RF windows are used to separate vacuum from atmosphere in high power microwave systems, such as klystrons. RF breakdowns in these megawatt power environments could damage the window. An S-band RF window was designed to have reduced electric and magnetic field in the waveguide joints and the ceramic. Specifically the electric field on the ceramic was minimized and a traveling wave was created inside the ceramic by optimizing the shape of the window and the geometry of the joint between the circular waveguide to the rectangular waveguide. A prototype of this window is being made at SLAC for high power tests. Show Partial Abstract