We report measurements of the positronium (Ps) $2^{3}S_{1}\ensuremath{\rightarrow}2^{3}P_{2}$ interval in which atoms traveling free space were irradiated with microwave radiation generated using a horn antenna. Previous this transition, performed waveguides, exhibited asymmetric and shifted line shapes. In free-space we here, much smaller line-shape asymmetry was observed, but large frequency ...