A differential antenna includes a substrate formed of non-radio-frequency material and a pair of conductive microstrip lines formed on the substrate. A metallic sheet is supported and spaced apart from the pair of conductive microstrip lines by a plurality of support elements to form an air gap. The metallic sheet is patterned to include a plurality of differential radiating gaps disposed along the longitudinal axis of the antenna and above a gap between the pair of conductive microstrip lines. Multiple rows of metallic vias are formed in the substrate disposed and spaced apart laterally with respect to the gap between the pair of conductive microstrip lines to define two propagation air cavities in which radiation can propagate. The differential antenna is configured such that the radiation is differential-mode, second-order radiation, which is emitted from the differential antenna at the differential radiating gaps in the metallic sheet.