The measurement of radiation patterns of antennas in air is relatively straightforward. In contrast, the measurement of the underground pattern for ground-penetrating radar (GPR) antennas poses particular challenges. Since GPRs are equipped with transmitting and receiving paths, the combined pattern is the most useful. To measure this pattern, a probe (scatterer) can be used to reflect part of the received signal back to the receiving antenna. However, the processing on the receiving end must determine whether or not that signal comes from the probe (“desired”) or from the soil or other objects (“undesired.”) These two issues can be addressed by using a modulated scatterer, i.e., a scatterer that is modulated at a frequency much less than the carrier frequency. The modulation can be realized either electrically or optically. The advantage of the optical approach is that spurious reflections are greatly reduced since an optical fiber is used instead of current-carrying metallic cables. The electrical approach, however, allows for deeper modulation levels, which increases the level of “desired” signal at the receiver. Another issue is related to the bandwidth of the scatterer. Since GPRs are generally very broadband, it is of interest to measure their broadband radiation patterns. The scatterers in the present work are successfully made broadband by resistively loading them. The results and trade-offs resulting from this technique are shown. In summary, the modulated scatterer technique is verified to be useful for these purposes. Experiments are realized in air and underground and the corresponding radiation patterns of a set of GPR antennas are shown.