Negative-refractive-index Transmission-line Research Articles

Reconfigurable inductorless negative‐refractive‐index transmission‐line metamaterial phase shifter for 5G antenna beam‐steering applications

AbstractA reconfigurable inductorless negative‐refractive‐index transmission‐line (NRI‐TL) metamaterial phase shifter for sub‐6 GHz 5G antenna applications is proposed. The design consists of a microstrip transmission line loaded both in series and in parallel with varactor diodes to provide continuous tuning of the phase of the transmitted signal by varying the bias voltage applied to the varactors, without the need for variable inductor components. The total measured differential phase shift is 190° at 3.2 GHz, with a measured insertion loss of 1.5–2.7 dB in the frequency range of 3.2–3.6 GHz, when the biasing voltage of the varactors is varied between 0 and 7 V. The total size of the microstrip NRI‐TL phase shifter is 19.4 mm × 24 mm. The proposed reconfigurable phase shifter has been incorporated into the feeding network of a two‐element antenna array to demonstrate its use in beam‐steering applications. The measured results for the reconfigurable array show good impedance matching between 3 and 3.35 GHz for all biasing values, allowing continuous beam steering over a scanning range of 60°.

A NRI-TL Metamaterial Leaky-Wave Antenna Radiating at Broadside With Zero Beam-Squinting

An implementation of a planar fixed-beam leaky-wave antenna that radiates at broadside is presented, consisting of a periodic arrangement of negative-refractive-index transmission-line (NRI-TL) metamaterial unit cells. Using an experimental prototype of the antenna, it is shown that each branch of the antenna radiates a beam in opposite directions, and the combination of the two beams results in a symmetric broadside beam over a wide measured zero-beam-squinting bandwidth of 1.1 GHz (23%) around the design frequency of 5 GHz. This is well within the 80% measured −10 dB reflection coefficient bandwith. Furthermore, the antenna has a size of 3.42λ0 × 0.125λ0× 0.025λ0 at 5 GHz.

A compact microstrip crossover using NRI‐TL metamaterial lines

AbstractA practical implementation of a miniaturized crossover is presented using negative‐refractive‐index transmission‐line (NRI‐TL) metamaterial lines. NRI‐TL metamaterial theory has demonstrated that by loading a conventional transmission line with lumped LC elements, a large phase shift per‐unit‐length can be achieved. Employing this technique, the conventional microstrip transmission lines in a three‐section branch‐line coupler crossover are replaced with 90° NRI‐TL lines, and thus a compact NRI‐TL crossover with an area 47 times smaller than the area of its conventional counterpart is demonstrated. Measured results demonstrate that the proposed NRI‐TL metamaterial crossover exhibits a −10 dB |S11| bandwidth of 33% and a |3 dB| |S31| bandwidth of 26% around 1 GHz.

A Compact Reconfigurable NRI-TL Metamaterial Phase Shifter for Antenna Applications

This communication presents a compact reconfigurable phase shifter based on negative-refractive-index transmission-line (NRI-TL) metamaterial unit cells. Two interswitchable NRI-TL metamaterial unit cells are collocated within the same board area, and can be reconfigured based on the biasing polarity of embedded p-i-n diodes to provide two discrete phase states. The p-i-n diodes are located on the shunt branches of the metamaterial line in order to reduce losses in the direct signal path. Design limitations in terms of return loss and insertion loss are discussed in relation to the two phase states. A proof-of-concept module with a size of $17.8 \times 21.6~ \text {mm}^{2}$ is designed at 1 GHz, with measured insertion phases of −14.5° and +58.5° in the two phase states leading to a differential phase shift of 73°, while exhibiting insertion losses of 1.43 and 0.89 dB, respectively. The combined bandwidth for which the reflection coefficient in both states remains simultaneously below −10 dB is 0.46 GHz (46%), from 0.83 to 1.29 GHz.

