We experimentally compared the performance of semi-insulating (SI)-GaAs stripline antennas with different gaps, electrode widths, and electrode materials. Large gap antennas have high electrical-to-terahertz (THz) and optical-to-THz conversion efficiencies. The peak frequency slightly decreases with the increase of the antenna gap. The stability of an antenna with narrower electrodes is less than that of an antenna with wider electrodes because the wider electrodes can remove more heat from the antenna. We made and placed two metal pads beside the electrodes to act as a heat sink, which caused the performance of the antenna to be much more stable. The antenna with AuGeNi electrodes has a higher breakdown voltage and better stability than the antenna with $\mathrm{Ti}∕\mathrm{Au}$ electrodes. The dynamic range of an antenna with a $150\mu \mathrm{m}$ gap, $100\mu \mathrm{m}$ AuGeNi electrodes, and a heat sink is 6300 without purging.