An apparatuses and methods for phase matching power combined signals are described. A typical apparatus includes at least a waveguide portion in at least a first branch conducting a first electromagnetic signal of a combiner, the waveguide portion having an effective size and a thermal control system effecting a temperature change in the waveguide portion to alter the effective size. Altering the effective size of the waveguide portion adjusts phase matching between the first electromagnetic signal of the first branch and at least a second electromagnetic signal of a second branch of the combiner. High thermal expansion coefficient materials including silver plated polyetherimide can be used. In addition, composite materials having anisotropic thermal expansion may be used.