For several years, ground-penetrating radar (GPR) has been used to search for buried landmines. Most of the evaluation effort on complete detection systems has focused on end-to-end performance metrics (e.g., Pd and Pfa). Here, we focus on the specific performance of one critical component of GPR systems-the antennas. This is the first in a series of papers that will compare the following parameters of many different antennas: (1) the most useful bandwidths, (2) the role of polarization and polarization diversity, (3) spurious coupling effects, and (4) phase-correction considerations. This paper compares four types of Planning Systems, Inc., antennas that were developed for current and past GPR systems. These are a 5.5-in. log-spiral antenna without balun or spiral-arm terminations; 5.75-in. log-spiral antenna with tapered balun and arm termination; 5.5-in. Archimedean-spiral antenna with tapered balun, but without arm terminations; and 5.75-in. Archimedean-spiral antenna with tapered balun and arm terminations. Three main tests were performed to compare the antennas: (1) S11, to show overall matching bandwidth and to reveal discontinuities in the balun-antenna-termination structure; (2) S21, to measure undesired direct-path coupling relative to intended target scattering; and (3) S21, to show direct coupling vs. antenna spacing.