The fact that this surprising behavior does not violate the requirements of relativistic causality is illustrated with time-domain simulations, which show that discontinuities in the pulse waveforms are traveling at exactly the speed of light in vacuum. The pulse-reshaping mechanism underlying this behavior is also illustrated using time-domain simulations.

Negative refraction and focusing in hyperbolic transmission-line periodic grids

We propose a class of anisotropic periodic structures with spatial hyperbolic dispersion characteristics, synthesized by arranging unloaded transmission lines (TLs) in a two-dimensional planar grid. These planar periodic grids support the formation of sharp beams, called resonance cones. Analogous resonance-cone effects are observed in highly anisotropic resonant plasmas. By interfacing transposed versions of these anisotropic periodic grids, negative refraction and focusing of resonance cones can be achieved. Simulation results are presented for ideal TL grids and are verified experimentally using planar microstrip grids. These grids are easy to fabricate at a low cost. Moreover, since the TLs are unloaded and the periodicity is comparable to the wavelength, the structures are scalable from microwave to millimeter-wave frequencies and could be utilized for spatial filtering, multiplexing, and demultiplexing.

Resonant modes in continuous metallic grids over ground and related spatial-filtering applications

We use the theory of periodic structures, full-wave electromagnetic, and microwave circuit simulations to explain the resonant modes that propagate in metallic grids having rectangular unit cells constructed over a ground plane. We show that these metallic grids can support two types of resonant modes that have rectangular and hyperbolic isofrequency dispersion contours. By exploiting the spatial dispersion properties of these modes, a microwave 3GHz∕6GHz harmonic splitter and a highly selective 5.8GHz∕6.2GHz diplexer are designed and simulated. Furthermore, we provide experimental results for the diplexer and for the harmonic splitter, synthesized in microstrip technology. The proposed metallic grids utilize continuous unloaded transmission-line segments thus leading to spatial-filtering devices that are easy to fabricate and are scalable to terahertz frequencies and beyond.

Metamaterial for gain enhancement of printed antennas: Theory, measurements and optimization

Metamaterials have been shown to enhance specific performance parameters of low profile and high-profile antennas. Our focus in this paper on specifically increasing the gain of low-profile antennas and in particular the microstrip patch antenna. By placing a metamaterial slab above a microstrip patch antenna (as a superstrate), we show that the gain of the antenna can be enhanced appreciably. The key advantage of using the superstrate is to maintain the low-profile advantage of microstrip patch antennas. In previous works, different types of superstrates were proposed to enhance the gain of microstrip antennas, however, to the best of our knowledge, no theory was developed to understand the mechanism behind the enhancement in the gain. In this paper, we present a simple analytical formulation that provides a very accurate prediction of the gain when a superstrate is used. In fact, our analytical technique is capable of predicting the gain when a multilayer superstrate structures is used. To validate the theory of gain enhancement, antennas and superstrates using metamaterials were fabricated and tested in an echoic chamber. The metamaterials developed were based on split-ring resonators. Strong agreement was found between the measurements and full-wave simulation using commercial tools. Finally, we present optimization results to demonstrate the maximum gain enhancement potential that can be achieved when superstrates are used.

Resonance-cone focusing in a compensating bilayer of continuous hyperbolic microstrip grids

We propose a periodic structure synthesized by arranging continuous transmission-line segments in a two-dimensional planar grid over a ground plane. The periodic grid exhibits hyperbolic spatial dispersion characteristics, leading to the formation of highly directive beams called “resonance cones.” Simulations and experiments at 10GHz show negative refraction and frequency-dependent spatial focusing of the resonance cones, when two such grids with compensating phase properties are interfaced to form a bilayer. The proposed structures are deprived of loading elements or vias, thus leading to ease of fabrication and scalability with frequency for applications, such as spatial filters, multiplexers, and demultiplexers.

Frequency-Selective Energy Tunneling in Wire-Loaded Narrow Waveguide Channels

Frequency-dependent energy tunneling that results in full transmission of electromagnetic energy through wire-loaded sharp waveguide bends is demonstrated by full-wave simulations. The frequencies at which the tunneling takes place is predicted by a numerical method that involves a variational impedance formula based on Green function of a probe-excited parallel plate waveguide. Analogous tunneling efiects have also been previously observed in waveguide bends fllled with epsilon-near-zero media. However, since the frequency response in the wire-loaded waveguides can be tailored by simply modifying the lengths of the wires, the phenomenon is scalable over a broad range of frequencies and can be potentially exploited in various flltering and multiplexing applications.

Study of resonance-cone propagation in truncated hyperbolic metamaterial grids using transmission-line matrix simulations

Abstract In this paper, we utilize transmission-line matrix simulations to study the electromagnetic properties of hyperbolic metamaterial grids, which are synthesized by arranging unloaded transmission lines over electrical ground planes, such that the unit cells are rectangular in shape. The hyperbolic grids support Bragg and Hyperbolic resonant modes that have rectangular and hyperbolic dispersion surfaces, respectively. We demonstrate the formation of these resonant modes in the hyperbolic grids and their refraction, and spatial scanning properties via circuit simulations. The simulation results are compared with experimental results of a two-layer anisotropic hyperbolic metamaterial.