High-frequency leaky-mode excitation on a microstrip line

The excitation of leaky modes (LMs) and the continuous spectrum (CS) on a microstrip line at high frequencies from a delta-gap source is studied. The delta-gap source models a practical source or discontinuity on the line. It is shown that the current excited from the source exhibits spurious effects at high frequencies due to the excitation of the CS (radiation spectrum), which may or may not be dominated by a physical LM, depending on the frequency range and the substrate permittivity. In some cases, the spurious effects are due to a physical LM, while in other cases the effects are due to the excitation of one or more "residual-wave" (RW) currents, which have not been previously studied for open microstrip lines. There are two types of RW currents: a free-space type and a surface-wave type. Depending on the frequency and the structural parameters, either of these may be the more dominant. At certain frequencies, weakly attenuated high-order LMs may also be excited, in which case spurious effects are observed out to large distances from the source     

The influence of a top cover on the leakage from microstrip line

When a microwave integrated circuit is enclosed in a package, the top cover causes the transmission lines used in the circuit to become leaky at a lower frequency than otherwise. This effect is investigated in detail for microstrip line, and is found to be particularly dramatic. The leakage is strong enough to produce spurious effects that can ruin the performance of the circuit. The amplitude behavior is obtained by numerically solving for the current on a covered microstrip line due to a delta-gap source excitation. The results clearly show that a strong leaky mode (LM) is excited and that spurious effects due to the LM and from direct radiation from the source generally become more severe at higher frequencies and when the top cover is brought nearer to the strip     

High-Speed Computation of Single and Coupled Microstrip Parameters Including Dispersion, High-Order Modes, Loss and Finite Strip Thickness

Based on an optimized rigorous hybrid mode solution for covered/open zero thickness microstrip patterns the following frequency dependent single and coupled line data are evaluated with very short CP-times: The characteristic impedances of the even and odd quasi-TEM modes, the propagation/attenuation constants and associated strip current density components of these and the higher order modes, the loss of the dominant modes under consideration of nonuniform strip current, substrate surface roughness and dielectric loss tangent. Finite strip thickness is introduced by a correction of the strip width input values.     

Excitation of leaky modes on multilayer stripline structures

A quasi-analytical method for calculating the excitation of leaky modes on multilayer stripline structures by a finite source is presented in this paper. Simple sources such as an infinitesimal dipole near the conducting strip or a delta-gap feed on the conducting strip of the transmission line are considered. The method uses a numerically constructed Green's function for the source in the presence of the conducting strip, which is calculated from Fourier transform theory in terms of a one-dimensional Green's function for a line source in the presence of the conducting strip. The numerical Green's function involves a one-dimensional integration in the longitudinal wavenumber plane. The residue contributions from the poles of the Green's function define the excitation amplitudes of the leaky and bound modes that exist on the structure. The numerical Green's function is also used to numerically calculate the complete current on the strip excited by the source. The correlation between the leaky-mode current and the complete current is used to define the extent of the physical meaning of the leaky mode. The generalized pencil of functions (GPOF) method is used to study this correlation by resolving the complete current on the strip into exponential waves, which are then compared with the current of the leaky mode. The physical meaning of the leaky modes is also analytically examined by consideration of the branch cuts in the longitudinal wavenumber plane for the numerical Green's function integration. A “path consistency condition” is established as a necessary condition for the physical meaning of the leaky mode     

Suppression of waveguide modes in strip transmission lines

Undesired waveguide modes can be excited in a strip transmission line shielded in a wide metallic channel. A mode suppressor which suppresses selectively the waveguide modes over a certain frequency range is described. This technique allows a substantial increase of the channel and dielectric substrate dimensions.     

The theory of surface-wave and space-wave leaky-mode excitation onmicrostrip lines

This paper discusses the excitation and physical validity of both surface-wave and space-wave leaky modes on microstrip lines. This is done by analyzing the discrete modal spectrum excited by a realistic source on or near an infinite microstrip line. A semi-analytical three-dimensional (3-D) Green's function is used for this purpose, which provides the current excited on the conducting strip due to the source. The 3-D Green's function is in the form of a spectral integration (inverse Fourier transform) in the longitudinal wavenumber plane. The poles of the integrand directly determine the excitation amplitudes of the modes on the structure that are launched by the source. The integrand also has different types of branch points, and the location of the poles on the various Riemann sheets is used to determine the physical significance of the leaky modes. Although the theory presented here is illustrated for a microstrip line, the conclusions apply in general to open printed-circuit structures     

Novel analytical representations of the continuous-spectrum current in multilayer stripline structures

