Simon Otto

Transmission Line Modeling and Asymptotic Formulas for Periodic Leaky-Wave Antennas Scanning Through Broadside

It is shown, using three specific examples-a series fed patch (SFP) array, a phase reversal (PR) array and a composite right/left-handed (CRLH) antenna-that one-dimensional periodic leaky-wave antennas scanning through broadside build a class of leaky-wave antennas sharing qualitatively similar and quantitatively distinct dispersion and radiation characteristics. Based on an equivalent transmission line (TL) model using linearized series and shunt immittances to approximate the periodic (Bloch) antenna structure, asymptotic TL formulas for the characteristic propagation constant, impedance, energy, power and quality factor are derived for two fundamentally different near- and off-broadside radiation regimes. Based on these formulas, it is established that the total powers in the series and shunt elements are always equal at broadside, which constitutes one of the central results of this contribution. This equal power splitting implies a severe degradation of broadside radiation when only one of the two elements series or shunt efficiently contributes to radiation and the other is mainly dissipative. A condition for optimum broadside radiation is subsequently established and shown to be identical to the Heaviside condition for distortionless propagation in TL theory. Closed-form expressions are derived for the constitutive (LCRG) parameters of the TL model for the specific SFP, PR and CRLH antenna circuit models, and quantitative information on the validity range of the TL model is subsequently provided. Finally, full-wave simulation and measurement LCRG parameter extraction methods are proposed and validated.

Extended composite right/left-handed (E-CRLH) metamaterial and its application as quadband quarter-wavelength transmission line

A novel extended composite right/left-handed (E-CRLH) transmission line (TL) metamaterial structure, constituted by the combination of the conventional CRLH (C- CRLH) and the recently introduced dual CRLH (D-CRLH) prototypes, is proposed. This E-CRLH metamaterial is characterized by eight LC parameters (four C-CRLH and four D-CRLH parameters), which allow unprecedented diversity in the manipulation of the dispersion relation of the resulting TL structure. In particular, an E-CRLH TL metamaterial, under an extended balance condition, exhibits two frequencies of infinite wavelength propagation. In addition, the E-CRLH is intrinsically a quadband (arbitrary quadruplet of frequencies) structure. The latter property is exploited here into the design of a quadband quarter- wavelength transformer, and may be applied in principle to any TL-based microwave component.

Composite right/left-handed /spl lambda/-resonator ring antenna for dual-frequency operation

A novel ring structure operating as a dual-frequency resonating antenna is presented. The ring analysis and design is based on the concept of the composite right/left-handed transmission line (CRLH TL), with the particularity that the ends of the line are connected together so as to form a closed loop. As in the case of conventional ring resonators, multiples of full wavelength (n/spl lambda/) field distributions are supported. The key feature of the proposed resonator is that an identical field distribution along the ring is achieved at two different operating frequencies. This phenomenon is due to the dual nature of the CRLH dispersion characteristic. The lower operating frequency corresponds to the /spl lambda/ mode in the left-handed (LH, backward wave, /spl beta/<0) range, while the higher resonance frequency corresponds to the /spl lambda/ mode in the right-handed (RH, forward wave, /spl beta/>0) range. The main benefit of having the same field distribution is that identical performances (radiation, polarization and matching) are achieved at each of the two frequencies. The proposed antenna structure has been simulated with commercial MoM/FEM tools and validated with measurements on fabricated prototypes.

Highly Directive Resonator Antennas based on Composite Right/Left-Handed (CRLH) Transmission Lines

A composite right/left-handed (CRLH) series mode zeroth order resonator antenna (ZORA) and a CRLH half-wavelength antenna (HWA) with tunable directivity are presented and demonstrated to exhibit substantially higher directivity than a conventional patch antenna in the case of a 2.4 GHz (WLAN) design. Due to the special dispersion relation of a CRLH transmission line (TL), the length of CRLH TL resonators can be enlarged while keeping their resonance frequency constant, which yields enhanced directivity. The ZORA has been shown to have a higher efficiency (etaZORA 70%) than the CRLH HWA (etaHWAap40%), comparable to that of a patch antenna, as a result of the perfectly uniform current distribution in the zeroth order mode. In particular, the ZORA due to its versatile characteristics and high performance, is expected to find wide applications in the future.

Circular Polarization of Periodic Leaky-Wave Antennas With Axial Asymmetry: Theoretical Proof and Experimental Demonstration

This paper includes two contributions. First, it proves that the series and shunt radiation components, corresponding to longitudinal and transversal electric fields, respectively, are always in phase quadrature in axially asymmetric periodic leaky-wave antennas (LWAs), so that these antennas are inherently elliptically polarized. This fact is theoretically proven and experimentally illustrated by two case-study examples, a composite right/left-handed (CRLH) LWA and a series-fed patch (SFP) LWA. Second, it shows (for the case of the SFP LWA) that the axial ratio is controlled and minimized by the degree of axial asymmetry.

