## 1 Introduction  
A high-gain pencil beam radiation pattern, often required in satellite communications and radar systems, can be synthesized by large planar arrays. These antennas, where hundreds of radiating elements are arranged, show appealing features such as low profile and light weight [1]. However, the insertion loss due to the feeding networks long electrical paths causes radiation efficiency and gain degradations. One promising approach to reduce such loss is to divide the planar array into modules where microwave power is delivered by a low-loss hybrid feeding network. This can be made of planar circuits such as substrate integrated waveguides (SIWs), grounded coplanar waveguides (GCPWs) or quasi-planar structures such as air-filled rectangular waveguides (RWGs), connected together with low-loss transitions. SIWs have received much attention in recent years [2; 3]. It consists of two rows of periodic holes embedded in a dielectric substrate, electrically connecting two parallel metal plates. Some advantages of SIW technology include low loss, high Q-factor, low profile, light weight, high-density integration with planar circuits, low cost, and easy fabrication with standard printed circuit board (PCB) technology. On the other hand, the traditional RWG, due to the absence of the dielectric substrate, features a much lower insertion loss especially at high frequency bands and a higher power handling capability. Moreover, even if the SIW can potentially replace the RWG in some applications, a large number of commercial and existing products are in waveguide format, thus an RWG interface is anyway needed. Finally, RWGs are more rigid and therefore more suitable for the design of complex 3D feeding structures. In hybrid feeding networks both the SIW and RWG technologies are employed, thus a broadband and low-loss transition is necessary in order to connect the two transmission lines. There are two main geometrical configurations for RWG-to-SIW transitions, namely the in-line and the right-angle configurations, which differ from one another because of the longitudinal axes of both waveguides being respectively collinear or perpendicular. In-line transitions [4; 5; 6; 7; 8; 9; 10; 11] use probes or probes together with waveguide tapers and exhibit wide bandwidths at the expense of bulky volumes, thus making this geometry not suitable when compactness is required. Moreover, in-line transitions are not compatible with subarray in the specific scenario of antenna arrays based on a tile architecture. For these reasons, right-angle transitions may also be required. Such configurations employ a coupling aperture etched on the broad wall of the SIW, leading to greater compactness and good system layout, at the expense of narrower bandwidths. The coupling aperture comes in the form of a single slot in [12], which is designed according to slotted waveguide theory. Bandwidth is limited due to the slot resonant behavior, though it can be improved using a pair of slots resonating at two close frequencies as in [13]. Slot-to-slot coupling between the SIW and the RWG ground planes is employed in [14], but again the frequency range is still limited for many applications. The slots bandwidth can be extended with the same principle of aperture-coupled patch antennas by introducing additional substrate layers with printed parasitic patches [15; 16]. Wider bandwidths can be found in [17; 18; 19], where transitions are matched by means of properly shaped apertures, stepped ridges, and SIW inductive posts. The most broadband performance can be obtained when the device is designed similarly to an E-plane waveguide bend: Very good matching is ensured by a stepped transition region where the steps are realized with additional substrate layers [20] or integrated in the RWG flange [21]. In this paper a novel right-angle RWG-to-SIW transition is presented, which makes use of a distributed impedance matching network, in the form of symmetric E-plane irises. Bandwidth performance depends on the number of matching elements introduced.