Slotline impedance

A theoretical model is presented to compute the characteristic impedance and wavelength in a slotline printed on or embedded in a dielectric substrate. In this treatment the effects of the fringing field caused by the finite width of the conducting strips are taken into account. The main task is to calculate the capacitance per unit length of the slotline. First, the Green's function for the potential of a pair of filament sources in a dielectric substrate is solved; this is used as a building block to construct the overall solution of the boundary value problem. Then, the surface charge density on the conducting strips is found by using a moment method and imposing the source condition (equal potential) on the conductors. From the surface charge density, the characteristic impedance of the slotline is computed for various input parameters. The formulation is applicable to a single-sided, sandwiched, or double-sided slotline printed on or embedded in a dielectric substate     

中波天线输入阻抗计算机分析

通过对76m天线、138m天线输入阻抗数据分析，对天线输入阻抗与平均特性阻抗、波长缩短系数以及天线等效直径的关系进行了探讨，分析了对用传输线法计算天线输入电抗误差较大的原因，并对输入电抗的计算提出了修正。     

Fundamental interaction impedance of a helix surrounded by a dielectric and a metal shield

The fundamental interaction impedance for a helix surrounded by a dielectric medium and a conducting shield is computed and the result is found to be in good agreement with measurements on several TWT structures. Results obtained in this paper indicate that the addition of a conducting shield around a helix reduces the interaction impedance; this impedance reduction becomes more severe as the shield is brought closer to the helix and as γa, the ratio of helix circumference to guide wavelength, is made smaller. A sheath helix surrounded by a dielectric medium and a conducting shield is used to obtain a factor representing the impedance reduction caused by the shield. This impedance reduction factor is then combined with Tien's factors to compute the helix impedance.     

Scanning and impedance properties of TEM horn arrays for transientradiation

A general concept for ultrawide-band array design using interconnected transverse electromagnetic (TEM) horns is described. At high frequencies (wavelength small compared to unit cell dimensions), the mutual coupling between elements is small and, consequently, the input impedance depends only on the lattice dimensions and not on either scan angle or frequency. At low frequencies (wavelength large compared to unit cell dimensions), the mutual coupling is purposefully made large, by interconnecting the elements to maximize the low-frequency performance. This paper presents the results of analyses using a periodic hybrid finite-element approach to calculate input impedance and scanning performance of generic TEM horn arrays. The limiting case, the planar bicone, is shown to have the frequency-independent property of a self-complementary antenna, making it a useful case for establishing the effects of feed region geometry. Although it radiates bidirectionally, it has the interesting property that its broadside-scan frequency response in the array environment is absolutely flat up to the grating lobe onset limit. A TEM horn array is more unidirectional, but as a consequence suffers both oscillatory variations in the input impedance with frequency and increased limits on minimum achievable rise time     

扫频仪测圆环天线输入阻抗值的可行性研究

用扫频仪测量天线的输入阻抗时，在同轴传输线与天线间应按平衡—不平衡变换器。针对圆环天线的输入阻抗为复阻抗，提出了一种新的四分之一波长平衡一不平衡变换器，得到了实验结果，并进行了讨论。     

Ground influence on the input impedance of transient dipole and bow-tie antennas

In this paper, the influence of a lossy ground on the input impedance of dipole and bow-tie antennas excited by a short pulse is investigated. It is shown that the ground influence on the input impedance of transient dipole and bow-tie antennas is significant only for elevations smaller than 1/5 of the wavelength that corresponds to the central frequency of the exciting pulse. Furthermore, a principal difference between the input impedance due to traveling-wave and standing-wave current distributions is pointed out.     

Finite element modeling of electrode-skin contact impedance inelectrical impedance tomography

In electrical impedance tomography (EIT), the measured voltages are sensitive to electrode-skin contact impedance because the contact impedance and the current density through it are both high. Large electrodes were used to provide a more uniform current distribution and reduce the contact impedance. A large electrode differs from a point electrode in that it has shunting and edge effects that cannot be modeled by a single resistor. The finite-element method (FEM) was used to study the electric field distributions underneath an electrode, and three models were developed: a FEM model, a simplified FEM model, and a weighted load model. The FEM models considered both shunting and edge effects and closely matched the experimental measurements. It is concluded that FEM models of electrodes can be used to improve the performance of an electrical impedance tomography reconstruction algorithm     

Determining electrical ground constants from the mutual impedance of small coplanar loops

Curves are presented for inverting measured mutual impedance to yield the conductivity and permittivity of an equivalent half-space. The displacement currents are allowed for and the loop separation is specified to be either one tenth or one twentieth of a free-space wavelength.     

