A co-axial cavity type Overhauser sensor for geomagnetic field measurement has been analyzed. The cavity with free radical solution has inductance of 32 nH. A Radio Frequency (RF) matching network has been designed to couple the RF power at proton polarization frequency of 40 MHz. The tuned circuit has quality factor of 150. The required voltage for the geomagnetic field measurement, which is less than 50 V has achieved at RF input power of less than 2 watts at 40 MHz.