The effective bandwidth of a travelling-wave slotted waveguide array antenna is improved by using a proposed broadband single radiating element based on a pair of longitudinal slots CNC-machined in the broad wall of the waveguide. The TE10 mode excites two resonances generated by each slot, providing an effective bandwidth improvement. As a proof of concept, an array antenna formed by 16 double longitudinal parallel-slot elements is designed. A prototype has been manufactured in printed technology in order to experimentally validate the antenna performance. Measurements show high stability of efficiency, maximum gain and radiation patterns in a broadband higher than 20% in the K-band.