Johannes Müller

Directional Coupler Compensation With Optimally Positioned Capacitances

An accurate design synthesis for the phase velocity compensation in coupled line microstrip couplers by means of parallel capacitances is presented. In contrast to previous approaches, an a priori arbitrary placement of the capacitances along the coupled line structure is considered. By optimizing these positions, the directivity-bandwidth performance is significantly improved. Cases with two and three capacitances are treated extensively. The findings are used to generalize the compensation scheme to any number of capacitances. Throughout the analysis, the parasitic even-mode capacitance is taken into account using a realistic model. Several design examples are presented. Simulation results are confirmed by measurements. They compare favorably with those reported previously.

Stepped impedance microstrip couplers with improved directivity

This paper presents the design of microstrip couplers with improved directivity. In order to compensate for the unequal phase velocities of even- and odd-mode in inhomogeneous media, the coupler is subdivided into multiple sections. It is shown that by properly choosing the lengths, the impedances, and the coupling coefficients of the corresponding sections, the couplers directivity can be optimized. The design procedures can easily be applied using commercially available simulation software. Measurements confirm the design methodology and show good agreement with the simulation results.

Complex compensation of coupled line structures in inhomogeneous media

This paper presents a novel method for the compensation of unequal phase velocities of coupled line structures in inhomogeneous media. By means of a rigorous scattering parameter analysis, ideal values for compensating impedances, connected in series to one or several ports of the coupler, are found. It is shown that, by properly choosing the values of the impedances’ real and imaginary part, perfect isolation can theoretically be obtaind at any frequency. Simulation and measurements are compared to previously published results, demonstrating the advantage of this novel approach.

Sensitivity Analysis of Microwave Sensors for the Permittivity Measurement of Liquids

This paper presents an advanced microwave sensor for the complex permittivity measurement of liquids. By means of a rigorous mode matching technique, the sensitivity of the microwave sensor is analyzed and its structure is successfully optimized for the use at low frequencies.