Contents 1 Introduction
Nowadays microstrip technology is commonly used in antennas designated for application in modern electronic devices, due to the requirements of low-cost, low-profile and light-weight solutions, allowing for integration of passive and active components [1]. In some specific applications a broad impedance bandwidth and a broadside beam are essential, for example in such applications as FMCW radar systems. The progress made by researchers over the years in the antenna design field allows for enhancement of various properties with the use of proper design techniques. The bandwidth in terms of reflection coefficient can be improved using for example appropriate excitation techniques or thick dielectric substrate having low permittivity [2], whereas the broadside beam in a wide frequency range might be achieved with the use of a corporate feeding network [3], symmetrical layout of the antenna or utilizing broadband phase shifters. The 180-deg phase shifters are used in planar antenna arrays, which utilizes ‘through-element’ series feeding technique, in order to feed the pairs of antenna subarrays with appropriate signals. Such a phase shift can be performed with the use of a meandered transmission-line section, a broadband Schiffman phase shifter [4] or alternatively with the use of slot-coupled power divider. Such power dividers were previously presented in [5] [8]. In [9] a 4-way power divider has been proposed, in which the input open-ended microstrip line was slot coupled to the pair of microstrip lines, parallel to the input line and located on the same metallization layer. The signal obtained at the ends of the output lines have equal amplitudes and are out-of-phase in a broad frequency range.
