AbstractThe input impedance of directive electromagnetic bandgap (EBG) resonator antennas is studied theoretically and experimentally at millimeter‐wave frequencies. These antennas consist of a printed primary source illuminating a metallo‐dielectric EBG radome through a free‐space coupling region. A Fabry–Perot (FP)‐type resonator with highly reflecting metallic strip gratings is used. The variations of input impedance, resonant frequency, and return loss at resonance are investigated as functions of the thickness of the coupling region, for various topologies of (i) printed sources (single‐ and double‐layer patch antennas and arrays) and (ii) EBG cavities with a variable Q factor (that is, directivity). We also demonstrate that the coupling conditions are optimized when the resonator is fed by a relatively directive primary source (here, a 2 × 2 aperture‐coupled patch‐antenna array), rather than by a patch or a wire antenna, as is usually done. In that case, the required accuracy of the coupling region's thickness is much less stringent, which is essential from a practical point of view. These theoretical results are experimentally validated at 60 GHz through the characterization of several configurations of EBG antennas. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 369–375, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20144