  We report on a planar microwave resonator providing arbitrarily polarized
oscillating magnetic fields that enable selective excitation of the electronic
spins of nitrogen-vacancy (NV) centers in diamond. The polarization plane is
parallel to the surface of diamond, which makes the resonator fully compatible
with (111)-oriented diamond. The field distribution is spatially uniform in a
circular area with a diameter of 4 mm, and a near-perfect circular polarization
is achieved. We also demonstrate that the original resonance frequency of 2.8
GHz can be varied in the range of 2-3.2 GHz by introducing varactor diodes that
serve as variable capacitors.
