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Microwave and millimeter wave imaging has shown significant potential in various applications. An imaging system commonly consists of a sensitive electric field mapping array devised to measure the spatial distribution of the scattered field from an object to be imaged. One of the most prominent methods used to realize a cost-effective real-time imaging system is the modulated scatterer technique (MST). Although the conventional MST, using small loaded dipole antennas, performs well at lower microwave frequencies, its utility at high microwave and millimeter wave frequencies is limited. To improve upon the conventional MST, a novel modulated elliptical slot antenna, loaded with a PIN diode, is introduced and analyzed in this paper. The modulation-depth (effectiveness in modulating the slot), current distribution around the slot, and the influence of PIN diode bias structure are discussed based on numerical simulation results and experiments. Finally, the efficacy of the proposed slot for electric field measurements at 24 GHz is demonstrated using a prototype slot.