Lionel E. Davis

Finite-element method with edge elements for waveguides loaded with ferrite magnetized in arbitrary direction

In this paper, finite-element method (FEM) formulations with edge elements for directly calculating the phase constants of ferrite-loaded waveguides with arbitrarily magnetized directions are presented. Dispersion characteristics are calculated for the partially-filled ferrite rectangular waveguide structure, where the dc field is in any arbitrary direction including parallel to any of the three axes. The variation of phase constants with the direction of dc magnetic field is illustrated. In order to solve the quadratic eigenvalue equation, which appears in the case where the magnetized direction is not parallel to the propagation, a simple and effective approach is proposed with no increase in the size of the matrices.

Microstrip and stripline ferrite-coupled-line (FCL) circulator's

Measurements on an 11-GHz four-port microstrip ferrite-coupled-line (FCL) circulator, employing a broad-band hybrid coupler and an improved air bridge, confirm that only a weak biasing field of 3-4 kA/m (35-50 Oe) is required. By cascading the S-matrices of the FCL section and hybrid coupler using signal flow graphs, the measured and predicted bandwidth and losses are compared for the first time. Simulated results for stripline FCL sections exhibit broader bandwidths than those obtained with microstrip FCL sections. Using such a structure, the simulated performance of a three-port circulator indicates that, in principle, bandwidths of at least 3:1 or 4:1 may be possible.

Microwave-tomographic system for oil- and gas-multiphase-flow imaging

Since the early 1980s, a number of electrical imaging techniques based on capacitance, resistance, or inductance measurement at low frequencies have been developed for the monitoring of industrial processes, such as oil- and gas-multiphase flows. In principle, microwave tomography would produce higher resolution images than these low-frequency techniques. But it has mainly been studied for medical applications over the past two decades and is less developed for industrial applications. In this paper, the development of an experimental microwave-tomography system intended for oil- and gas-flow measurements is described, which includes the hardware for data acquisition and the numerical algorithm for image reconstruction. The investigation of the system for the imaging of static?dielectric phantoms modelling oil- and gas-flow distributions is reported together with the images obtained at two different microwave frequencies: 2.5 GHz and 4 GHz. It has been demonstrated that images of the dielectric phantoms can be reconstructed using the system, with the images obtained at 4 GHz having better quality and higher resolution.

A Ka-band indium-antimonide junction circulator

Following a brief overview of the underlying theory, experimental results are presented for the first time showing circulator action in a semiconductor junction structure. An axially magnetized indium-antimonide disc fixed in a three-port finline structure and cooled to the temperature of boiling nitrogen, 77 K gives circulation across K/spl alpha/-band. For a dc magnetic bias of 0.73 T, a 15-dB isolation is recorded from 28 to 40 GHz, or a fractional bandwidth of at least 35%. Typical insertion loss is less than 1.5 dB from the WG22 reference plane at the test fixture ports. Continued operation above 40 GHz is predicted, but has not yet been measured. Measurement suggests that circulation is evident even where the effective propagation constant is imaginary, although better theoretical agreement is achieved when this is a real quantity. This new device makes millimeter-wave broad-band circulation a possibility and confirms the current model based upon the Drude-Zener approximation. A theoretical example is then given for a design operating to 140 GHz, yielding a fractional bandwidth of 110%.

Nonreciprocity and the optimum operation of ferrite coupled lines

The first full-wave normal-mode analysis of ferrite coupled lines (FCLs) magnetized in the longitudinal direction is presented in this paper. It is found that the tangential and axial components of the guided electric and magnetic fields undergo a different change in the process of reversing the direction of magnetization. These changes cause the same input wave to decompose into the eigenmodes of the FCL differently for different direction of magnetization and, consequently, cause the nonreciprocal behavior of the magnetized FCL. A new optimum nonreciprocal operation condition is obtained, and applications to FCL circulators built on microstrips and striplines are discussed.

