This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS 1 Full-Band Air-Filled Waveguide-to-Substrate Integrated Waveguide (SIW) Direct Transition Juan Luis Cano, Angel Mediavilla, and Ana R. Perez Abstract—A 45% bandwidth in-line air-ﬁlled waveguide-to-sub- strate integrated waveguide (SIW) direct transition is designed to greatly improve the performance of existing conﬁgurations. The transition, based on a four-section height-stepped waveguide, includes a single-step widening transformer that enables full-band operation without using any dielectric probe. The absence of the probe reduces signiﬁcantly the insertion loss and makes this transition simple, bandwidth controllable and easily scalable to the millimeter-wave frequency range. A back-to-back transition is designed to cover the 32–50 GHz band showing a return loss of 15 dB and a mean insertion loss of 0.68 dB in the whole bandwidth. Index Terms—Air-ﬁlled waveguide, rectangular waveguide, sub- strate integrated waveguide (SIW), waveguide transition. I. INTRODUCTION A IR-FILLED waveguide technology has been widely used since decades for applications demanding their well-known characteristics such as high -factor, power handling capability, very low insertion loss and mechanical rigidity. However, when a large number of components need to be integrated, this bulky and expensive technology may limit the number of affordable circuits. In the last years, the use of substrate integrated waveguides (SIW) overcomes these limitations providing good -factor and low insertion loss characteristics with a high integration capability at a reduced manufacturing cost [1], [2]. Therefore, the availability of high-performance transformers between hollow waveguide and SIW providing full-band operation, low insertion loss, and mechanical simplicity to work in the millimeter-wave frequency bands is of great interest. Many interesting transitions can be found in the literature grouped in two main conﬁgurations. Right-angle transitions, where the two axes are perpendicular and they meet in one or two coupling slots cutting the broad wall of the SIW [3], [4]. These simple transitions exhibit limited bandwidth due to their inherent resonant property and they are not suitable for parallel integration of subsystems as in multi-receiver instruments. On the other hand, in-line conﬁgurations, where the air-ﬁlled wave- guide and the SIW follow the same orientation, enable parallel Manuscript received July 28, 2014; accepted November 15, 2014. This work was supported by the Ministerio de Ciencia e Innovación, Spain, under the CONSOLIDER-INGENIO 2010 Program under project CSD2010-00064. The authors are with the Departamento de Ingeniería de Comunica- ciones, Universidad de Cantabria, Santander 39005, Spain (e-mail: juan- luis.cano@unican.es). Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LMWC.2014.2372480 Fig. 1. Sketch of the back-to-back air-ﬁlled waveguide-to-SIW direct transi- tion. integration with wider frequency bandwidths. In [5] a four-sec- tion height-stepped waveguide transition is designed to con- nect a SIW to a pyramidal horn antenna. The presented simula- tions, the only available data, show a bandwidth around 30% in Ka-band while the simulated insertion loss is not well reported. Also, imaginative dielectric probes together with different tran- sition conﬁgurations have been used. An optimized radial probe is inserted into a height-tapered waveguide [6], an antipodal quasi-Yagi dielectric antenna probe is introduced into a rect- angular waveguide [7], and a parallelepiped alumina probe is designed together with a two-section height-stepped transition [8], reaching a 33%, a 59% and a 7% fractional bandwidth, re- spectively. Furthermore, the use of these dielectric probes com- plicates the design process and raises the insertion loss to levels that could compromise the performance of the overall system. In this letter, a simple and compact four-section height- stepped waveguide direct transition is internally combined with a single-step widening transformer for the two last sections (see Fig. 1). This novel solution combines the bandwidth control capability, using a short number of sections, with a simple and scalable structure. Moreover, since the proposed architecture does not require the use of a dielectric probe to improve the bandwidth, the overall insertion loss is kept extremely low compared with other published conﬁgurations, which makes it suitable to be used in the millimeter-wave frequency range. II. TRANSITION DESIGN Monomode operation in rectangular waveguides is carried out with the fundamental mode . SIW structures can only support modes [9], being the fundamental one. Therefore, a broadband transition between both transmission 1531-1309 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.