AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES Vol. 10, No. 4 ISSN: 1998-4456 Page 536 Copyright  2017 Al-Qadisiyah Journal For Engineering Sciences. All rights reserved. EVALUATION OF THE EFFECT OF LENGTH ON THE PERFORMANCE OF RECTANGULAR TO RECTANGULAR WAVEGUIDE TAPER Hasan F. Khazaal, Wasit University, College of Engeneering, Electrical Engineering, Iraq. E-mail: hf1964@yahoo.com Haider Th. Salim ALRikabi, Wasit University, College of Engeneering, Electrical Engineering, Iraq. E-mail: haiderelectrical@gmail.com Mansour S. Farhan, Wasit University, College of Engeneering, Electrical Engeneering, Iraq. E-mail: mansur545@yahoo.com Received on 14 September 2017 Accepted on 31 October 2017 Published on 20 February 2018 DOI: 10.30772/qjes.v10i4.503 Abstract: RF Waveguide taper have a significant role in radar or communication systems through the use of it for joining two waveguides that have different cross sections, and this difference may be in dimensions of cross section or in the shape of it. the tapering may be from rectangular to rectangular waveguide (i.e. each waveguide operating at a certain frequency), or transition from rectangular to circular or elliptical waveguide. In this paper; two rectangular to rectangular waveguide tapers have been fabricated to join waveguide operating at C band frequencies with another one operating at X band frequencies. The fabricated tapers have the same shape but differ in length (8 cm, and 16 cm) in order to measure the effect of length on the most important parameters that have an effect on the performance of the taper, like voltage standing wave ratio "VSWR", return losses, and input resistance. Many tests have been done using two - port network analyzer. The results of the tests indicate that there is no effect on the performance of the waveguide taper although there is an extra 8 cm in length. Keywords: waveguide, radio frequency, VSWR, return losses, input resistance. INTRODUCTION This RF waveguide transition or tapers are used to connect waveguide components with different cross sectional sizes and shapes. There are many kinds of waveguide tapers, like rectangular-to-rectangular of radio frequency waveguide transition adapters which designed to connect one rectangular waveguide to