A Compact Multiband Printed Dipole Antenna Loaded With Two Unequal Parallel NRI-TL Metamaterial Unit Cells

A negative-refractive-index transmission-line (NRI-TL) metamaterial-loaded dipole antenna is proposed, which has a fully printed configuration and exhibits multiband performance. The antenna consists of a conventional dipole antenna loaded with two unequal parallel NRI-TL metamaterial unit cells which have the same electrical length, but each is loaded with a thin inductive strip that has a different length and therefore a different effective shunt inductance. An equivalent circuit is extracted to investigate the performance of the proposed metamaterial-loaded dipole antenna by modeling each of the dipole arms as a transmission line. This circuit is then used to verify that the resonant behavior of the proposed antenna can be altered by simply adjusting the lengths of the thin inductive strips. A fabricated prototype has compact dimensions of $40 \mathrm{mm} \times 10 \mathrm{mm}\times 1.6 \mathrm{mm}$ , and exhibits good agreement between the measured and simulated results.

Electron Beam Coupling to an NRI Transmission-Line Metamaterial

A backward-wave amplifier topology based on a coaxial negative refractive index transmission-line (NRI-TL) metamaterial is introduced. The dispersion relation of the isolated coaxial metamaterial is derived rigorously using a mode-matching technique. The structure is then homogenized to derive its effective material parameters. The electron beam is subsequently introduced by means of an anisotropic, spatially dispersive permittivity. With the electron beam present, the homogenized metamaterial is analyzed to obtain a coupled-mode dispersion relation that accounts for electron beam–metamaterial interaction. Solutions to this dispersion relation yield complex coupled modes. The analytical solutions are compared with full-wave simulation of the NRI-TL with electron beam present, and show close agreement.

New π –T generalised metamaterial negative refractive index transmission line for a compact coplanar waveguide triple band pass filter applications

This study presents, for the first time, a novel π − T generalised negative refractive index (NRI) transmission line (TL) configuration. It is innovated for increasing the possible passbands in designing microwave components. It is proved that the proposed transmission line can introduce up to seven passbands for completely unbalanced transmission line condition. The detailed mathematical analysis of that configuration based on the periodic network analysis and equivalent circuit model is introduced. As an application for the novel π − T generalised NRI TL configuration, a new compact triple band coplanar waveguide (CPW) metamaterial filter is introduced. The proposed triple bands CPW filter design steps are introduced and its performance is validated using equivalent circuit model simulation, electromagnetic full wave simulation and experimental measurements. Good agreements between the circuit modelling, electromagnetic full wave simulation and practical measurement results are achieved. All the results confirm that the proposed filter has triple passbands from 900 to 960 MHz, 1.4 to 1.48 GHz and 2.4 to 2.45 GHz. Moreover the filter size is compact whose length is only 12.1% λg at first passbands, 18.2 λg % at second passbands and 31.7 λg % at third passbands.

Leaky-Wave Antennas Using Negative-Refractive-Index Transmission-Line Metamaterial Supercells

Leaky-wave antennas (LWAs) are designed based on supercells which are expanded versions of conventional negative-refractive-index transmission line (NRI-TL) unit cells. In the proposed LWAs with NRI-TL supercells, the number and values of inductors are reduced by half, decreasing the complexity of their design and reducing the Ohmic loss, which increases the antenna gain and efficiency. Compared with conventional NRI-TL unit cells, NRI-TL supercells operate over narrower bandwidth while showing proper Bloch impedance behavior over both left-hand (LH) and right-hand (RH) regions, leading to broadband impedance matching. Using coplanar waveguide (CPW) technology, closed-stopband LWAs based on NRI-TL supercells are designed, and their performance compared with that of the conventional unit cell LWAs. Finally, by using CPW supercells with a backing ground and vias, high-gain LWAs with sharp beams and suppressed backlobes are designed and realized.

