Coplanar-strip horn antennas are switched photoconductively to generate picosecond bursts of freely propagating electromagnetic energy with bandwidth covering 15-75 GHz. The antennas are fabricated on GaAs grown by molecular beam epitaxy at low substrate temperatures. These antennas are used to perform transient scattering measurements from slit-coupled circular and coaxial cavities; Prony's method [IEEE Trans. Antennas Propagat. 23, 777 (1975)] is used to extract cavity resonances from the measured late-time scattered signal.