A modified double-ridged horn structure is proposed to obtain ultrawideband antenna characteristics over a bandwidth ratio of greater than 40:1 for impulse radar systems. The Vivaldi-shaped TEM horn feeder has been designed to extend the lower cut-off frequency and the partial dielectric loading technique using a small lens inward while the aperture has been implemented to enhance the gain of the standard double-ridged horn antenna without changing the physical dimensions. The starting frequency of the antenna operation band is lowered from 800 MHz to 400 MHz. Moreover, approximately 5 dB gain increment level is achieved from 4 to 10 GHz. It is shown that proper combination of the partial dielectric loaded TEM and ridged horn antennas can be suitable for multiband ground-penetrating radar operations that provide high resolution imaging at different depths. The antenna gain and input reflection performance measurements are presented with comparisons in both the frequency and time domains.