Multiband Compact Printed Dipole Antennas Using NRI-TL Metamaterial Loading

A compact, fully-printed negative-refractive-index transmission-line (NRI-TL) metamaterial-loaded dipole antenna is proposed, which exhibits multiband resonant characteristics that are not harmonically related. The multiband behavior is achieved by loading a host dipole antenna with series capacitive gaps and shunt inductive strips, to form a two unit-cell NRI-TL loaded antenna. By modeling the right-handed dipole sections as biconical transmission lines, the rich dispersion properties of the NRI-TL structure can be exploited, while still creating an effective radiator. An equivalent circuit model of the NRI-TL antenna is developed, and it is shown that its performance agrees well with the results obtained using full-wave simulations. It is also shown that the resonant behavior of the antenna can be adjusted by changing its geometrical parameters, which in turn tune the loading parameters of each NRI-TL unit cell. As such, both dual-band and triband versions of the antenna are designed and fabricated. The dual-band version exhibits three distinct resonances at 1.15, 2.88, and 3.72 GHz, with measured -10 dB bandwidths of 37 and 1150 MHz. The lowest resonance at 1.15 GHz corresponds to a 47% reduction in the resonant frequency compared to that of a reference unloaded dipole antenna, which translates into a size miniaturization factor of approximately two. The antenna exhibits stable dipolar radiation patterns with a linear electric field polarization, and has a total size of 50 × 10 × 0.79 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> or 0.19λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.04λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.003λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at 1.15 GHz. Throughout the operating bands the measured gain and radiation efficiency vary from 0.11 to 3.26 dBi and 49.5% to 95.6%, respectively.

Arbitrary dual-band unequal Wilkinson power divider based on negative refractive index transmission lines

This paper focused on the application of negative refractive index transmission line (NRI-TL) in dual-band unequal Wilkinson power divider (WPD) with controllable frequency and power dividing ratio. Theory and design procedure of the dual-band NRI-TL are presented in details. For demonstration, two dual-band unequal Wilkinson power dividers (WPDs) with power dividing ratio of 2:1 and operating frequencies of 0.9 and 1.8GHz are designed, fabricated and tested. The first unequal divider is based on 2-stage NRI-TLs and the second one is based on 4-stage NRI-TLs. In addition, these two types of NRI-TLs are presented to demonstrate that by increasing the number of NRI-TL unit-cells the phase response of the NRI-TLs converge to the desired characteristic. The good agreement between measured and simulated results confirmed the design concept and derived closed-form design equations. Measurements show that the first divider has 18.37% and 21.86% relative bandwidths and the second one has 33.52% and 29.12% relative bandwidths at 0.9 and 1.8GHz, respectively. The design concept of this paper can be extended to equal dual-band power dividers with arbitrary frequency ratio.

Theoretical and experimental study of the backward-wave radiation using resonant-type metamaterial transmission lines

We investigate theoretically and experimentally the backward radiation of a type of negative-refractive-index (NRI) resonant-type metamaterial transmission lines (TL). The NRI TL goes beyond the bulk metamtaterials made of original split-ring resonator/wire and the NRI TLs using inter-digital capacitors and shorted stub inductors. The operation mechanism is investigated in depth behind a general composite right/left handed concept based on a TL theory and is interpreted from the perspective of field distributions, showing that the antiparallel phase and group velocities give rise to the full-space beam scanning peppery from backward, through broadside and finally to forward as frequency increases. In addition, we discuss the possible realizations by using complementary split ring resonators and other electrically smaller fractal-shaped resonators with different configurations. The fractal strategy certainly fulfills the challenging necessity of drastically increasing left handed capacitances and inductances in the homogenization process. In all cases, the resultant radiation is analogous to the reversed Cherenkov radiation. Moreover, the comprehensive theory established for a generic type of TL metamaterial paves the way for a round of antenna design, which is of a reference value.

Design of a printed dual-band coupled-line coupler with generalised negative-refractive index transmission lines

A dual–band printed metamaterial coupler made of a microstrip line and a generalised negative-refractive-index (NRI) line is presented. Owing to the presence of two left-handed bands in the generalised negative-refractive-index transmission line (NRI-TL) unit cell, backward coupling occurs over two frequency ranges centred on 2.7 and 4.7 GHz. Measured results for an edge-coupled device are reported; peak coupling levels of −3.5 and −4.4 dB are observed.

Effective-Medium Properties of Cylindrical Transmission-Line Metamaterials

This letter presents a cylindrical negative-refractive-index transmission-line (NRI-TL) metamaterial composed of radially oriented NRI-TL layers. This arrangement introduces radially inhomogeneous and anisotropic effective permeability and permittivity, whose profiles may be extracted from full-wave simulations using a discrete multilayer approximation. It is shown that a cylindrical NRI-TL metamaterial of single-unit-cell thickness may be described as a homogeneous, anisotropic effective medium exhibiting resonances akin to those observed in isotropic cylindrical structures, which may be used to enhance the radiated power of nearby antennas. An analytical formulation describing the near fields, radiation patterns, and power-ratio enhancement displays very good agreement with full-wave simulations.

