Darko Kajfez

Q-Factor Measurement with Network Analyzer

Ginztons impedance method of Q-factor measurement is adapted to network analyzer techniques. The circuit model of the resonator incorporates also an external reactance which varies linearly with frequency to take into account the effects of the coupling mechanism and the influence of the distant resonant modes.

Computed Modal Field Distributions for Isolated Dielectric Resonators

Electric- and magnetic-field patterns for five of the lowest resonant modes in cylindrical dielectic resonators are displayed in various planes of intersection. The computational procedure is based on a method moments solution of the surface integral equation for bodies of revolution. Improvement of the numerical stability through the normalization of the matrix is discussed, and an algorithm for the evaluation of the modal field components is described.

Numerical analysis of stacked dielectric resonator antennas excited by a coaxial probe for wideband applications

The objective of the present study is to improve the bandwidth of the dielectric resonator antenna (DRA) excited by a coaxial probe by using a stacked DRA configuration above an infinite ground plane. The DRA is axisymmetric and a coaxial probe is placed off the antenna axis to excite the HEM/sub 11/spl delta// mode in the DRA, resulting in a broadside radiation pattern. A surface integral equation formulation and the method of moments are used for the numerical analysis. The input impedance and the far field radiation patterns have been computed and the effects of different parameters on the antenna performance have been investigated. With the proper excitation and selection of the resonator parameters, a bandwidth of 35% has been achieved for the stacked DRA configuration based on a -10 dB reflection coefficient on a 50 /spl Omega/-transmission line. An equivalent circuit model is postulated to describe the dual-resonance behavior of the stacked antenna system.

Chip Impedance Matching for UHF RFID Tag Antenna Design

Passive UHF RFID tag consists of a microchip attached directly to an antenna. Proper impedance match between the antenna and the chip is crucial in RFID tag design. It directly influences RFID system performance characteristics such as the range of a tag. It is known that an RFID microchip is a nonlinear load whose complex impedance in each state varies with the frequency and the input power. This paper illustrates a proper calculation of the tag power reflection coefficient for maximum power transfer by taking into account of the changing chip impedance versus frequency.

MICROSTRIP LINE AND CPW FED ULTRA WIDEBAND SLOT ANTENNAS WITH U-SHAPED TUNING STUB AND REFLECTOR

MICROSTRIPLINEANDCPWFEDULTRAWIDEBANDSLOTANTENNASWITHU-SHAPEDTUNINGSTUBANDREFLECTORR.Chair,A.A.Kishk,K.F.Lee,C.E.Smith,andD.KajfezDepartmentofElectricalEngineeringCenterforAppliedElectromagneticSystemsResearch(CAESR)UniversityofMississippiUniversity,MS38677,USAAbstract—Widebandprintedrectangularslotantennasbackedwithreflectorsforunidirectionalradiationpatternsareinvestigated. AU-shaped tuning stub is used to improve the matching. Two differentfeeding mechanisms are introduced. A rectangular slot excited bymicrostriplinefeedwithaU-shapedtuningstubgivesanimpedancebandwidth of 110% (

Uncertainty analysis of the transmission-type measurement of Q-factor

The transmission-type measurement of the Q-factor of a microwave resonator is analyzed with a goal of establishing uncertainty limits on the unloaded Q-factor. The three major sources of systematic errors which are discussed are: the instrumentation inaccuracy, the inequality of the input and output coupling coefficient, and the effect of coupling losses. It is shown that, for tightly coupled resonators, the uncertainty of the transmission-type method is significantly increased.

A numerical study of a dielectric disk antenna above grounded dielectric substrate

An antenna made of a dielectric disk with a high permittivity mounted on top of a grounded dielectric substrate of low permittivity is analyzed. A numerical procedure based on surface integral equations, derived from the equivalence principle, is used to compute the natural resonant frequencies for the HEM/sub 11/ mode from which the radiation Q factor of the antenna is obtained. Then the radiation pattern of the antenna, operating at the resonant frequency evaluated previously, is computed with an electric dipole excitation located within the dielectric substrate under the dielectric disk. The effect of various parameters on the radiation characteristics of the antenna is studied, and presented in the form of diagrams. The low values of the radiation Q, combined with the high values of the dielectric Q and conductor Q, indicate that this antenna promises to be more efficient then the microstrip antenna. >

Linear fractional curve fitting for measurement of high Q factors

A procedure is described for a precision measurement of the unloaded Q factor and of the coupling coefficient for microwave resonators. The complex reflection coefficient data are processed by the linear fractional curve-fitting routine on a personal computer. >

Effect of fabrication imperfections for ground-plane-backed dielectric-resonator antennas

The effect of imperfections in terms of surface roughness which could lead to the introduction of air gaps between the dielectric resonator (DR) and the contacting conducting surfaces is investigated. Results of studies which illustrate the effect of air gaps upon the input impedance and resonant frequency of cylindrical dielectric resonator antennas (CDRAs) operating in the TM01 and HEM11 modes are presented.

PERMITTIVITY MEASUREMENT WITH A NON-STANDARD WAVEGUIDE BY USING TRL CALIBRATION AND FRACTIONAL LINEAR DATA FITTING

Modifications in the measurement of the complex permittivity are described, based on the transmission and reflection coefficients of a dielectric slab. The measurement uses TRL two- port calibration to bring the reference planes accurately to the sample surface. The complex permittivity as a function of frequency is computed by minimizing the difference between the measured and the ideal scattering parameters. An alternative procedure for determining the complex permittivity uses the fractional linear data fitting to a Q- circle of the virtual short-circuit and/or virtual open circuit data. In that case, the sample must be a multiple of one-quarter wavelength long within the measured range of frequencies. Comparison with results obtained by other traditional procedures is provided.