This dissertation contains two significant investigations. One is the development of the broadband microwave bandpass filters with high out-of-band performance. The other is the development of low-loss hybrids. These researches are parts of the National Aeronautic and Space Administrator (NASA)s mission to explore the universe. The former is focused on the techniques used in microstrip line bandpass filter design that help achieving both low in-band insertion loss and high out-of-band attenuation level. Moreover, these filters achieve very broadband out-of-band attenuation bandwidth. These techniques are related to the improvement of stepped impedance resonators, coupling between resonators and effective methods to allocate transmission zeros to suppress filters out-of-band spurious responses. The later is focused on the techniques used in planar magic-T designs such that the developed magic-T obtains high isolation between port E (difference port) and port H (sum port). Moreover, it obtains low-loss and broadband characteristics. These techniques are related to the development of the low-loss broadband microstrip-toslotline (MS-to-SL transition and the magic-T with a highly symmetric structure. The theoretical analysis and experimental measurements have been performed. The experimental results of both the filter and magic-T researches show significant improvement over their prior state-of-the-art designs by number of magnitude. The designs also reduce fabrication complexity.