Generalized negative-refractive index transmission line based on defected ground structure

The planar generalized negative-refractive index transmission line (G-NRI TL) is proposed using only distributed structures, such as defected ground structure, stub with rectangular patch, wire-bonded interdigital capacitor, and short stub.The characteristics, such as dispersion relation and frequency response of the G-NRI TL are analyzed by circuit analysis, full wave simulation, and measurement. The results show a good agreement. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2223–2225, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25467

Spatial Harmonics and Homogenization of Negative-Refractive-Index Transmission-Line Structures

In this paper, we discuss negative-refractive-index transmission-line (NRI-TL) metamaterial structures with respect to spatial harmonics. The power in the fundamental spatial harmonic is shown to yield a measure that relates to the homogeneity of the metamaterial. It is shown that NRI-TL metamaterials yield highly homogeneous field distributions if both the unit cell is short compared to the guided wavelength, and if the unloaded unit cell is short compared to the “free space” wavelength. Based on the homogeneity, we provide practical design choices for NRI-TL metamaterial unit cells. Under the restriction of a high homogeneity we derive expressions for extracting an effective permittivity and permeability. Circuit theory results and full-wave eigenmode simulations of two NRI-TL structures are presented.

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.

A Dual-Band Bandpass Filter Based on Generalized Negative-Refractive-Index Transmission-Lines

A dual-band bandpass filter is presented. The filter is composed of generalized negative-refractive-index transmission-lines (NRI-TL) realized in microstrip. The filtering mechanism relies on all four of the generalized NRI-TLs' left-handed and right-handed passbands. Under the closed stopband condition, two passbands are achieved. A complete design procedure is described. A fabricated example measures 0.25lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> 0.25lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , where omega <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> is the wavelength of the center frequency of the stopband. Maximum insertion loss in the passbands (centered around 4 GHz) is measured at 1.5 dB.

A Broadband Dual-Mode Monopole Antenna Using NRI-TL Metamaterial Loading

A printed monopole antenna is proposed which uses negative-refractive-index transmission-line (NRI-TL) metamaterial loading in order to achieve a broadband dual-mode operation. The metamaterial-loaded monopole supports a predominately even-mode current at 5.5 GHz, which allows the antenna to be modeled as a short folded monopole. Around 3.55 GHz, the metamaterial-loaded monopole acts as a balun for the ground plane currents, therefore rendering the entire top edge of the ground plane as the main radiating element. This in turn radiates a dipolar mode that is orthogonal to the folded-monopole mode at 5.5 GHz. By virtue of the orthogonality between the two radiating modes, the metamaterial antenna exhibits a return-loss characteristic with a dual resonance, and therefore a very wide measured impedance bandwidth of 4.06 GHz. The total size of the antenna is only 22 times 30 mm, and the measured efficiency is on the order of 90% at both 3.55 and 5.5 GHz.

Negative-Refractive-Index Transmission Lines With Expanded Unit Cells

A new ldquoexpandedrdquo negative-refractive-index transmission line (NRI-TL) metamaterial unit cell is introduced. This expanded unit cell minimizes the number and reduces the size of the shunt inductors needed to achieve a certain phase-shift per unit length. As in standard NRI transmission lines, both backward and forward bands are supported, with the possibility of closing the stopband between these two bands. The reduction of the shunt inductors is especially advantageous for microstrip realizations. However, this advantage comes at a tradeoff with bandwidth. An example structure is designed and fabricated in microstrip and the measured results show good agreement with simulations.

New transmission lines of negative refractive index based on periodically loaded negative-impedance-converted inductors and capacitors

A new class of negative refractive index (NRI) transmission line is proposed. Differential operational amplifiers are used to form the required lumped elements, i.e., negative-impedance-converted (NIC) inductors and capacitors. When the gain of the differential operational amplifiers in the NIC circuits is very large over the entire frequency band, the equivalent material parameters are non-dispersive, and the phase response of the new NRI transmission line can be complimentary to that of a conventional transmission line over the entire frequency band. As an application example, a broadband 1:4 power divider based on the proposed NRI transmission line is studied.