In this work, we present the development of a low-power and high-sensitivity electromagnetic field (EMF) sensor operating in the n258 FR2 band. The design of the sensor architecture and its sub-systems are discussed, and the sensor performance is characterised using over-the-air measurements. For the front end, a dual-polarised leaky-wave antenna feed is coupled to a dielectric lens and implemented in a multilayer PCB stack. The elliptical lens is 3D-printed using commercially available ABS. The back-end RF electronics are integrated on the same PCB to realise low-power direct down conversion sensing. The components include low noise amplifiers (LNA), digital step attenuators (DSA) and root-mean-square (RMS) detectors. An onboard 12-bit digitiser provides signal quantisation. The measured radiation patterns of the lens antenna agree well with the simulations with 17.18 dBi directivity. The responsivity is −57.7 dBm and the power consumption is about 0.6 W for a single polarisation.