Predicted performance of semiconductor junction circulators with losses

A study of the circulation properties of a gyroelectric medium consisting of a high-quality n-type semiconductor is given. Losses due to electron collisions are modeled by inclusion of the collision frequency nu /sub c/ in the relative permittivity tensor. Broadband low-loss operation of a semiconductor slotline junction circulator above the extraordinary wave resonance frequency f/sub res/ appears possible at near-millimetric frequencies. Larger, applied static magnetic fields enable narrowband low-loss operation at frequencies below the resonance at f/sub res/. A 40-GHz design is described for GaAs cooled to 77 K. The minimum inband insertion loss is 0.82 dB. InSb theoretically possesses still lower losses for a reduced applied magnetic field. An example at an operating frequency of 75 GHz in InSb is given. >

Broadband theoretical gyroelectric junction circulator tracking behavior at 77 K

The perfect circulation conditions for the gyroelectric circulator are given for a gyroelectric ratio with magnitude in the range zero to two. Values of this ratio above unity given in this paper correspond to the frequency regions where the effective permittivity is negative. The subsequent Greens function analysis employs the modified Bessel function. In accordance with the Drude model of semiconductors, a theoretical low-loss GaAs design is presented with a 20 dB isolation bandwidth of approximately 90% at an operating temperature of 77 K. The theoretical broadband circulation tracking behavior of this design is demonstrated for gyroelectric ratios which may exceed a magnitude of unity. The operating frequency range for this particular circulator design is below the extraordinary wave resonance frequency. In order to measure the microwave properties of the magnetised semiconductor disk, a two port analysis is performed based upon the Drude model of semiconductors.

Normal-mode analysis of ferrite-coupled lines using microstrips or slotlines

A normal-mode analysis of a pair of axially-magnetized ferrite-coupled lines (FCL) is presented for the first time. This normal-mode method, as opposed to the coupled-mode method, will permit optimization of propagation characteristics and impedance-matching. Also, the finite element solution of the normal modes can be used to obtain the field distribution to assist suitable placement of ferrite for device applications. Potential applications include a novel 4-port distributed microstrip circulator which may have advantages over junction devices at millimetric wavelengths. Optimum normal-mode conditions for the use of the FCL as a component in the distributed circulator are derived, and the design procedure for microstrip FCL is presented for the first time.

Sequential quadratic programming method for solution of electromagnetic inverse problems

In this paper, a new algorithm, namely, a reduced Hessian sequential quadratic programming (SQP) method, for solving electromagnetic inverse problems is proposed. The electromagnetic inverse problem is considered to be a constrained nonlinear programming. The reduced Hessian SQP method finds the solution of this constrained nonlinear programming by solving a sequential of quadratic programming subproblems. The reduced Hessian scheme is applied to reduce the requirement of computational memory of the basic SQP method for large inverse problems. Numerical results are presented to demonstrate the efficiency and accuracy of the proposed method, and some comparisons show that the proposed method has a better convergence and a faster speed than the previous methods.

Tunable microwave components based on dielectric nonlinearity by using HTS/ferroelectric thin films

This study provides the possibility of developing tunable microwave components based on the dielectric substrate nonlinearity, with the conducting surfaces made of a superconductor. The displacement vector, the dipole moment, polarization, polarizability, susceptibility, and relative permittivity concepts are used for ferroelectrics; and for superconductors, Bose statistics and the Gorter and the Casimir model for a two-fluid model, Londons equations, and the classical skin effect for the normal component of the current are used. A sinusoidal wave solution is found for a planar superconducting transmission line. This solution gives expressions for the phase velocity and attenuation coefficient which are used to characterize the tunability of microwave components. The measured data in the literature have been used to compute the relative phase velocities and phase shift per cm versus temperature and the dc bias electric field E (kV/cm). It is shown that with a ferroelectric film of thickness of 140 nm, with /spl epsiv//sub /spl tau//=2/spl times/10/sup 3/ and tan /spl delta/=10/sup -3/ phase shifts and attenuation of the order of tens of degrees per centimeter and 5.76/spl times/10/sup -3/ dB/cm, respectively, at 10 GHz, can be obtained with tens of millivolts at 4 K.