Ruediger Vahldieck

Analysis and Simulation of a 1-18-GHz broadband double-ridged horn antenna

A 1-18-GHz broadband double-ridged horn antenna with coaxial input feed section is investigated. For the ridged horn antenna it is found that the radiation pattern, contrary to common belief, does not maintain a single main lobe in the direction of the horn axis over the full frequency range. Instead, at frequencies above 12 GHz, the main lobe in the radiation pattern starts to split into four large side lobes pointing in off-axis directions with a dip of up to 6 dB between them along the main axis. Although this type of horn is the preferred test antenna, which is in common use for over four decades, no explanation for this unwanted behavior was found in the open literature. To investigate this phenomenon in detail, a method of moments approach has been adopted to simulate the complete antenna system. The simulations are in good agreement with the measurements over the 1-18-GHz operational bandwidth and indicate that the use of this type of horn antenna in EMC applications remains questionable.

A closer look at reverberation Chambers - 3-D Simulation and experimental verification

This paper presents three-dimensional (3-D) full-wave simulations of the electromagnetic field inside a medium-sized reverberation chamber. A frequency-domain method-of-moments approach based on the electric field integral equation is used. A synopsis of the computational challenges particular for reverberation chambers is described and a detailed overview on the chamber modeling procedure is given. The electric field inside the chamber is computed and the influence of small geometric details and asymmetries is investigated as well as the effect of different excitations and stirrers. It is demonstrated that a statistics-based validation of reverberation chamber simulations is insufficient. To validate simulation results, therefore extensive near-field measurements inside the prototype reverberation chamber are performed. The complete 3-D reverberation chamber simulation, considering stirrers, door, and various practical excitations, accurately predicts the fields within the chamber in the important lower-to-medium frequency range and thus represents a reliable tool facilitating reverberation chamber optimization.

Comparison of the Radiation Efficiency for the Dielectric Resonator Antenna and the Microstrip Antenna at Ka Band

The radiation efficiency of a dielectric resonator antenna (DRA) and a microstrip antenna (MSA) at Ka band is investigated numerically and experimentally. For direct comparison, one cylindrical DRA and one circular disk MSA were designed with similar feeding networks for operation at around 35 GHz. The efficiency of both devices was measured using the directivity/gain (D/G) method and the Wheeler cap method, in both of which the losses in the test system and the feeding structure were taken into account for calibration purposes. A good agreement between measured and simulated results for both methods is found, when considering the effect of the sampling interval and cross-polarization in the D/G method and the effect of the metallic cap size in the Wheeler cap method. It is finally demonstrated that the radiation efficiency of the DRA is significantly higher than that of the MSA at millimeter wave frequencies.

A flexible wearable antenna

The paper describes the development of a flexible wearable antenna used in military and police work. The design is based on numerical simulations using commercial software. Simulations taking into account the curved-surface profile of the antenna and differently sized human bodies are presented. Experimentally obtained return loss and radiation patterns are shown in comparison with the computed results. Measured return loss better than 7 dB has been achieved across the frequency band of 360-460 MHz. The superiority of the antenna with respect to conventionally used antennas in the abovementioned applications is demonstrated.

The Trapezoidal Dielectric Resonator Antenna

The operational concepts of the trapezoidal dielectric resonator antennas (DRAs) are presented and their performance is studied theoretically and experimentally. The rectangular parallelepiped is a special case of the more general trapezoid and therefore a comparison between both is useful in order to highlight their similarities and differences. It is shown in this paper that the inverted trapezoidal DRA exhibits a significantly larger impedance bandwidth than the rectangular parallelepiped, while the rest of their properties remain similar. Based on that, two versions of a probe-fed wide-band linearly polarized (LP) DRA are designed by combing the resonances of the first two lower-order modes of the trapezoidal DRA. The resulting impedance bandwidth exceeds 55% (S11 < -10 dB) and the patterns are broadside with low cross-polarization. The antennas are simple to fabricate and they can be easily made to be mechanically stable. The numerical analysis was performed with a commercially available FEM solver and an in-house developed finite-volume-time-domain (FVTD) code. The measurements confirmed the results of the simulations.

Efficient procedures for the optimization of defects in photonic crystal structures.

Seven different stochastic binary optimizers--based on the concepts of genetic algorithms and evolutionary strategies--are developed, applied to determine defect locations in several photonic crystal structures that serve as test cases, and compared by extensive statistical analysis. In addition to the stochastic optimizers, a quasi-deterministic optimizer based on an algorithm inspired by hill-climbing algorithms was implemented. The test cases include the prominent 90 degrees photonic crystal waveguide bend and a photonic crystal power divider. The analysis of the results shows that many different photonic crystal structures with high transmission may be found for any operating frequency. All of the eight optimizers outperform standard codes-because they maintain an incomplete fitness table-and find the global optima with a high probability even when the number of fitness evaluations is much smaller than the number of potential solutions contained in the discrete search space. Based on the incomplete fitness table, an algorithm to estimate bit-fitness values is presented. The bit-fitness values are then used to improve the performance of some algorithms. The four best algorithms-an extended microgenetic algorithm, two mutation-based algorithms, and the quasi-deterministic algorithm inspired by hill-climbing algorithms-are considered to be of high value for the optimization of defects in photonic crystals and for similar binary optimization problems.

Dual Mode Slot Coupled Cylindrical Dielectric Resonator Antenna

A simple slot-coupled cylindrical dielectric resonator antenna (DRA) with wideband characteristics is studied theoretically and experimentally. In previous studies, the slot has been designed to radiate at the same frequency as the fundamental HEM11delta mode. In this paper, the partial independence of the slot mode from the DRA mode is discussed and a simple design is described, in which the slot mode and the DRA mode are offset, allowing an impedance bandwidth of 27%

Model-based parameter estimation (MBPE) for metallic photonic crystal filters

An efficient method for the accurate computation of the response of photonic crystal filters is obtained when model-based parameter estimation (MBPE) is combined with accurate field solvers. In this paper, MBPE is combined with MMP and MAS and the results are compared with results obtained from a commercial field solver. When metals are present in photonic crystal filters, strong material dispersion at optical frequencies causes nonlinearity of the filter response. It is demonstrated that MBPE is still useful although it is originally designed for linear systems.

An LTCC-Based folded Rotman lens for phased array applications

A novel planar Rotman lens design for LTCC (Low-Temperature-Cofired-Ceramics) integration is introduced. By folding the lens about the middle plane and utilizing the multilayer features (3D) of LTCC, extremely compact Rotman lenses can be achieved which are attractive for applications in the lower microwave range. Simulation results for the folded and unfolded Rotman lens show very similar results.

Broadband method of moment simulation and measurement of a medium-sized reverberation chamber

In this paper, we present three-dimensional full-wave simulations of the electromagnetic fields inside a medium-sized reverberation chamber. For the simulation, a method of moments based numerical approach was employed in the frequency domain. A short synopsis of the computational challenges particular for reverberation chambers is described and a detailed overview on the modeling procedure of the chamber is given. The electric and magnetic fields inside the chamber as well as the current density on the walls, the stirrer, and the antennas are computed and visualized. Different excitations of the chamber are simulated and parameters such as the rotational stirrer position or the influence of small geometric details are investigated. For the purpose of validation, extensive measurements inside a real reverberation chamber were taken and compared against the simulated results.