Joaquim P. Leite Neto

Bio: Joaquim P. Leite Neto is an academic researcher from National Institute for Space Research. The author has contributed to research in topics: Resonator & Metamaterial. The author has an hindex of 7, co-authored 22 publications receiving 181 citations.

Design of coaxial Bragg reflectors

Abstract: A coaxial line periodically loaded by circular disks on the inner conductor is specifically designed to act as Bragg reflector to prevent radio-frequency fields from leaking out of a 6.6-GHz monotron. Design criteria to ensure a stopband as wide as possible are presented and a discussion is given on how band gap width, center frequency, and reflectivity arise from the geometry of the periodic structure

Examining by the Rayleigh-Fourier method the cylindrical waveguide with axially rippled wall

Abstract: Axially corrugated cylindrical waveguides with wall radius described by R/sub 0/(1+/spl epsi/cos2/spl pi/z/L), where R/sub 0/ is the average radius of the periodically rippled wall with period L and amplitude /spl epsi/, have been largely used as slow-wave structures in high-power microwave generators operating in axisymmetric transverse magnetic (TM) modes. On the basis of a wave formulation whereby the TM eigenmodes are represented by a Fourier-Bessel expansion of space harmonics, this paper investigates the electrodynamic properties of such structures by deriving a dispersion equation through which the relationship between eigenfrequencies and corrugation geometry is explored. Accordingly, it is found that for L/R/sub 0//spl ges/1 a stopband always exists at any value of /spl epsi/; the condition L/R/sub 0/=1 gives the widest first stopband with the band narrowing as the ratio L/R/sub 0/ increases. For L/R/sub 0/=0.5 the stopband sharply reduces and becomes vanishingly small when /spl epsi/<0.10. Illustrative example of such properties is given on considering a corrugated structure with L/R/sub 0/=1,R/sub 0/=2.2 cm, and /spl epsi/=0.1, which yields a stopband of 1.5-GHz width with the central frequency at 8.4 GHz; it is shown that in a ten-period corrugated guide, the attenuation coefficient reaches 165 dB/m, which makes such structures useful as an RF filter or a Bragg reflector. It is also discussed that by varying L/R/sub 0/ and /spl epsi/ we can find a variety of mode patterns that arise from the combination of surface and volume modes; this fact can be used for obtaining a particular electromagnetic field configuration to favor energy extraction from a resonant cavity.

Analysis and Simulation of Reentrant Cylindrical Cavities

Abstract: Of applications ranging from electron spin resonance to detection of gravitational waves, reentrant circular cylindrical cavities are analyzed on the basis of a mathematically simple formalism extending the range of validity of expressions for resonant frequency and quality factor obtained from lumped RLC constant models. Several cavity configurations in the 1 – 3 GHz range are analytically examined in excellent agreement with frequencies obtained from Superfish code.

The sinusoid as the longitudinal profile in backward-wave oscillators of large cross sectional area

Abstract: High-power generation in backward-wave oscillators (BWO) of large section requires that the beam electrons flowing close to the corrugated wall interact efficiently with surface waves supported by a periodic structure. Such waves are described by the superposition of slow-wave space harmonics of the operating mode. The present paper reports on design tools for BWOs operating in symmetric TM modes since these modes are able to perturb the axial velocity and electron density on rectilinear beams confined by an external magnetic field in slow-wave systems. Here we investigate whether a cylindrical guide with sinusoidally rippled wall can provide strong coupling between the guide surface waves and mildly relativistic (» 500 keV) electron beams in the 8-9 GHz frequency range for BWOs of large diameter (D » 3‚). For this purpose, the characteristic equation of a sinusoidally corrugated structure is derived on the basis of the Rayleigh-Fourier method, whereby the field solution is represented by a single expansion of TM eigenmodes. From the dispersion diagrams thus obtained we infer the appropriate periodic length and ripple amplitude of the guiding structure that optimize the beam-wave interaction.

Experiments on wave propagation at 6.0 GHz in a left‐handed waveguide

Abstract: Microwave propagation in a metallic waveguide loaded with an array of split-ring resonators is examined both analytically and experimentally. The synthesized combined medium exhibits a frequency band around 6.0 GHz with simultaneously negative values of permittivity and permeability, which allow transmission in an otherwise forbidden band for the hollow waveguide. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2175–2178, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25435

Effective medium theory of left-handed materials

Abstract: We analyze the transmission and reflection data obtained through transfer matrix calculations on metamaterials of finite lengths, to determine their effective permittivity epsilon and permeability micro. Our study concerns metamaterial structures composed of periodic arrangements of wires, cut wires, split ring resonators (SRRs), closed SRRs, and both wires and SRRs. We find that the SRRs have a strong electric response, equivalent to that of cut wires, which dominates the behavior of left-handed materials (LHM). Analytical expressions for the effective parameters of the different structures are given, which can be used to explain the transmission characteristics of LHMs. Of particular relevance is the criterion introduced by our studies to identify if an experimental transmission peak is left or right handed.

Experimental observation of true left-handed transmissionpeaks in metamaterials

Abstract: We report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of 21.2 dB 20.3 dB cm was obtained. The experimental results agree extremely well with the theoretical calculations. © 2004 Optical Society of America OCIS codes: 120.7000, 260.2110.

Comparisons of microwave dielectric property measurements by transmission/reflection techniques and resonance techniques

Abstract: This review provides a general comparison of the two most commonly used techniques for measurement of complex permittivity at microwave frequencies: transmission/reflection and resonance. The transmission/reflectance techniques are analyzed using distributed and lumped impedance models. The resonance techniques are analyzed using both dielectric and cavity resonance models. The analysis, combined with experimental results, enables us to illustrate the advantages and disadvantages of the various techniques and provide guidance on which techniques to use under particular circumstances. In general, transmission/reflection techniques can be used over a broad band of frequencies, and are suitable for loss measurements on high loss materials. Resonance techniques do not have swept frequency capability, but have higher accuracy for measurement of the real part of permittivity and can measure the loss tangent of low loss materials with high resolution.

Terahertz Graphene Optics

Abstract: The magnitude of the optical sheet conductance of single-layer graphene is universal, and equal to e 2/4ħ (where 2πħ = h (the Planck constant)). As the optical frequency decreases, the conductivity decreases. However, at some frequency in the THz range, the conductivity increases again, eventually reaching the DC value, where the magnitude of the DC sheet conductance generally displays a sample- and doping-dependent value between ∼e 2/h and 100 e 2/h. Thus, the THz range is predicted to be a non-trivial region of the spectrum for electron transport in graphene, and may have interesting technological applications. In this paper, we present the first frequency domain measurements of the absolute value of multilayer graphene (MLG) and single-layer graphene (SLG) sheet conductivity and transparency from DC to 1 THz, and establish a firm foundation for future THz applications of graphene.