Contents I. Introduction
Microstrip antenna arrays are commonly used in modern telecommunication and radar systems offering low cost, low weight, and easiness of integration with hardware. Typically, corporate feeding networks are used in such antennas to provide in-phase excitation of all elements in wide frequency range having, however, relatively large dissipation losses. In applications in which low dissipation losses are a priority, a series feeding technique is used [1]. In such antennas, radiating elements are fed by coupling/connecting all radiating elements to a transmission line [2], [3], or alternatively by guiding signal through radiating elements [4], [5]. It needs to be underlined that, in such arrays, 180° phase shifters are often required [5]. Most often, such a phase shift is realized by insertion of a 180°-long transmission-line section being, nevertheless, frequency-dependent [5]. In [6], the approach for realization of the broadband 180° phase shift has been proposed with the use of slot-coupled power dividers, which has been further developed in [7]. The slot coupler presented in [7] has been utilized as a four-port power divider, which not only provides equal-amplitude and out-of-phase signals to the two outputs placed above the slot, but also is capable of providing appropriate power division in the feeding network below the slot. The adjustment of power division ratio is achieved by change of the transmission-line impedance and its inclination with respect to the slot.
