This article presents a complete design procedure for planar slotted waveguide antennas (SWA). For a desired sidelobe level ratio (SLR), the proposed method provides a pencil shape pattern with a narrow half power beamwidth, which makes the proposed system suitable for high power microwave applications. The proposed planar SWA is composed of only two layers, and uses longitudinal coupling slots rather than the conventional inclined coupling slots. For a desired SLR, the slots excitation in the radiating and feeder SWAs are calculated based on a specified distribution. Simplified closed-form equations are then used to determine the slots nonuniform displacements, for both the radiating and feeder SWAs, without the need to use optimization algorithms. Using simplified equations, the slots lengths, widths, and their distribution along the length of the radiating and feeder SWAs can be found. The feeder dimensions and slots positions are deduced from the dimensions and total number of the radiating SWAs. An 8 × 8 planar SWA has been designed and tested to show the validity of the proposed method. The obtained measured and simulated results are in accordance with the design objectives. K E Y W O R D S antenna arrays, high power microwave applications, slotted waveguide antennas, sidelobe level ratio 1 INTRODUCTION High power microwave (HPM) technology is well known in both military and commercial applications. 1 An efficient antenna to be used as the radiating system for these applications is required to have a well directive pattern, large sidelobe level ratio (SLR), and high gain. It should also have high power handling capability required to extract the HPMs from their source. Slotted waveguide antennas (SWA) 2 are taking a major interest in this field, with their major ability of beam pointing and HPM handling capability. 3 SWAs can be also directly connected to Reltron HPM sources, without the need for additional mode converter, which makes them suitable for HPM applications. SWAs are also used in wireless technologies, maritime, and space applications. SWAs are made of rectangular waveguides, having slots cuts used to radiate the energy. 4 The conventional cuts have a rectangular shape. The slots can be made either on the broadwall or the narrow wall of the waveguide, with the most This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.