Sabine Dieter

Reconfigurable Folded Reflectarray Antenna Based Upon Liquid Crystal Technology

A reconfigurable antenna based on the liquid crystal technology is presented in this paper. The antenna comprises a planar lower reflector with an incorporated feed at its center and a polarizing grid on top as an upper reflector. The lower reflector is utilized to collimate the beam and to twist the polarization. The polarizing grid selects the polarization for the transmission and reflects the orthogonally polarized waves toward the lower reflector. Combining reflector elements with a polarizing grid allows performing additional phase adjustment on the upper reflector for beam steering. Reconfigurability is maintained by the upper reflector, in which a liquid crystal mixture is used as a tunable substrate. The liquid crystal layer is tuned with a bias voltage configuration to obtain an appropriate phase adjustment for the beam steering. As a proof of concept, the beam steering capability of the antenna is demonstrated by steering the main beam to -6°, 0 °, and 6 ° at 78 GHz. The measured gain at 78 GHz is 25.1 dB. The proposed antenna configuration is a promising candidate for reconfigurable, high-gain, low-profile, and low-cost antennas.

High-Resolution Probes for Near-Field Measurements of Reflectarray Antennas

This letter proposes three different near-field-measurement probes in order to characterize individual patches on the surface of reflectarray antennas, both in phase and amplitude. The first realized structure is a dielectrically filled waveguide probe, the second is a probe ending in a dipole structure, and the third is a substrate-integrated waveguide probe. Requirements for near-field probes in this application are high resolution and reduced influence of the probe on the examined structure. The ability of the presented probes has been investigated by simulations and validated with measurements at a frequency of 35 GHz at reflectarray structures, placing the probes a few millimeters above the antenna surface.

Realization and characterization of a 77 GHz reconfigurable liquid crystal reflectarray

A tunable microstrip reflectarray with beam reconfiguration capability at 77 GHz is presented. Tunablility is achieved by using anisotropic liquid crystal as RF-substrate, tunable by a biasing voltage below 15V. The reflector has been characterized by utilizing a lens setup forming a gaussian beam. The reflector properties has been extracted in terms of its reflected phase and magnitude: While it is possible to tune about 280. of phase, the associated losses are still quite high. Nevertheless, pattern measurements of the reflectarray showed distinct beam collimation towards different angles when appropriate DC control voltages are applied.

Characterization of Reconfigurable LC-Reflectarrays Using Near-Field Measurements

A new measurement method for characterization of liquid crystal reflectarrays (LC) is presented. The phase and amplitude characteristics are determined by near-field measurements close to the antenna surface, using high resolution probes, which have been developed to resolve individual patches on the reflectarray. Measurement results showed a phase angle range of 280° of individual detected LC patches. The measurement setup and corresponding measurement results at 35 GHz are presented. The procedure used for measurement automation is also explained.

A 77 GHz Near-Field Probe with Integrated Illuminating Waveguide used for Reflectarray Characterization

Abstract In this paper, a near-field probe with integrated illuminating waveguide for frequencies around 77 GHz is introduced. For such high frequencies, alignment of transmitter and receiver probes in the near-field measurement setup are difficult. So the purpose of this work is to integrate both devices into one setup for an easy and reproducible measurement procedure. The measurement probe is used to characterize individual patches on the surface of reflectarray antennas, placing the device a few millimeters above the antenna surface. This requires a high spatial resolution and a insignificant influence of the probe on the examined structure. The ability of the developed probe has been investigated in simulations and verified in near-field measurements at reflectarray structures for 77 GHz. In this extended paper version, the setup is used to investigate a test-array with polarization twisting elementary cells and the lower reflector of a folded antenna, whose characteristics are based on a bifocal design process.

Investigations of advanced folded reflectarray antennas

This paper reports on recent research on folded ref-lectarray antennas done at the University of Ulm. The first area of work deals with the investigations of reflector elements with improved phase angle range and the influence of different effects on bandwidth and antenna gain including cell type, size, or substrate layer structure. A second area of work is the precise characterization of reflector elements, both with respect to the phase angle as well as the polarization twisting. Two methods have been developed for such characterization at frequencies around 77 GHz, one based on a quasi-optical Gaussian beam setup, another one based on near-field probing.

Planar lens using embedded quasi-lumped-element stripline filters

Lenses are an alternative antenna type for narrow beam antennas. Dielectric lenses, however, typically are relatively thick, heavy, and complicated to fabricate due to their specific contour. A possible solution for this problem are planar printed antennas which are thin, lightweight, and can be fabricated easily. This paper describes the design of a planar lens based on an array of slot-coupled planar filter structures; the required phase delay is adjusted by different center frequencies of the respective filters. An example lens at 35 GHz has been designed, fabricated, and tested.