Marco A. Antoniades

Compact linear lead/lag metamaterial phase shifters for broadband applications

A compact one-dimensional phase shifter is proposed using alternating sections of negative refractive index (NRI) metamaterials and printed transmission lines (TL). The NRI metamaterial sections consist of lumped element capacitors and inductors, arranged in a dual TL (high-pass) configuration. By adjusting the NRI-medium lumped element values, the phase shift can be tailored to a given specification. Periodic analysis is applied to the structure and design equations are presented for the determination of the lumped element parameters for any arbitrary phase shift. To validate the design, various phase shifters are simulated and tested in coplanar waveguide (CPW) technology. It is demonstrated that small variations in the NRI-medium lumped element values can produce positive, negative or 0/spl deg/ phase shifts while maintaining the same short overall length. Thus, the new phase shifter offers some significant advantages over conventional delay lines: it is more compact in size, it exhibits a linear phase response around the design frequency, it can incur a phase lead or lag which is independent of the length of the structure and it exhibits shorter group delays.

A compact and low-profile metamaterial ring antenna with vertical polarization

A compact (/spl lambda//sub 0//11 footprint) and low-profile (/spl lambda//sub 0//28 height) metamaterial ring antenna is proposed using two metamaterial unit cells. Each constituent unit cell consists of negative-refractive-index (NRI) microstrip transmission lines (TL) designed to incur a zero insertion phase at the antenna design frequency. This allows the inductive posts to ground, which act as the main radiating elements, to be fed in phase. Hence, the antenna operates as two closely spaced monopoles that are fed in phase through a compact feed network. An embedded matching network ensures good VSWR performance. The theoretical performance of the antenna is verified by full-wave simulations and experimental data obtained from a fabricated prototype at 1.77 GHz. The antenna offers a 120 MHz -10 dB bandwidth and a measured efficiency exceeding 50%.

A broadband series power divider using zero-degree metamaterial phase-shifting lines

A metamaterial 1:4 series power divider that provides equal power split to all four output ports over a large bandwidth is presented, which can be extended to an arbitrary number of output ports. The divider comprises four nonradiating metamaterial lines in series, incurring a zero insertion phase over a large bandwidth, while simultaneously maintaining a compact length of /spl lambda//sub 0//8. Compared to a series power divider employing conventional one-wavelength long meandered transmission lines to provide in-phase signals at the output ports, the metamaterial divider provides a 165% increase in the input return-loss bandwidth and a 155% and 154% increase in the through-power bandwidth to ports 3 and 4, respectively. In addition, the metamaterial divider is significantly more compact, occupying only 2.6% of the area that the transmission line divider occupies. The metamaterial and transmission line dividers exhibit comparable insertion losses.

A broadband Wilkinson balun using microstrip metamaterial lines

A metamaterial balun that converts a single-ended input to a differential output over a large bandwidth is presented. The device also exhibits excellent return loss, isolation, and through characteristics over the same frequency band. The balun comprises a Wilkinson divider, followed by a +90/spl deg/ negative-refractive-index (NRI) metamaterial (MM) phase-shifting line along the top branch, and a -90/spl deg/ MM phase-shifting line along the bottom branch. Utilizing MM lines for both the +90/spl deg/ and -90/spl deg/ branches allows the slopes of their phase responses to be matched, resulting in a broadband differential output signal. The theoretical performance of the balun is verified through circuit simulations and measurements of a fabricated prototype at 1.5 GHz. The MM balun exhibits a measured differential output phase bandwidth (180/spl deg//spl plusmn/10/spl deg/) of 1.16 GHz (77%), from 1.17 to 2.33 GHz. The measured isolation and return loss for all three ports remain below -10 dB over a bandwidth in excess of 2 GHz, while the output quantities |S/sub 21/| and |S/sub 31/| remain above -4 dB from 0.5 to 2.5 GHz.

