Continuous-spectrum Current Research Articles

Excitation of an Infinite Microstrip Line With a Vertical Coaxial Feed

An efficient method is presented for the analysis of a vertical coaxial probe excitation of an infinite microstrip line. The novel feature of the method is that it uses a semianalytical Green's function that is derived for current sources in the presence of the infinite microstrip line. Hence, in a method of moments approach, unknown currents need only be placed on the conducting probe feed and not on the infinite strip surface. The method also utilizes an attachment mode at the contact point between the probe and line so that the correct Kirchhoff condition is automatically satisfied. Once the surface current density on the probe is known, the surface current density on the strip conductor can be readily obtained using the Green's function of the background grounded substrate. The method is valid even at high frequency, where simple transmission line theory fails to account for effects such as the continuous-spectrum current that is excited on the line. After validating the method with various commercial software simulation packages, results are presented to study the fundamental behavior of the input impedance, probe current, and current launched on the microstrip line, and to examine the high-frequency behavior of these currents.

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.

Direct modal transition from space wave to surface wave leakage on microstrip lines

In this paper it is shown for the first time that a novel transition may occur for dominant (quasi transverse electromagnetic (quasi‐TEM)) leaky modes on microstrip line, between a leaky mode that leaks into the TM0 surface wave mode and one that leaks both into the surface wave and into space. The modal evolution of leaky modes as a function of frequency is studied by means of a full‐wave spectral domain approach, and it is shown that such transitions occur on microstrip lines that have a critical strip width, which is relatively large. When the strip width is equal to the critical value, a transition frequency will exist at which the attenuation constant of the leaky mode drops exactly to zero and the leaky mode has a real propagation wave number exactly equal to the free‐space wave number. At this frequency, the leaky mode transitions from one that leaks into only the surface wave to one that also leaks into space. In a frequency neighborhood of the transition frequency, very large spurious effects may be produced because of interference between the fundamental quasi‐TEM mode and the continuous spectrum current.

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.

Spurious radiation from a practical source on a covered microstrip line

The TM/sub 0/ parallel-plate mode field that is radiated from the currents induced on a covered microstrip transmission line by a finite-gap voltage source is studied. The behavior of the total radiation field (the field radiated by the total strip current) is investigated, along with the field radiated by the constituent current components that make up the total current, namely the bound-mode (BM) and continuous-spectrum currents. The continuous-spectrum current is further resolved into the sum of a physical leaky-mode current and a residual-wave current, and the fields radiated by each of these separate components are examined. It is determined that leaky-mode fields can contribute to crosstalk and other interference effects near the source and within an angular leakage while the radiation field from the BM current is the predominant mechanism for these effects further away from the gap source, outside the leakage region. The field radiated from the residual-wave current can be quite strong in the spectral-gap region, which is the frequency region where the leaky mode is nonphysical, and therefore the leaky mode does not contribute directly to the spectrum of current on the strip in the decomposition used here.

Spurious transmission effects due to the excitation of the bound mode and the continuous spectrum on stripline with an air gap

The nature of the strip current excited by a practical delta-gap source on a stripline structure with an air gap above the strip conductor is studied. This structure supports the existence of a dominant leaky mode (LM) in addition to a bound mode of propagation. It is shown that the total current on the strip excited by the source can be resolved into the sum of a bound-mode current and a continuous-spectrum current, with the latter current further represented as the sum of all physical LM currents and a "residual-wave" current. The roles of the various current components are investigated numerically as the frequency and air-gap thickness change. The appearance of a spurious sharp dip in the transmission response is demonstrated numerically and explained theoretically, and numerical results are compared with measurements. Since such a transmission dip can always occur for an appropriate set of parameter values, it is important to be able to understand and predict this effect.

Current induced on a pair of wires above earth by a vertical electric dipole for grazing angles of incidence

Exact expressions for the current induced on a pair of horizontal wires above the earth by a vertical electric dipole have been available for some time. It is well known that the wire currents can be expanded as a continuous spectrum current plus a sum of discrete modal currents. Each current consists of a monofilar and a bifilar component. In this paper, conditions for which the currents can be approximated either by the discrete modal currents or a simple approximation to the continuous spectrum current are found. For simplicity it is assumed that the earth is perfectly conducting and that the wires are thin and can be characterized by a surface impedance boundary condition. Using steepest descent integration, the case for grazing incidence (distance from source to observation point R greater than the perpendicular distance from the source to the center of the wire array r) is examined. It is found that either the monofilar or the bifilar component of current can be written as a single term which is proportional to 1/R if R is of the order of r or to 1/R5/2 if R is ‘very large.’ In the intermediate range the discrete modal currents dominate the solution. Transition solutions valid between these ranges are given.