Effect of finite size of ground plane on the impedance of a monopole immersed in a lossy medium

The effect of finite size of the ground plane on the impedance of a monopole immersed in a lossy medium is studied. The theoretical results show that for water medium and for the ratio of the ground-plane radius to free-space wavelength equal to and greater than 0.2, the effect of ground plane on the monopole impedance is negligible.     

Compact printed monopole antenna with 24:1 impedance bandwidth

A compact tapered CPW-fed hollowed elliptical printed monopole antenna with extremely wide bandwidth is proposed. The results demonstrate that this antenna achieves a measured impedance bandwidth from 0.44 to 10.6 GHz (24.1:1) for VSWR les 2, and exhibits a nearly omnidirectional radiation pattern, while its area is only about 0.18 times 0.13lambda_l, where lambda_l is the wavelength of the lowest operating frequency.     

Monopole impedance and gain measurements on finite ground planes

The possibility of making acceptably accurate monopole input impedance and gain measurements on a reduced-size ground plane is discussed. Existing measured and calculated data show that the diameter of a highly conducting ground plane should be at least 4λ (where λ is the wavelength) for measuring the input impedance of quarter-wavelength monopoles. At 25 MHz, the lowest frequency considered here, such a ground plane would require a space at least 48 m in diameter. The model impedance measurements and calculations presented imply that a reduced space of only 10 m by 11 m is required if 16 resistively loaded wire radials are connected to a 3.66 by 4.88-m rectangular aluminum ground plane, thus effectively `extending' the ground plane by the resistive loading. Measured insertion-loss data acquired using a 1:5 scale-model ground plane with resistively loaded radials indicate that the plane is sufficiently large for gain measurements as well. Measured and calculated monopole standing-wave ratio and insertion loss on a full-scale ground plane verify the results of the model measurements     

A finite-element study of the effects of electrode position on themeasured impedance change in impedance cardiography

Traditional impedance cardiography (ICG) technique uses band electrodes both for delivering current to and measuring impedance change in the thorax. The use of spot electrodes increases the ease of electrode placement and comfort level for patients. Research has shown that changes in thoracic impedance can have multiple causes. In this study, we used finite element modeling to investigate the sources of impedance change for both band-electrode and spot-electrode ICG, and focused on how differences in electrode location affect the contribution of different sources to changes in impedance. The ultimate purpose is to identify the optimal electrode type and placement for the sensing of stroke volume (SV). Our models were built on sets of end-diastolic and end-systolic magnetic resonance images of a healthy human subject. The results showed that the effect of ventricular contraction is opposite to that of the other changes in systole: the expansion of major vessels, decrease in blood resistivity due to increased blood flow velocity, and decrease in lung resistivity due to increased blood perfusion. Ventricular contraction, the only factor that tends to increase systolic impedance, has a larger effect than any of the other factors. When spot electrodes are placed on the anterior chest wall near the heart, ventricular contraction is so dominant that the measured impedance increases from end-diastole to end-systole, and the change represents 82% of the contribution from ventricular contraction. When using the common band-electrode configuration, the change in measured impedance is a more balanced combination of the four effects, and ventricular contraction is overcome by the other three factors so that the impedance decreases. These results suggest that the belief that ICG can be used to directly measure SV based on the change in the whole thoracic impedance may be invalid, and that spot electrodes may be more useful for understanding local physiological events such as ventricular volume change. These findings are supported by previously reported experimental observations.     

