  We report on the design and characterization of a compact microwave antenna
for atomic and molecular physics experiments. The antenna is comprised of four
loop antennas arranged in cloverleaf shape, allowing for precise adjustment of
polarization by tuning the relative phase of the loops. We optimize the antenna
for left-circularly polarized microwaves at 3.5 GHz and characterize its
performance using ultracold NaCs molecules as a precise quantum sensor.
Observing an unusually high Rabi frequency of $2\pi \times 46$ MHz, we extract
an electric field amplitude of 33(2) V/cm at 22 mm distance from the antenna.
The polarization ellipticity is 2.3(4) degrees, corresponding to a 24 dB
suppression of right-circular polarization. The cloverleaf antenna is planar
and provides large optical access, making it highly suitable for quantum
control of atoms and molecules, and potentially other quantum systems that
operate in the microwave regime.
