The spatial structure of waves and plasma uniformity in microwave discharges using a multi-slotted planar antenna are investigated experimentally and by calculation. The wave field amplitude and phase are measured in the axial and azimuthal directions, and are compared with three-dimensional finite-difference time-domain calculation result. The wave mode in the plasma does not change with various densities, and, as a consequence, spatially uniform plasmas are obtained for a wide range of parameters being free from abrupt changes associated with surface wave eigenmodes.