A new concept of an ultrawide bandwidth 180°-hybrid-coupler is presented. The ultrawideband design approach is based on the excitation of a coplanar waveguide (CPW) mode and a coupled slot line (CSL) mode in the same double slotted planar waveguide. The coupler is suitable for realization in planar printed circuit board technology. For verification of the new concept a prototype was designed for the frequency range from 3？GHz to 11？GHz, built, and measured. The measurement results presented in this paper show a good agreement between simulation and measurement and demonstrate the very broadband performance of the new device. The demonstrated coupler with a size of 40？mm？×？55？mm exhibits a fractional bandwidth of 114% centered at 7？GHz with a maximum amplitude imbalance of 0.8？dB and a maximum phase imbalance of 5°. 1. Introduction The main task of a 180°-hybrid-coupler is the division of an input signal into two autonomous signals, which possess the same amplitude and are in-phase or out-of-phase. The phase relation between the output signals depends on the feeding port, referred to as -port or -port, respectively. A 180°-hybrid-coupler consists all in all of four ports, two inputs and two outputs. The division into in-phase signals introduces a principle of a basic power divider, which can be easily realized even for very large bandwidths. The creation of differential signals can be achieved by a differential power divider. Bialkowski and Abbosh [1] describe such a differential power divider for UWB technology. A 180°-hybrid-coupler combines the two aforementioned power dividing principles in one single device. 180°-hybrid-couplers are used in many microwave circuits such as push-pull amplifiers [2], balanced mixers [3], and pattern diversity antennas [4]. A further application of such couplers is in the monopulse radar technique, where sum and difference beams are created for an accurate angular tracking of the target [5]. The possibility of the creation of a sum and difference beam over UWB bandwidth is verified by the authors in [6]. The general advantage of the 180°-hybrid-coupler over conventional or differential power dividers is the possibility to process sum and differential signals in the same device. In narrowband systems, those kinds of couplers are well known as rat-race couplers or magic-tees. However, there is a demand for systems combining the new broadband possibilities (e.g., UWB technology) with traditional narrowband concepts (e.g., monopulse radar) [7]. Hence, hybrid-couplers that cover the UWB frequency band, for example, from
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