Performance of the Impedance Boundary Conditions for Eddy-Current Problems
Impedance boundary conditions are used to derive approximate expressions for the magnetic vector potential, field quantities, Joule losses and forces for conducting spheroids in the presence of circular turns carrying sinusoidal with time currents. Numerical results are generated to determine, by comparison with the results obtained applying the exact boundary conditions, the improvement achieved with the surface impedance model with respect to the simpler perfect conductor model and the range of validity of these models. The study performed is useful for evaluating various approximate numerical solutions for real world conductors
Benchmark computations of the fields, losses, and forces for conducting spheroids in the proximity of current-carrying turns
We derive exact analytical expressions for the quasi-stationary field quantities due to circular turns carrying current in the presence of conducting prolate and oblate spheroids. We use these expressions to generate numerical results of specified accuracy for the field quantities inside and outside the spheroids, as well as for the Joule losses inside them and the forces acting upon them. We present selected results in a normalized form for determined, extended ranges of the spheroid axial ratio, the ratio of the depth of penetration to the semi-minor axis, and the position of the inducing turns relative to the spheroids. The results constitute reference data for comprehensive comparisons of results from approximate volume methods or from boundary impedance models used for real-world conductors in the presence of nonuniform external magnetic fields.
Slotted Waveguide Antenna With Two Radiation Nulls
A design of a linear array antenna having two radiation nulls and otherwise omnidirectional radiation pattern is reported. A new configuration of a waveguide broad wall longitudinal slot doublet with positive and negative offsets with respect to the waveguide center line is used to design the array. This type of doublet produces two radiation nulls. Each of the null directions can be shifted at least up to plusmn25deg by proper choice of slot offset. Resonance and radiation characteristics of the proposed doublet are also presented. Finally, one linear array antenna consisting of seven doublets has been fabricated. Measurements show that the 15 dB null widths are 14.2deg and the null depths are at least 24.8 dB. Maximum gain ripple in the omnidirectional region is 1.75 dB.