  We present an easily reproducible inexpensive microwave antenna that can
generate a strong and homogeneous magnetic field of arbitrary polarization,
which enables fast and coherent control of electron spins over a large volume.
Unlike preceding works, we present a resonant antenna that maintains its
resonant behaviour regardless of the proximity of other experimental hardware
components. This robustness is crucial as it enables, amongst others, using
microscope objectives with short working distances to perform wide field
imaging/sensing with bulk diamonds. The antenna generates a magnetic field
strength of 22.3 A/m for 1 W total driving power, which doubles the power
efficiency compared with previously reported patch antenna designs. The
magnetic field homogeneity in a volume of $0.3 \text{mm}^3$, $0.5 \text{mm}^3$
and $1 \text{mm}^3$ is within 6\%, 8\% and 13\%, respectively. The antenna has
a full width at half maximum bandwidth of $\sim$160 MHz and its resonant
frequency can be tuned over a 400 MHz range via four capacitors or varactors.
The antenna has been tested and found to remain within safe handling
temperatures during continuous-wave operation at 8 W. The files required to
reproduce this antenna, which can be built on a standard and affordable double
sided PCB, are provided open-source. This work facilitates a robust and
versatile piece of instrumentation, being particularly appealing for
applications such as high sensitivity magnetometry and wide field
imaging/sensing with Nitrogen Vacancy centers.
