Fred E. Gardiol

Reflection of an open-ended coaxial line and application to nondestructive measurement of materials

Reflection measurement techniques require a way to correlate measured reflection factors with the permittivity of the materials. This relationship is derived for the open-ended coaxial line propagating the TEM mode.

A Dynamical Radiation Model for Microstrip Structures

Publisher Summary This chapter deals with the models currently used to improve the quasistatic approximation of microstrip structures. The chapter presents a brief review of the microstrip fundamentals; a comprehensive account of these dynamical models. A particular model based on the calculation of the electric surface currents in the structure is introduced in the chapter. The Greens functions involved are constructed and numerically evaluated using the exact formulation for dipole potentials in a stratified medium. New evaluation techniques developed for Sommerfeld integrals permit a very accurate computation of Greens functions. Their properties are then extensively discussed; the validity of the quasistatic approximations is checked and the importance of the surface wave is pointed out in detail. Finally, the integral equations for the currents are set up and solved by numerical methods in that also give several practical applications. The chapter concludes the model is that the surface currents on the antenna are directly determined and the near field can then be determined easily, that becomes most important when microstrip structures are utilized as radiators or biological applicators.

The Reflection from an Open-Ended Rectangular Waveguide Terminated by a Layered Dielectric Medium

The measurement of reflection from an open-ended waveguide is a simple and nondestructive technique for determining the dielectric properties of materials. A flange-mounted waveguide is considered, the flange being pressed on an unknown material which may be of finite or infinite thickness. The relationship linking the reflection coefficient to the dielectric properties is obtained from a theoretical analysis of the electromagnetic field in the vicinity of the aperture. The theory includes the effects of both cross polarization and higher order modes. An integral equation is obtained, the kernel of which is the dyadic Green function in each medium. The method of characteristic modes is used for the numerical computation. The theoretical results are in good agreement with experimental measurements. Futhermore, a simple and handy technique for data inversion is provided.

A small microstrip patch antenna with a convenient tuning option

The paper addresses two aspects of resonant microstrip patch antennas, namely, miniaturization and resonant frequency tuning. First, a patch geometry which allows a controllable size reduction over a limited range is presented. The basic shape is circular with slits cut into it. Modification of the slit geometry leads to both linear as well as circular polarized (CP) operation. Second, the use of another patch of a specific shape as a superstrate layer in a stacked configuration allows tuning over a relatively large frequency range as compared to the patch bandwidth. Tuning is accomplished by a simple rotation of the superstrate layer. The use of another superstrate layer allows tunable CP operation. Details on the antenna characteristics have been worked out for two examples, and computations have been compared with measurements where possible. Some design guidelines have also been included.

Higher-Order Modes in Dielectrically Loaded Rectangular Waveguides

The problem of propagation in a waveguide containing E-plane slabs of dielectric was considered previously by several authors. However, their treatment was limited to the TE/sub mo/ modes. A more general and complete derivation of the dispersion equations for all the modes existing in these structures is presented here. It is shown, on the basis of theoretical and experimental results, that the frequency bandwidth is much smaller than the values previously obtained, considering only the TE/sub mo/ modes, for many cases of practical interest

Simple Nondestructive Method for the Measurement of Complex Permittivity

A simple nondestructive method is presented for the measurement of the complex permittivity ?r of materials. The reflection coefficient ? of a flanged open-ended rectangular waveguide placed, next to a flat large sample is computed by a variational method as a function of the dielectric constant. Computer-generated charts and an optimization computer program are elaborated. ?r can then be easily determined from the measured amplitude and phase angle of the reflected wave in the waveguide. The method is best suited for materials having high dielectric constants and large losses.

Improvement of a Class-C Transistor Power Amplifier by Second-Harmonic Tuning

Considerations for the effects of second-harmonic reactive terminations on the performances of a UHF class-C transistor power amplifier are presented. An experimental amplifier circuit design using coupled-TEM-bar transmission lines is described. This circuit can vary the fundamental and the second-harmonic impedance terminations of the amplifier independently. With this amplifier circuit, significant improvement in the performance characteristics of a class-C power amplifier were achieved by presenting proper values of second-harmonic reactive terminations, both at the input and the output of the transistor.

An efficient method to determine Green's functions of a two-dimensional photonic crystal excited by a line source - the phased-array method

A novel and efficient method to determine Greens functions in photonic crystals (PCs), i.e., the phased-array method (PAM), is presented. The PAM is a combination of the plane-wave method and the array-scanning method, which is both more flexible and computationally faster than the eigenmodes expansion method. A complete derivation of the electric- and magnetic-field Greens functions associated, respectively, with an infinite electric and magnetic current line exciting a two-dimensional PC is given. Although the developments are presented only for a line source, the PAM can be extended to a dipole source. Thus, the PAM represents a promising method for the analysis of printed-circuit elements or antennas on PC materials. Numerical results for the Greens functions are shown for different positions of the source and a discussion about radiation patterns, asymptotic behaviors, and convergence characteristics is proposed.

Computer Analysis of E-Plane Resonance Isolators

E-plane ferrite resonance isolators are commonly used in most medium-power waveguide systems, allowing to effectively decouple successive stages. The study of propagation in this type of structure leads to a set of transcendental equations, for which exact theoretical results have not been available so far. The design of these devices has been done experimentally, which is time consuming and does not allow for achievement of optimal performance due to the large number of parameters involved. The present study considers the TE/sub m0/, modes in an isolator structure. A matrix formalism is used to derive the dispersion relation, which is then solved with the help of a computer program. Experimental results for a simple structure show good agreement with the computed values. The influence of ferrite parameters on the isolation and forward losses is presented. Higher order modes in the structure and the methods to avoid them are discussed. Finally, the field distribution in the device is determined, showing a large concentration of the fields within the loading material.

Near fields of microstrip antennas

The paper presents an iterative method for the computation of the near fields of single layer microstrip patch antennas. A judicious combination of the advantages of the MPIE (mixed potential integral equation) method, the FFT algorithm and the biconjugate gradient leads to an efficient numerical solution. The scheme can deal with patches of arbitrary shapes and is capable of dealing with a very large number of unknowns. Near field computation is done in a straightforward manner by computing the weighted near fields. For illustration, three patches belonging to the family of generalized annular sector have been chosen. Computed results of input impedances with a coaxial probe excitation have been compared with measurements. Computed near field amplitudes have also been compared with measured results where possible. Computed far field characteristics have been correlated with the near field information. >