Electrical impedance

About: Electrical impedance is a research topic. Over the lifetime, 36015 publications have been published within this topic receiving 371891 citations. The topic is also known as: electrical impedance & complex impedance.

Analysis of microstrip patch antennas using finite difference time domain method

Abstract: The study of microstrip patch antennas is directly treated in the time domain, using a modified finite-difference time-domain (FDTD) method. Assuming an appropriate choice of excitation, the frequency dependence of the relevant parameters can readily be found using the Fourier transform of the transient current. The FDTD method allows a rigorous treatment of one or several dielectric interfaces. Different types of excitation can be taken into consideration (coaxial, microstrip lines, etc.). Plotting the spatial distribution of the current density gives information about the resonance modes. The usual frequency-dependent parameters (input impedance, radiation pattern) are given for several examples. >

Multilayer piezoelectric ceramics for two-dimensional array transducers

Abstract: In medical ultrasound imaging, 2-D array transducers have become essential to implement dynamic focusing and phase-correction in the elevation dimension as well as real-time volumetric scanning. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near resonance. These elements have poor sensitivity because their impedance is much higher than the electrical impedance of the transmit and receive circuitry. Sensitivity can be improved by using an N layer structure of PZT ceramic with the layers connected acoustically in series and electrically in parallel. For the multilayer ceramic (MLC), the damped capacitance is multiplied by a factor of N/sup 2/ and the electrical impedance by 1/N/sup 2/ compared to a single layer element of the same dimensions. A 3/spl times/43 phased-array transducer has been fabricated using 3 layer PZT-5H material. Each element had a thickness of 0.66 mm and an area of 0.37/spl times/3.5 mm. The MLC was manufactured using thick film technology with plated-through vias to electrically interconnect the electrode layers. The completed transducer was compared to a single layer control array of similar dimensions. With a light epoxy backing and a /spl lambda//4 matching layer, the MLC array elements had an impedance of 100 /spl Omega/ at series resonance of 2.25 MHz, compared to 800 /spl Omega/ for the control elements. The lower impedance of the MLC elements resulted in a minimum round-trip insertion loss of 24.0 dB, compared to an 34.1 dB for the control array elements. These results were consistent with KLM modeling. B-scan images were made of cysts in a tissue-mimicking phantom and of the left kidney in vivo. The images clearly showed a higher signal-to-noise ratio for the MLC array compared to the control. As a result, 2-D arrays made of multilayer ceramics can be used to form images at a higher frequency and greater range than single layer arrays. >

Synthesizing Microstrip Branch-Line Couplers With Predetermined Compact Size and Bandwidth

Abstract: A new method for designing the microstrip branch- line couplers with predetermined compact size and bandwidth is proposed in this paper. With the proposed approach, the size of the quarter-wavelength transmission line in the branch-line coupler can be reduced greatly. In addition, the proposed couplers can be easily fabricated on the printed circuit board without any lumped element. A chart concludes the relationship between bandwidth and size reduction rate. It shows that open stubs with low impedance perform better than those with high impedance; moreover, the more open stubs with low impedance utilized, the broader the bandwidth will be. Furthermore, the measured frequency responses show good agreement with the theoretical results.

Input impedance and radiation pattern of cylindrical-rectangular and wraparound microstrip antennas

Abstract: The radiation from a cylindrical microstrip antenna excited by a probe is analyzed. Both the cylindrical-rectangular and the wraparound elements are discussed. The current distribution on the patch is rigorously formulated using a cylindrically stratified medium approach. A set of vector integral equations is derived which governs the current distribution on the patch. The set of equations is then solved using a moment method. The input impedance and the radiation pattern are derived both exactly and in the small substrate thickness limit. >

Impedance of thin-wire loop antennas

Abstract: THE THIN-wire loop is one of the first antennas to receive theoretical consideration, having been discussed by Pocklington1 in 1897. Pocklington treated a closed loop excited by a plane wave; he obtained an exact solution for the current on the loop in the form of a Fourier series. More recently, Hallen2 considered a driven loop and obtained a solution, again in the form of a Fourier series, for the current and the impedance. However, Hallen pointed out that the coefficients of this series contained a singularity which made the series only quasiconvergent and hence useful only for loops small in comparison to a wave length. Moreover, the individual terms were complicated and their evaluation and a summation involved a somewhat difficult numerical task.