Ultra-wideband (UWB) antennas have been widely used in short range radar applications such as ground penetrating radar (GPR). There are numerous reported studies addressing the design of different types of antennas, loading methods and feed structures to achieve the required UWB performance. However, there are few reports on the relationship between the source impedance of the antenna feed and the UWB performance. This paper investigated the effects of the source impedance (changing from 50 ohms to 350 ohms) on the time-domain performance of bowtie antennas. The results were validated by both simulation and experiment. For bowtie antennas, the time-domain signal settles quicker with the increasing source impedance from the output of the balun structure, however the signal magnitude was reduced. A good compromise source impedance was found to be around 250 ohms.