A Compact Tri-Band Monopole Antenna With Single-Cell Metamaterial Loading

A compact tri-band planar monopole antenna is proposed that employs reactive loading and a ¿defected¿ ground-plane structure. The reactive loading of the monopole is inspired by transmission-line based metamaterials (TL-MTM), which enables the loaded antenna to operate in two modes. The first resonance exhibits a dipolar mode over the lower WiFi band of 2.40 GHz - 2.48 GHz, and the second resonance has a monopolar mode over the 5.15-5.80 GHz upper WiFi band. Full-wave analysis shows that the currents of the two modes are orthogonal to each other, resulting in orthogonal radiation patterns in the far field. The feature of a ¿defected¿ ground-plane, formed by appropriately cutting an L-shaped slot out of one of the CPW ground-planes, leads to the third resonance that covers the WiMAX band at 3.30-3.80 GHz. Air bridges at the intersection between the antenna and the CPW feedline ensure a balanced current. A fabricated prototype has compact dimensions of 20.0 mm × 23.5 mm × 1.59 mm, and exhibits good agreement between the measured and simulated S parameters and radiation patterns. The measured radiation efficiencies are 67.4% at 2.45 GHz, 86.3% at 3.50 GHz and 85.3% at 5.50 GHz.

A Folded-Monopole Model for Electrically Small NRI-TL Metamaterial Antennas

An improved model for analyzing electrically small NRI-TL antennas is proposed, that highlights the methods that enable these antennas to offer a good impedance match and a high radiation efficiency compared to previously reported designs. An even-odd mode analysis reveals that the antenna supports a predominately even-mode current on the vertical vias, allowing the antenna to be modeled using a multiple folded monopole topology, which provides a substantial increase in the radiation resistance of the antenna. This, together with the top-loading effect of the microstrip line on the vias, enables the antenna to be matched to 50 Omega without the use of an external matching network, while maintaining a high radiation efficiency. The validity of the proposed model is confirmed with a fabricated prototype, that consists of four microstrip zero-degree NRI-TL unit cells with dimensions of lambda0/10 times lambda0/10 times lambda/20 over a 0.45lambda0 times 0.45lambda0 ground plane. The antennas performance is verified by full-wave simulations and experimental data obtained at 3.1 GHz, which yield a vertical linear electric field polarization, a measured -10dB return-loss bandwidth of 53 MHz and a measured efficiency of 70%.

A Compact Multiband Monopole Antenna With a Defected Ground Plane

A compact multiband antenna is proposed that consists of a printed circular disc monopole antenna with an L-shaped slot cut out of the ground, forming a defected ground plane. Analysis of the current distribution on the antenna reveals that at low frequencies the addition of the slot creates two orthogonal current paths, which are responsible for two additional resonances in the response of the antenna. By virtue of the orthogonality of these modes the antenna exhibits orthogonal pattern diversity, while enabling the adjacent resonances to be merged, forming a wideband low-frequency response and maintaining the inherent wideband high-frequency response of the monopole. The antenna exhibits a measured -10 dB S 11 bandwidth of 600 MHz from 2.68 to 3.28 GHz, and a bandwidth of 4.84 GHz from 4.74 to 9.58 GHz, while the total size of the antenna is only 24 times 28.3 mm. The efficiency is measured using a modified Wheeler cap method and is verified using the gain comparison method to be approximately 90% at both 2.7 and 5.5 GHz.

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.

A CPS Leaky-Wave Antenna With Reduced Beam Squinting Using NRI-TL Metamaterials

A reduced beam-squinting printed leaky-wave antenna (LWA) is proposed, comprising cascaded negative-refractive-index transmission-line (NRI-TL) metamaterial unit cells. Each NRI-TL unit cell is implemented in co-planar strip (CPS) technology and consists of a host TL loaded with series interdigitated capacitors and shunt meandered inductors. Periodic analysis is applied to the NRI-TL unit cell in order to extract the dispersion and Bloch impedance characteristics. Subsequently, the angular variation of the main radiated beam with frequency or ldquobeam squintingrdquo is derived, based on the expression for the Bloch propagation constant of the NRI-TL line. It is shown that by operating the LWA in the upper right-handed band where the phase and group velocities are closest to the speed of light, the beam squinting that the antenna experiences can be minimized. The theoretical performance of the LWA is verified through full-wave simulations and measurements of a fabricated prototype designed to produce a radiated beam at an angle of at GHz. This 20-element NRI-TL LWA exhibits a measured return-loss bandwidth below dB of 0.91 GHz (18.2%), and an average beam squint of 0.031/MHz. The proposed NRI-TL LWA is uniplanar, differential and broadband, and therefore suitable for integration with other microwave components and devices.

Negative-refractive-index transmission-line metamaterials and enabling electromagnetic applications

We present the latest results for two RF/microwave devices and structures that we are developing at the University of Toronto, based on transmission-line NRI metamaterials. The first example is a super-resolving planar lens around 1 GHz and the second is a low-profile antenna, radiating vertical polarization, around 30 GHz.