The problem of the realization of broadband quadrature directional couplers in UMS (United Monolithic Semiconductors) PH25 is thoroughly investigated. The limitations of the selected technology are discussed with respect to directional couplers' design. It is shown that, in such technologies, two major problems have to be overcome to achieve the high-performance of the resulting integrated couplers, i.e., the realization of appropriate coupling together with appropriate characteristic impedances, and the equalization of inductive and capacitive coupling coefficients that are inherently different in inhomogeneous dielectric structures. Three different solutions that allow for achieving strong coupling in the selected UMS PH25 process were selected and experimentally investigated, showing the possibility of such circuits' realization. In each of the presented integrated couplers, capacitive compensation techniques were applied for coupling coefficients' equalization.

A new approach to the synthesis of broadband branch-line couplers with arbitrary coupling level was investigated in this paper. It was shown that the operational bandwidth of a classic branch-line coupler can be increased by utilizing N-section impedance transformers added to each of the coupler’s ports. Furthermore, the obtained response can be approximated by the Chebyshev polynomial. Moreover, it was proven that for such couplers a range of coupling coefficient values can be obtained by the modification of classic branch-line topology. The analysis of the electrical parameters of the proposed branch-line couplers was comprehensively investigated. To verify the correctness of the proposed design procedures, 3-dB, and 6-dB broadband branch-line couplers operating at the center frequency of 2 GHz having return losses greater than 20 dB were designed, fabricated, and measured.

A novel concept of a differential feeding network for four-node antenna/antenna arrays is proposed. The network is constructed out of a four-strip coupled-line directional coupler operating in a mixed differential-nodal mode enabling direct interfacing of differential front-ends with single-ended antennas. It is shown, that such a configuration provides at the outputs of the proposed feeding network four signals, which are equal in amplitude and with phase progression of 90°. Thus, such a network can be utilized for generating either dual-circular polarization from a 2 x 2 antenna array or can constitute simultaneously two out-of-phase power dividers and a directional coupler resulting in a two-beam antenna array in one cut-plane and twoelement array in the other one. The dual-circular excitation is of particular interest for modern-day transceivers as it enables polarization-division-duplex communication. The presented concept is confirmed by measurements of two different antenna arrays operating at 2.5 GHz frequency range featuring dual-circular and dual-beam properties, respectively.INDEX TERMS Differential coupler, differential feeding network, polarisation-division-duplex, polarization-reconfigurable antenna, multi-beam antennas.

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