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A passive probe antenna for cw near-field microscopy at millimeter and submillimeter wavelengths is defined. It is based on the coupling between a free-space linearly polarized propagating beam to a wire mode. This is obtained efficiently owing to a discontinuous phase plate. This passive “optical” structure allows either the generation of a subwavelength confinement of the longitudinal electric field (polarized along the wire antenna) or, due to reciprocity, the collection of the longitudinal component of the electric field (along the wire antenna) with subwavelength resolution. The emission and collection properties of the proposed antenna have been demonstrated experimentally using a preliminary realization designed to work at 0.1 THz. Experimental results are well supported by calculations.