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A mutual-synthesis approach is presented for the design of subsystems combining two microwave circuits. In this paper, we are focused on subsystems combining a filter and an antenna. In a first step, the methodology consists of synthesizing the global subsystem, which realizes both filtering and radiation functions, with respect to overall specifications. Considering circuits interactions within the subsystem, the resulting constraints are distributed optimally between the two circuits. The latter mutual synthesis then leads, on one hand, to a set of optimal impedances for their connection and, on the other hand, to the corresponding ideal characteristics of each circuit. A solution can then be selected by carrying out a compromise between the simplicity of achieving the characteristics of each circuit and the performances and compactness of the subsystem. In a second step, the circuits are designed independently with respect to the synthesized characteristics. Compared to a classical synthesis approach, where circuits are synthesized independently with respect to a common reference impedance (generally 50 or 75 Omega), the proposed mutual synthesis allows to simplify the global subsystem since interactions between the two circuits are considered and used optimally. The technique is illustrated with the design of a filter-antenna at Ku-band.