Empty Substrate Integrated Waveguide Technologies: A Major Step-Forward Towards Realizing Low-Cost and Low-Loss Microwave Circuits

Substrate integrated circuits (SICs) [1] have attracted much attention in the last several years because of their great potential. The first implementation of this concept can be found in [2], as a feeding mechanism for a slot array antenna. Deslandes and Wu [3], [4] used the term substrate integrated waveguide (SIW) to define this waveguide integrated in a substrate where the lateral walls are synthesized with metallized via holes, and the upper and lower walls are the top and bottom metallization of the substrate. Many other SICs have been developed since the appearance of the SIW. In the substrate integrated slab waveguide (SISW) [5], the guide is synthesized on a substrate by cutting an air hole in the center of a SIW, and the frequency operating bandwidth is increased. The ridge substrate integrated waveguide (RSIW) [6] also increases the operation bandwidth by drilling cylindrical air holes in the middle of the SIW. The substrate integrated non-radiative dielectric waveguide (SINRD) [7] uses a specific pattern of air holes as lateral walls, and creates a dielectric waveguide channel reducing the radiation losses with respect to the SIW. The half mode substrate integrated waveguide (HMSIW) [8] reduces the size of the SIW by half, by exploiting the fact that there is a symmetry plane along the propagation direction that is equivalent to a magnetic wall. The main advantage of this new concept of substrate integrated waveguide, when compared with classical planar circuits, is that it is low cost and easy to manufacture with standard printed circuit board (PCB) machinery. Moreover, it presents a higher quality factor for resonators and filters than other planar transmission lines. Many transitions have been developed from different planar lines (microstrip, coplanar, etc.) to each type of substrate integrated circuit, and so the SICs can be embedded in a substrate and easily connected to other planar circuits. When compared to the classical rectangular waveguide, SICs present smaller quality factor and greater insertion losses, but they are low cost, low profile, and can be easily embedded in a PCB. A full suite of microwave devices has been developed with SICs, such as filters [9], [10], couplers [11], [12], phase shifters [13], antennas [14], [15], and tunable devices [16]. All of them prove the interesting features of SICs for being used in the microwave industry. However, the presence of a dielectric in these devices limits their performance, especially at high frequencies, due to the losses in the substrate that significantly increase the total insertion losses and reduce the related quality factor. In order to solve this problem, a new type of substrate integrated waveguide has appeared where the dielectric is removed, while maintaining the advantages of low cost, low profile, easy manufacturing and integration in a PCB. The first attempts to combine planar circuits and empty waveguide devices are the so-called surface-mountable devices, or surface-mounted waveguides (SMW). In [17], a waveguide bandpass filter integrated within an MMIC is mounted on top of a planar substrate using a silicon pedestal. Then the filter is coupled with input and output microstrip lines through microstrip to waveguide transducers. In [18] another bandpass filter is mounted on top of a planar circuit. This time the filter is implemented in a reduced height metal waveguide fabricated with micromachining. The filter is connected to the planar substrate using slot coupling, and it is coupled to input and output microstrip lines via slots in the ground plane. These slots act as the first and last inverter of the filter. In [19], a pyramidal horn antenna micromachined in a metal body, and fed with a rectangular waveguide, is mounted on top of a substrate, which is coupled to a SIW line in this substrate by means of a quarter-wave stepped impedance transformer. A theoretical comparison between the same filter implemented in stand-alone SIW, and in a surface-mountable waveguide coupled to feeding SIW lines is presented in [20]. Finally, another surface-mounted waveguide filter is presented in [21]. In all these cases the surface mounted devices allow the propagation of waves through air, and this leads to lower insertion losses, and higher quality factors in filters. However, this is not a fully substrate integrated solution, requires non-planar manufacturing processes and transitions, thus not being a low-profile and low cost integrated solution.