Ralf Stephan

An Eigen-Analysis of Compact Antenna Arrays and Its Application to Port Decoupling

Placing the radiators of antenna arrays closer than aggravates the problem of power mismatch. Based on efficiency considerations, a general analysis of this effect is presented, putting forward a simple tool to quantify, compare, and optimize the performance of antenna arrays. This analysis is not restricted with respect to the number of radiators or the degree of compactness. In order to improve power matching, a systematic approach for the design of lossless decoupling and matching networks based on 180 directional couplers is suggested for up to eight radiators. Implications of network losses, which have not yet received appropriate attention by researchers in the past, will be analyzed and discussed by means of a manufactured three-element prototype array.

Two-dimensional electron gas based actuation of piezoelectric AlGaN/GaN microelectromechanical resonators

Free-standing piezoelectric AlGaN/GaN beam resonators have been prepared on silicon substrates. The two-dimensional electron gas at the interface of the III/V heterostructure has been employed to act as back electrode for the piezoelectric active layer. The fundamental mode as well as higher order resonant modes of flexural vibration has been excited piezoelectrically and analyzed using optical laser–Doppler vibrometry. The experimental investigations were carried out under normal ambient conditions. The specific piezoelectric actuation scheme is described and the dependence of the measured resonant frequencies between 0.2 and 8.1 MHz on geometry and material parameters is investigated.

Strain- and pressure-dependent RF response of microelectromechanical resonators for sensing applications*

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. A semiconductor fabrication process has been applied to prepare resonant AlN and SiC beams operating at frequencies between 0.1 and 2.1 MHz. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. For systematic studies of the resonant frequencies and quality factors, the induced voltage was measured using time domain and frequency domain techniques. Resonator geometry, material and ambient pressure were varied to attain a generalized understanding of the RF performance. The dependence of the resonant frequency on tensile axial strain has been derived analytically and extended to include highly strained beams. Based on these formulas, accurate detection of the residual layer strain after fabrication is presented. To describe the quality factor a chain of beads model has been applied successfully. The influences of the beam width and the pressure-dependent viscosity on the model parameters are analyzed.

Pulsed mode operation of strained microelectromechanical resonators in air

A pulsed mode magnetomotive operation of micro- and nanoelectromechanical devices in air is demonstrated, where viscous damping determines the quality factor of the device. An enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed. Internal strain is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates. Comparing unstrained and strained resonators, an increase of the quality factor by one order of magnitude from about 30 to 300 was achieved. This increase will improve the sensing performance of such resonant structures for an operation in ambient environment.

Broadband Decoupling and Matching of a Superdirective Two-Port Antenna Array

Decoupling and matching networks may be used to improve the performance of compact antenna arrays where mutual radiator coupling has caused a degradation of the diversity capabilities. A popular network consists of a 180deg rat-race directional coupler, which decouples the antenna ports, followed by impedance matching networks at each port. Researchers, however, usually neglect the presence of losses both within the antenna array and the decoupling and matching network. For this reason, we have built various narrowband and broadband matching networks and compare their performances with the help of calibrated far-field measurement data.

60 GHz Ultrawideband Polarimetric MIMO Sensing for Wireless Multi-Gigabit and Radar

Polarimetric radio wave processing becomes of increasing interest for very high-data rate wireless transmission and for short-range radar at millimeter-waves (mm-W). This goes along with the huge bandwidth of 7 to 9 GHz, which is available worldwide in the 60 GHz unlicensed band. In this paper, we propose a 60 GHz ultra-wideband (UWB) polarimetric multiple-input-multiple-output (MIMO) sensing system architecture and polarimetric signal processing for short-range communications and radar. Demonstration measurements were made by using an UWB radar interface. By measurements in multipath rich environments it is demonstrated that tap-wise polarimetric filtering in delay domain can enhance the 60 GHz link budget by filtering some paths and then reducing shadowing due to human activity. Additionally, optimum MIMO polarimetric filtering is proposed to reduce heavy clutter for mm-W radar, increasing by about 30 dB the signal-to-clutter-plus-noise-ratio.

Design and application of dielectrically scaled double-ridged horn antennas for biomedical UWB radar applications

Ultra-wideband sensing begins to play an important role in biomedical diagnostic systems. Promising and relevant applications include remotely monitored vital functions as well as the characterization of tissues and organs. The acquisition of such physiological signatures requires small and radiation-efficient antennas, designed for ultra-wideband frequency operation. We have developed physically small and adjustable double-ridged horn antennas with which we could demonstrate the specific advantages of miniaturized, dielectrically matched sensor elements in a direct mode compared to remote sensor applications. As a logical consequence of these results, we have considered to replace the lossy high-permittivity liquid by low-loss high-permittivity solid ceramic material to improve the degree of miniaturization and the radiation efficiency further. Some unexpected peculiarities related to this approach are discussed.

Physically small and adjustable double-ridged horn antenna for biomedical UWB radar applications

Biomedical applications of ultra-wideband radar promise a very important means to remotely characterise tissues and organs. The acquisition of such physiological signatures requires small and efficient antennas, designed for ultra-wideband frequency operation. We have designed and characterised physically small and adjustable double-ridged horn antennas for frequencies from 1 to 10 GHz. The miniaturisation of the radiating elements was accomplished by immersion into a high permittivity liquid dielectric. The effect of dielectric scaling on size, input matching, radiation patterns, and gain has been evaluated by comparison with a double-ridged horn antenna designed for operation in air.

Eigenmode decoupling of miniaturised diversity antennas using compact quasi-lumped networks

This paper investigates a three-port antenna array with an element separation of lambda/10. A compact decoupling and matching network, realised by quasi-lumped elements, compensates for the problem of mutual coupling between the radiators. One special feature of this network is the excitation of the eigenmodes of the antenna array, which results in a broad decoupling bandwidth. The physical properties of such a system as well as practical realisation strategies are discussed. Measured data are presented for the power matching and the efficiency of the system.

Compact planar L-band antenna arrays with optimal diversity performance

At L-band (1-2GHz) due to significantly large free-space wavelengths, compact antenna arrays with small inter-element separation i.e., d < λ/2, are a suitable choice for overall size reduction. However, mutual coupling becoming prominent in compact arrays results in a degradation of the diversity degrees-of-freedom, which are required for beamforming capabilities in modern receivers. In this paper we discuss a potential approach to mitigate this limitation. We present empirical results for an eigenmode analysis applied to the radiation matrices of compact planar arrays, derived from the far-field integration of complex realised-gain matrices. Furthermore, optimal arrangements for compact planar arrays with respect to the highest possible value of minimum eigenmode efficiency are discussed. It could be shown that planar arrays have higher efficiencies and lower radiation correlation, hence better diversity degrees-of-freedom than linear arrays, particularly in a compact configuration.