A 3-D-printed long-slot traveling-wave antenna (L-STA) design exhibiting a low output-beam squint-angle variation as a function of frequency is presented. The wave velocity within the rectangular waveguide is tailored by tuning the distance between the conductor’s surfaces. A surface is corrugated at the bottom of the waveguide for fine tunning and efficiency. WR-28 commercial launchers are used to connect the excitation of the antenna on one side of the waveguide and a matched load on the other side. The design is adapted to make a smooth transition between the WR-28 and the proposed antenna design. By properly choosing the direction of the main beam, the first mode excites the antenna and achieve the desired directivity angle. A prototype of the presented design as well as a reference design are fabricated, tested, and compared. Experiments confirm that the antenna performs as designed. The same directivity direction of the main beam is found at 31 and 35 GHz. In the simulations, the directivities for the reference antenna are 16.06 and 16.89 dBi for 31 and 35 GHz, respectively, while our design show directivities of 15.03 and 15.77 dBi for the same frequencies.