We consider a multilayer stripline structure excited by a delta-gap source and we study the continuous-spectrum current, which may be responsible of several spurious transmission effects such as interference with the quasi-TEM mode, undesired radiative effects, and crosstalk. In particular, a uniform asymptotic analysis of such continuous spectrum is presented, based on two alternative closed-form representations. Both representations are accurate also very close to the source and are valid inside a wide frequency range starting from DC. One formulation has a direct implementation, since it needs only knowledge of the spectral Green's function of the background structure. The other formulation requires knowledge of the exact complex (leaky) poles of the stripline Green's function and of their residues, but provides more physical insight since it gives quantitative information on the role of leaky waves in the continuous-spectrum representation in the entire considered frequency range. Numerical results which validate the accuracy and efficiency of the proposed approaches are provided at different frequencies for covered-microstrip structures with different strip widths.     

Analytical study on guided modes in vertical strip line embedded in NRD guide

Guided modes in a vertical strip line embedded in a non-radiative dielectric (NRD) guide were analysed based on the two-dimensional finite-element method. The dominant mode in this guiding structure is the TEM wave, and furthermore, it is evident that a quasi-LSM/sub 11/ odd mode occurs when a wide metal strip is used, whereas a quasi-LSE/sub 01/ even mode is generated when a narrow strip is employed.     

Electromagnetic scattering of finite strip array on a dielectricslab

A feast recursive algorithm is used to compute the scattering properties of a finite array of strip gratings on a dielectric slab. this algorithm has a computational complexity of O(N log² N) for one incident angle and O(N ² log N) for N incident angles. It uses plane wave basis for expanding the incident wave and the scattered wave. The scattered wave is expanded in terms of a Sommerfeld-type integral with spectral distribution along a vertical branch cut, rendering its expansion very efficient. To validate the scattering solution obtained using the recursive algorithm, comparisons with the method of moments are illustrated. The current distributions on the strips and scattering patterns are both presented. Since this algorithm has reduced computational complexity and is fast compared to other conventional methods, it can be used to analyze very large strip arrays. Scattering solution of a 50-wavelength wide strip is illustrated     

Upward travelling current wave excitation of overhead cable

We carry out a self-consistent analysis of the excitation of current on an overhead cable by a vertical current source. The ground plane is assumed to be a perfect conductor, but otherwise the solution obtained is quite general. It is shown, for the cable model adopted, two dominant modes are excited; one is the expected transmission line mode between the cable and the ground plane and the other is the coaxial mode between the center conductor and sheath. When generalized, results for the traveling wave source can simulate lightning, but this task is left for the future     

Effects of Losses on the Current Spectrum of a Printed-Circuit Line

This paper studies the effects of practical conductor and dielectric losses on the high-frequency current excited on a microstrip line by a gap voltage source. The analysis shows that whereas losses cause an exponential decay in the propagating bound mode (as expected), the continuous-spectrum current is much less influenced by the presence of material losses. As a consequence, the nature of the strip current far away from the source is dramatically affected by the presence of losses, and will be dominated by the continuous spectrum. This results in unusual behavior that is observed for the strip current far away from the source.     

Fundamental properties of radiation from a leaky mode excited on a planar transmission line

By using a specific normalization, it is shown that the TM/sub 0/ radiation field from a leaky mode that is excited by a source on any printed-circuit structure can be represented in a unified manner. The general properties of the normalized leaky-mode radiation field are examined in detail for a variety of phase and attenuation constants, as well as distances from the source, and for different types of source excitations. The normalized leaky-mode radiation field is compared to similarly normalized geometrical-optics and far-field radiation expressions to provide further insight into the nature of the radiation fields in the near- and far-field regions. The results provide a general view of leaky-mode radiation properties independent of the type of planar transmission line or background structure.     

Leakage of the dominant mode on stripline with a small air gap

In addition to the expected bound (proper) dominant mode, an independent leaky, (improper) dominant mode has been found to exist on a conventional stripline that has a small air gap above the conducting strip. Such an air gap often occurs during the fabrication process, and has in the past been suspected as the cause of spurious performance. This newly discovered leaky dominant mode leaks into the fundamental TM ₀ parallel-plate mode of the background structure, which is a parallel-plate guide with an air gap. Furthermore, it is found that the leaky dominant mode, not the bound dominant mode, is the continuation of the stripline TEM mode that exists with no air gap. Hence, it is the leaky mode that is excited predominantly by a conventional feed for the small air-gap structure. The general properties of both the bound and leaky dominant modes are obtained by using a full-wave spectral-domain approach. The primary purposes of this paper are to discuss the nature of the leaky dominant mode, and to show that its presence is indeed responsible for spurious transmission-line performance, such as unexpected loss and crosstalk, and interference between the bound and leaky dominant modes. These conclusions are verified experimentally     

Microstrip Open-Loop Resonator With Multispurious Suppression

A compact open-loop resonator with multispurious suppression is proposed. When excited symmetrically, the resonator shows two tunable transmission zeros. By adjusting the open-end gap capacitance, one of the zeros is placed near the passband and the other is tuned to collocate with the leading two degenerated higher order resonances, so that the circuit has a sharp transition as well as a wide upper stopband. The experimental circuit shows the first spurious peak occurs at four times the passband frequency (4f_o), and the measurement shows good agreement with the theoretic prediction.     

