  We report on a microwave planar ring antenna specifically designed for
optically-detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers
in diamond. It has the resonance frequency at around 2.87 GHz with the
bandwidth of 400 MHz, ensuring that ODMR can be observed under external
magnetic fields up to 100 G without the need of adjustment of the resonance
frequency. It is also spatially uniform within the 1-mm-diameter center hole,
enabling the magnetic-field imaging in the wide spatial range. These features
facilitate the experiments on quantum sensing and imaging using NV centers at
room temperature.
