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The demand for reliable high power solid-state sources at upper microwave and millimeter-wave frequencies has been the main driving force behind research on spatial power combining techniques. In order to combine power in space, the use of alternative, tile and tray, configurations of active planar antenna arrays (of oscillators or amplifiers) has been proposed and demonstrated. We focus on the tray configuration which, employing end-fire-type antennas, places no restriction on space for active stages and offers a higher operational bandwidth. Our investigations concern the passive structure of an X-band spatial power combiner consisting of seven trays of planar quasi-Yagi antennas (Deal, W.R. et al., IEEE Trans. Microwave Theory Tech., vol.48, no.6, p.910-18, 2000) connected back-to-back for the inclusion of active stages. Hard horns, for signal launching and receiving, and Schiffman phase shifters within the trays are used to provide the uniform field magnitude and phase across the trays required to achieve the large dynamic range of the combiner. Performance comparison of the power combining structure with and without Schiffman phase shifters is measured and discussed. Using Schiffman phase shifters helps to reduce insertion losses and increases the operational bandwidth of the tray-type power combiner over its straight through counterpart.