This paper proposes a dielectric window for Standing Wave Linear Accelerators (LINAC’s) which can be tuned after brazing and welding, in order to compensate the electro- magnetic alteration due to these production processes. This study investigates the in-frequency return loss behavior of the LINAC, in order to improve matching and transmitting conditions while maintaining the optimum coupling between LINAC and High Power Microwave (HPMW) source. Device design is single-frequency based and considers the dielectric window interface as an Input Matching Network (IMN) at the LINAC normal mode working frequency. Thus, design formulas are provided and Computer Aided Design techniques are proposed. A prototype has been made and tested by performing cold S-parameter measurements of a LINAC with the proposed dielectric window and with a traditional dielectric window. The proposed device offers more energy transport attitude over the traditional dielectric window, as shown by a return loss increase of 167% of the complete system.