A power combining technique using a Fabry-Perot resonator with many more active devices is investigated. The Fabry-Perot power combiner consists of two mirrors with a circular groove mounted with the active devices. The power combiner has an axially symmetrical structure and operates at an axially symmetrical TEM01n mode so that uniform device-field coupling required for perfect power combining can be realized. By numerical calculation using the boundary element method, it was shown that high combining efficiency can be obtained when the active devices are mounted in the circular groove of larger radius on either of the two mirrors. In experiments at X-band, power combining efficiency over 90% was obtained for the case of twelve devices on either of the mirrors and the output powers of the twenty or twenty-four devices on both the mirrors were almost perfectly combined.

We propose an axially symmetrical Fabry-Perot multiple-device oscillator operating at an axially symmetrical TEM01n-mode, which has an excellent feature of uniform device-field coupling required for high efficiency power combining. By carrying out the boundary element analysis, it is shown that a plane-mirror output structure is remarkably advantageous compared with a concave-mirror output structure to obtain an adequate output coupling and to enable stable operation characteristic with respect to the axial mode number n. Experiments in X-band confirmed this excellent performance and achieved almost perfect power combining of efficiency as high as 106% and 99% for six- and eight-device case, respectively.