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A high-impedance ground plane (HIGP) is used for a circular waveguide antenna. The benefits of using such a HIGP are related to the suppression of surface waves excited by the circular waveguide mouth, because the HIGP has a very obvious surface wave bandgap. Suppression or reduction of surface waves is expected to improve antenna efficiency and to bring a reduction of sidelobe level due to diffraction of the surface waves. In agreement with the expected result, the measured radiation patterns show that a 2 dB addition in the antenna gain and a 10 dB reduction in the back lobes, if the circular waveguide antenna working frequency is in the HIGP surface wave bandgap.