In this work we compare the characteristics of asymmetrically excited small-aperture antenna-type pulsed terahertz emitters fabricated using an ion implantation process. Our photoconductive materials consist of high resistivity GaAs substrates. Multienergy implantations of arsenic (1.2 and 2MeV) and oxygen (180, 450, and 700keV) have been used to obtain an almost uniform density of vacancies over the optical absorption depth in bulk GaAs substrates. Terahertz pulses are generated by exciting our devices with ultrashort laser pulses. Ion implantation followed by a thermal annealing process introduces nonradiative centers in our substrates which reduce the carrier lifetime and modify the shape of our terahertz pulses. Results obtained as functions of the laser excitation power and bias voltage are discussed and a comparison of the performance of these devices with conventional small-aperture antennas is given.