Design of high-temperature superconducting low-pass filter forbroad-band harmonic rejection

A new type of low-pass filter based on a coupled line and transmission line theory is proposed for suppressing harmonics and spurious frequency. The seventh-order coupled-line low-pass filter with strip line configuration was fabricated using a high-temperature superconducting (HTS) thin film on MgO(100) substrate. The seventh-order low-pass filter has five attenuation poles in the stop band. Its cutoff frequency is 1 GHz with a 0.01-dB ripple level. The fabricated filter exhibits a characteristic stop band in the range from 1 to 7 GHz. A measured insertion loss at the pass-band of the HTS low-pass filter is 0.2 dB at 77 K, while the maximum return loss is 17.1 dB. These results match well with those obtained by the EM simulation. The new low-pass filter structure is shown to have attractive properties such as compact size, wide stop band range, and low insertion loss     

一种宽频带方形槽圆极化天线设计

文中提出了一种加载微扰元素的新型宽频带圆极化方形槽天线,它由一个倒L 型微带馈电线、一个L 型枝节、一对矩形槽和一个方形槽地板组成。用这些缝隙槽和枝节作为微扰元素来调节天线表面电流分布,可以激 发出多个圆极化谐振模式,从而实现了宽频带圆极化辐射。为了验证其合理性,加工并测试了天线模型。测量结果 表明,实测小于-10 dB 的阻抗带宽为74. 9%,小于3 dB 的轴比带宽为84. 7%。此外,测量和仿真的峰值增益分别为 4. 88 dBi 和4. 86 dBi。因此,文中设计的圆极化方形槽天线具有宽的圆极化带宽特性。     

A full-wave model for EMI prediction in planar microstrip circuitsexcited in the near-field of a short electric dipole

A full-wave model to evaluate the interference induced in planar microstrip lines of arbitrary shape exposed to the near-field of an elementary electric dipole is presented. The analysis of the line response, consisting in the evaluation of the surface current excited along the microstrip line, is based on the electric dyadic Green's function method. The surface current induced on the metal strip which, for the sake of generality, is considered as embedded in the dielectric substrate, is obtained solving by means of the spectral-domain approach (SDA) the electric integral equation, which enforces the boundary condition of zero tangential electric field on the surface of the strip. The induced current is computed for different line geometries and loads, and for various positions (inside or outside the dielectric substrate) and orientations of the dipolar source. Indications towards reducing the level of the signal induced on the loads of the line are inferred     

Analysis of spherical annular microstrip antennas

The spherical annular microstrip antenna is analyzed. The antenna is excited by a coaxial line. The general transmission line model (GTLM) is used to compute the input impedance. The radiating TM₁₂ mode is considered because of its wide band. The effect of the other radiating modes and the nonradiating modes on the TM₁₂ mode is investigated. The effect of different parameters on the input impedance is considered. The radiation patterns are computed using the method of moments     

On spontaneous emission

The problem of the spontaneous emission from a single excited two-level atom in the presence of N unexcited similar atoms is discussed. A formalism is presented for evaluation of the time-development operator (and thus the density matrix) which has certain desirable features for time-dependent problems in the area, e.g. of atom-field interactions, when the off-diagonal elements are large compared to the diagonal elements of the Hamiltonian. By use of this formalism, it is shown that the spontaneous emission from a single atom is often drastically altered in the presence of unexcited atoms with similar resonances. When the number of atoms present becomes very large relative to the number of modes of the electromagnetic field, the excited atom becomes trapped in its excited state     

Accurately Approximate Formula of Effective Filling Fraction for Microstrip Line with Isotropic Substrate and its Application to the Case with Anisotropic Substrate

The accurately approximate formula of effective filling fraction q/sub w/ is obtained for the microstrip line with zero-thickness strip and isotropic substrate. The line capacitance per unit length C/epsilon/sub 0/ for the case with anisotropic substrate can easily be obtained by using the approximate formula of q/sub w/ and C/sub 0//epsilon/sub 0/ tabulated for the wide range of shape ratio w/h. The parameters Z, v, and lambda for such a line can be calculated by using C/ epsilon/sub 0/ and C/sub 0//espsilon/sub 0/.