  In this article we discuss the design and implementation of a novel
microstrip resonator which allows for the absolute control of the microwaves
polarization degree for frequencies up to 30 GHz. The sensor is composed of two
half-wavelength microstrip line resonators, designed to match the 50 Ohms
impedance of the lines on a high dielectric constant GaAs substrate. The line
resonators cross each other perpendicularly through their centers, forming a
cross. Microstrip feed lines are coupled through small gaps to three arms of
the cross to connect the resonator to the excitation ports. The control of the
relative magnitude and phase between the two microwave stimuli at the input
ports of each line allows for tuning the degree and type of polarization of the
microwave excitation at the center of the cross resonator. The third (output)
port is used to measure the transmitted signal, which is crucial to work at low
temperatures, where reflections along lengthy coaxial lines mask the signal
reflected by the resonator. EPR spectra recorded at low temperature in an S=
5/2 molecular magnet system show that 82%-fidelity circular polarization of the
microwaves is achieved over the central area of the resonator.