复杂截面同轴线特性阻抗、截止波长的计算

应用静电场的观点和导波传输理论,用有限元法（FEM）分别计算了各种复杂截面同轴线特性阻抗、截止波长。计算结果表明,该方法计算精度高,通用性强,简便易行,可以用于传输线工程问题的设计和计算。     

The impedance of a center-fed strip dipole by the Poynting vectormethod

The impedance of a center-fed strip dipole of width w and thickness t, with t≪w, is obtained by the Poynting vector method. With w also taken to be small with respect to the operating wavelength (w≪λ) the surface current on the dipole is modeled by the classical sinusoidal standing wave along its length. There are, however, appropriate transverse singularities assumed at the edges of the strip. The Poynting vector method leads to a two-dimensional integral in the transverse coordinates involving the classical mutual impedance between filamentary dipoles located on the surface of the strip itself as well as the above mentioned transverse edge singularities. A Legendre series expansion of this mutual impedance (the coefficients obtained by numerical integration) leads to a rapidly convergent series solution (eight terms) for the overall impedance of the strip dipole. Numerical results are provided For a variety of strip widths, thicknesses and lengths. They are compared with the impedances of equivalent circular dipoles of radius a=(w+t)/4 and good agreement occurs except for quite wide strips (w=0.05λ)     

Input impedance of a monopole antenna at the center of a finite ground plane

A technique for computing the input impedance of a monopole on a finite ground plane is provided. The case of an infinite ground plane is first examined, and then the concept of using a magnetic edge current as an equivalent source of diffraction on the limited size ground plane is introduced. The technique used is serf-consistent in that the circular ground plane considered is the limit of a polygonal ground plane as the number of sides tends to infinity. Good agreement has been demonstrated when the nearest edge of the ground plane is at least 0.3 wavelength from the base of the monopole.     

Mutual impedance between two short-circuited flat resonant dipoles

Explicit formulas are presented for the mutual impedance and coupling between two parallel or collinear short-circuited flat dipoles at resonance in terms of three dimensions with respect to wavelength. Experimental and theoretical results are in good agreement and show that the radiating dipoles are particularly well uncoupled and therefore are suitable for incorporation in short arrays for which superdirectivity can be achieved, or in large swept sector arrays.     

Electrical impedance imaging of the thorax

In this paper, we propose to develop a new imaging technology, computerized impedance tomography (CIT) for imaging the thorax. Our study involves reconstructing images of thoracic transverse plane impedance distributions noninvasively and nondestructively and exploring the potential of CIT. We overcome the reconstruction problem due to nonlinear and nonplanar current paths in impedance imaging by solving Laplace's equation numerically for every iteration and by using a new back-projection algorithm to modify the impedance profile. We discuss advantages and disadvantages associated with impedance imaging, the computer model, and back-projection algorithms used in reconstructing impedance images. We present reconstructed impedance images with 8 projection angles and different projection methods. We identify important variables affecting the quality of reconstructed images, and discuss the resolution and accuracy of this imaging technique. We summarize numerical aspects, computer requirement, and limitations of impedance imaging. We also discuss the future of impedance imaging.     

Surge impedance

In the conventional treatment of traveling waves on lossless lines, inductance and capacitance per unit length of line are used. Such treatment results in the usual concept of surge impedance and only certain aspects propagate with the velocity of light. The usual concepts of inductance and capacitance are defined in terms of their steady-state values. In the present paper, the treatment follows from electric field concepts in which the propagational time delays are taken into consideration through the use of retarded potentials. The current that flows is determined when a unit EMF is suddenly inserted in series in an infinitely long isolated conductor of zero resistance. The current equals the transient surge admittance and its reciprocal is the transient surge impedance. The surge impedance increases with time in a determinable manner from a very small value. When such a conductor forms only the connecting leads to a pair of infinitely long parallel conductors, the surge impedance approaches the conventional value for the parallel lines.     

Equivalent surface impedance of an infinite periodic array of slot impedance loads based on a semicylinder cavity

A problem of finding the equivalent surface impedance of an infinite periodic array of slot impedance loads based on a semicylinder cavity. The problem is solved by the integral-equation method. As the numerical method for solving the integral equation, the Krylov-Bogoliubov method is used. The results of the numerical experiment are presented in the form of dependences of the impedance on the cavity radius, slot width, conductor width, and angle of electromagnetic-wave incidence. The angular dependences of the impedance are compared to the earlier results obtained for an array of rectangular grooves and a single impedance load based on a semicylinder cavity.     

Characterization of single-mode fibers from wavelength dependence of modal field and far field

We present direct methods for determination of equivalent-step-index (ESI) parameters and file modal field by measurement of the wavelength dependence of the far-field intensity in single-mode fibers. A comparison is made with the spot-size method commonly used for characterization of such fibers. A new technique for measuring the LP11mode cutoff wavelength is also presented.