## 1 Introduction  
Synthetic Aperture Radar (SAR) has become nowadays one of the most important fields of remote sensing. Due to the penetration of the radar waves, it becomes a versatile technique for acquisition of information, weather independence, and it delivers complementary data to optical systems. As an active radar sensor with its own target illumination, it allows day and night operational capability. SAR provides high-resolution imaging whose geometric resolution can be independent of the acquisition distance. By means of exploiting the information given by the physical properties of the electromagnetic waves such as frequency, polarization, amplitude or phase, SAR systems offer high quality imaging data, covering a wide range of remote sensing applications. It is used for terrain topography, providing urban mapping and digital elevation models [1], oceanography, agriculture, traffic control [2] and ship detection [3], as well as for environment studies including deforestation [4], glacier and disasters monitoring, among many others. Its broad scope of research has greatly aroused the interest of the scientific community, and it has expanded its use for defense and commercial purposes. Especially advantageous are the applications of SAR for lower frequencies [5, 6] such as P-Band (216-450 MHz) or L-Band (1-2 GHz) [7]. Due to the penetrating nature of electromagnetic waves at higher wavelengths and by means of interferometry, SAR becomes an interesting way to monitor forest density and estimate forest height by using tomography [8], as well as to perform geology and glaciology studies for the measurement of snow, ice layer thicknesses and soil moisture. Furthermore, SAR polarimetry allows the use of polarization diversity in order to identify different scattering mechanisms, enhancing this way the retrieval of information [9]. As a remote sensing system, SAR sensors are typically on board flying platforms like satellites or aircraft depending on the application itself, which leads to different coverage areas and image resolutions. Well-known SAR spaceborne missions are BIOMASS, Sentinel, TerraSAR-X and TanDEM-X, which is an add-on of the TerraSAR-X system that allows the generation of global digital elevation model data of the Earth with an unprecedent accuracy [10]. The Microwaves and Radar Institute of the German Aerospace Center (DLR) has led the development and implementation of these two spaceborne missions, TerraSAR-X and TanDEM-X. Its research work in the field of remote sensing by means of Synthetic Aperture Radar is widely known. For the increasing demand of SAR applications and in order to support future spaceborne systems, airborne SAR provides an essential scope of research [11]. As a more flexible system, it is employed by the DLR in order to study signal processing concepts, to experiment with innovative techniques and methods, and to develop and test new components and hardware. The German Aerospace Center operates the so-called F-SAR, an enhanced airborne multi-frequency SAR system [12]. In order to take advantage of the aforementioned capabilities of SAR imaging at lower frequency bands, a new dual-polarized phased array antenna system in L-Band for the next generation DLR airborne SAR sensor is required, which becomes also relevant for upcoming spaceborne SAR missions [13]. Due to the space limitations on the aircraft, not only a solution that fulfills the electrical and airworthiness requirements is needed, but also a high degree of compaction has to be achieved, in order to exploit the restricted available area on the airplane [14; 15]. This leads to a careful selection of the implementation technology and innovative configurations for the antenna design [16; 17]. As a phased array system and in order to enhance its radiation properties, some design considerations have to be made to reduce the spacing between the array elements, and at the same time minimize the antenna interactions.