Radiation Efficiency of Longitudinally Symmetric and Asymmetric Periodic Leaky-Wave Antennas

This letter derives efficiency formulas and establishes fundamental limitations for longitudinally symmetric periodic leaky-wave antennas (LWAs). The proposed approach is based on an equivalent transmission-line model for periodic structures, which is derived from a lattice topology providing a perfect decoupling between the series and shunt immittances and their respective radiation contributions. A central result of this letter is that a 2-D array composed of uniformly excited 1-D LWAs cannot exceed a radiation efficiency of 50%. The presented theory is validated by comparing measured and finite-difference time-domain (FDTD) simulated radiation efficiencies with the ones predicted by the lattice model for several symmetrical and asymmetrical series-fed patch (SFP) array antenna and composite right/left-handed (CRLH) antenna configurations.

Transversal Asymmetry in Periodic Leaky-Wave Antennas for Bloch Impedance and Radiation Efficiency Equalization Through Broadside

This paper demonstrates that unit cell asymmetry with respect to the transversal axis -or transversal asymmetry- is an essential design parameter in periodic leaky-wave antennas (P-LWAs). Specifically, it shows that transversal asymmetry can be leveraged to fully and systematically solve the well-known radiation degradation of P-LWAs at broadside, where it provides both open-stopband closure and efficiency equalization. The problem is addressed via a generic equivalent circuit model composed of a series resonator, a shunt resonator and ideal transformers for modeling asymmetry by a single and simple parameter, namely the transformation ratio. Once the series and shunt frequencies have been balanced (frequency-balancing), equalization is ensured by adjusting the degree of asymmetry in the unit cell so to match the at-broadside Bloch impedance to the off-broadside Bloch impedance. This equalization condition is referred to as quality factor balancing ( Q-balancing) and it is related to the Heaviside condition (distortionless propagation) in homogeneous transmission lines. Based on this theory, optimization schemes for employing commercial fullwave eigenmode and drivenmode solvers are proposed to design unit cells with equalized efficiencies. Finally, two examples of P-LWAs are presented, a composite right/left-handed (CRLH) P-LWA and a series-fed coupled patch (SFCP) P-LWA, and verified to fully confirm the predictions of the theory obtained by circuit modeling.

Enlarged half-wavelength resonator antenna with enhanced gain

A concept of gain enhancement by radiation aperture enlargement has been presented. The nonlinear dispersion relation of a CRLH TL has been used to enlarge the antenna length keeping the resonance frequency nearly constant. It has been shown that the gain is strongly dependent on the size of the radiation aperture. This fact supports the idea that the x-component of the electric field in the interdigital capacitors of the unit cells is the main contribution to the radiation aperture and is also in agreement with the far-field polarization. A maximum gain of 10.7 has been reached for the largest antenna with 18 unit cells.

Dual mode zeroth order ring resonator with tuning capability and selective mode excitation

A novel ring configuration supporting two orthogonal unique zeroth order modes is presented. The analysis and the design of this ring are based on the artificially engineered composite right left handed (CRLH) transmission lines. A CRLH transmission line exhibits unusual and more diverse propagation characteristics. In the dispersion diagram a lower left hand (LH) and a higher right hand (RH) frequency range is separated by a propagation band gap. This band gap is confined by two zero phase origins (/spl beta/=0), associated with an infinite guided wavelength at non-zero frequencies. Using the CRLH-TL in a ring configuration with connected ends, both zero phase origins along with their corresponding frequencies form a valid resonance mode. One zeroth order mode comes with a constant transmission line voltage, a so-called voltage mode, while the transmission line current is zero. The other zeroth order mode is referred to as current mode and operates at resonance with a constant transmission line current and zero voltage along the resonator line. A simple tuning technique for the voltage modes resonance frequency is addressed. Different feeding strategies to excite both zeroth order modes simultaneously in a one port device as well as decoupled two port selective mode feeding (either voltage or current mode) are discussed.

A distributed attenuator for K-band using standard SMD thin-film chip resistors

This paper presents a distributed attenuator with standard SMD chip components. For low frequencies simple Pi and T-networks with chip resistors offer a good flexibility to design low cost attenuators in a microwave circuit. Nevertheless, with increasing frequency the parasitics of the chip resistors as well as the short additional line sections in the Pi and T network cause the design to fail. Here, a model of an 0201 chip resistor has been setup to show the limited operating range of the Pi and T-networks. Using a distributed attenuator design, two resistors separated by a quarter wavelength transmission line, can overcome these problems and extent the frequency range to K-band.