In this work, the impact of the antenna carrier, an aerodynamic attachment structure that allows the antenna installment on the aircraft, is analyzed. The resulting polarization-dependent edge diffraction effects of a flight-model L-band phased array antenna, embedded in the antenna carrier, is experimentally validated. Measurements show a pattern distortion in the vertical polarization along with a gain reduction of 1 dB in the elevation plane, in which the beam steering is performed, that also corresponds to the E-plane, thus becoming this polarization more sensitive to edge diffraction effects. Further analysis is performed increasing the electrical size of the antenna carrier diameter up to 11.0. A low-profile and planar solution, based on Electromagnetic Bandgaps, is presented, by which the induced surface currents on the antenna carrier for the vertical polarization are mitigated, thus reducing the impact of edge diffraction effects without interfering in the performance of the horizontal polarization. Thereby, the desired radiation characteristics for both polarizations can be fulfilled regardless of the antenna carrier structure.

Synthetic Aperture Radar (SAR) has become, nowadays, one of the most important techniques in remote-sensing, and the increasing interest in Earth monitoring reinforces this trend. Next-generation SAR sensors will enhance the radar resolution capabilities by means of digital beamforming (DBF) techniques along with multistatic systems. In order to support the technological development of future spaceborne SAR missions, airborne sensors become an essential scope of research. Thereby, future airborne SAR systems demand enhanced DBF capabilities that involve phased array antennas with a high density of array elements. However, the antenna aperture size is significantly limited in airborne applications, which leads to low-profile and highly integrated antenna solutions, becoming a more challenging task for lower frequency operations such as L-band. In this work, a compact L-band dual-polarized multilayer phased array antenna with beam steering in elevation, developed for the next-generation German Aerospace Center (DLR) airborne SAR system, is presented. The proposed design makes use of truncated cavities to improve the array element isolation and provides 66% more antenna elements than the previous L-band phased array of the current DLR airborne SAR sensor with the same antenna aperture size. Measurements of a manufactured prototype show an antenna bandwidth of almost 20%, matching levels better than 17 dB, up to 15 dB gain, and cross-polarization suppression values higher than 35 dB. Thus, the proposed work will allow the application of advanced DBF techniques in the upcoming first pulsed bistatic L-band airborne SAR sensor.

A novel planar phased array of 5×4 multilayer dual polarized aperture coupled stacked patch elements, operating in L-band, with beam steering in elevation and excited with an asymmetric amplitude distribution in azimuth, is presented in this work. The proposed design exploits the restricted available antenna size (2.45λ0×1.97λ0) maximizing the number of array elements by means of an interelement spacing of 0.48λ0 and the use of truncated walls. The measurements of the manufactured prototype show an antenna bandwidth of almost 20%, polarization isolation greater than 23 dB and directivity values above 15 dB. Despite the close proximity among the array elements, the measured coupling levels between the feeding ports are lower than −20 dB for the center frequency of operation 1.325 GHz, which makes the proposed work suitable for airborne Synthetic Aperture Radar systems, where a high degree of integration is required.

In this work, a novel rotation approach for the antenna elements of a linear phased array is presented. The proposed method improves by up to 14 dB the cross-polarization level within the main beam by performing a sequential 90° rotation of the identical array elements, and achieving measured cross-polarization suppressions of 40 dB. This configuration is validated by means of simulation and measurements of a manufactured linear array of five dual-polarized cavity-box aperture coupled stacked patch antennas operating in L-Band, and considering both uniform amplitude and phase distribution and beamforming with amplitude tapering. The analysis is further extended by applying and comparing the proposed design with the 180° rotation and non-rotation topologies. This technique is expected to be used for the next generation L-Band Airborne Synthetic Aperture Radar Sensor of the German Aerospace Center (DLR).

An extremely compact design of a power divider in Substrate Integrated Waveguide (SIW) and fed by means of a Coplanar Waveguide (CPW) line is presented in this paper. The proposed solution presents a relative low insertion loss and good adaptation (measurements show an S11 better than −20 dB for the centre frequency and below −15 dB for a 30% bandwidth) in X-band, despite the use of a substrate with a high dielectric permittivity (εr = 9.8). In order to validate the design, various topologies of the power divider are manufactured, measured and compared with the simulated data. These power splitters will be implemented in the feed network of a phased array for an airborne radar system with Digital Beamforming capabilities operated by the German Aerospace Center.

Phased array antennas are often built from sub-arrays with identical or symmetrical layout. At an early project stage, performance verification measurements of the sub-array are valuable to proof the single module design. However, the characteristics of the final antenna are questionable without further processing. This work presents a concept that is based on far-field measurements of a sub-array in a Compact Antenna Test Range (CATR) in conjunction with planar near-field (PNF) processing to synthesize the entire phased array antenna characteristics. The procedure is explained with an example of a dual linear polarized L-band planar phased array antenna for an airborne synthetic aperture radar application. It is shown that the measured sub-array can be complemented by the synthesized twin to evaluate the characteristics of a final antenna that is not yet available in this form. The resulting performance of the synthesized entire phased array is presented and compared with simulations. The presented post-processing method would be beneficial to characterizing radiation patterns of large phased arrays by measuring only sub-arrays in a limited test-zone with any measurement principle.

In this work, a new ultrawideband grounded coplanar waveguide (GCPW) to substrate integrated waveguide transition is presented. The proposed design improves the performance of the triangular tapered slot transition, by providing a return loss level better than 28 dB in the X-band frequency range (with enhancements of up to 29 dB at higher frequencies in such a band) and a flatter response for the insertion losses. The analysis of the new transition is performed using a dielectric material with high permittivity (ε r = 9.8), which leads to an extremely compact implementation. In order to validate the proposed design, several back-to-back transitions with different lengths were manufactured, measured, and compared with the simulated data. Further experimental results, after performing an additional thru-reflect-line calibration for deembedding undesired effects of involved connectors, are also shown.

The Microwaves and Radar Institute at the German Aerospace Center (DLR) operates an airborne versatile multi-frequency SAR sensor (F-SAR). Due to the applications of remote-sensing imaging at low frequencies, a new dual polarized L-Band antenna system for the next generation sensor is presented in this paper. Despite the restricted size of the used aircraft, the new antenna optimizes the available space in the antenna carrier by providing more antenna elements, in comparison with the current L-Band sensor, without increasing the inter-element mutual coupling and thereby enhancing the radiation properties of the antenna.

The Microwaves and Radar Institute of the German Aerospace Center (DLR) is known for its consistent work on the field of airborne Synthetic Aperture Radar and its application. Currently, the Institute is developing a new advanced airborne SAR system, the DBFSAR, which is planned to supplement its operational F-SAR system in near future. The development of DBFSAR was triggered by the various evolving digital beamforming (DBF) techniques for future space-borne SAR systems and the need for an airborne experimental platform for preparation of such missions. Additionally, there is a demand for very high resolution SAR imagery, which cannot anymore be fully satisfied with the existing F-SAR system. This paper should give an overview over the current status and performance of the DBFSAR system, including interferometirc results from test flights performed in spring 2017.