The near-field propagation behavior of terahertz (THz) pulses generated by a planar large-aperture photoconducting THz transmitter has been characterized. A simulation model based on Huygens-Fresnel diffraction theory has been developed that permits accurate prediction of the spatiotemporal profiles of the THz beam everywhere and gives excellent agreement with experimental measurements. Two key conclusions emerge from this research, namely, the realization that for practical laboratory setups one is always working in the near-field regime and that the proper temporal shape of the THz field at the antenna is one that rises rapidly but decays slowly.