###### Abstract
Nevertheless, this technique requires many plated via to be drilled around the whole structure and this can be complex and expensive for the development of compact ESIW filters. Gap waveguide (GW) technology was proposed in [21] to enhance the wave propagation in the air gap along the longitudinal direction of two parallel metallic surfaces. GW is based on two parallel plates and open sidewalls. One of the two parallel plates is composed of a conductive line centered between two periodic structures of metallic pins, which work as two artificial perfect magnetic conductor (PMC) sidewalls. The other one is a metallic plane, which behaves as a perfect electric conductor (PEC) surface. In this way, this structure eliminates the need for metallic contacts between the top and bottom parts, and the wave propagation takes place in the air groove area included between the two PMC sidewalls. These characteristics make this technology suitable for implementing millimeter and submillimeter band guided structures with low insertion loss and for manufacturing low-cost components. Based on the GW concept, a groove gap waveguide (GGW) was proposed in [22]. Recently, inverted microstrip and coplanar gap waveguides based on this idea were also presented [23; 24; 25]. However, the dielectric thickness in these new transmission lines negatively affects the losses with respect to the GGW. The GGW was designed as a potential alternative to the conventional RW at microwave/mm-wave frequency bands, but also it has a great potential in low-frequency applications (e.g., filters). GGW and RW have similar quality factor (\(Q\)) values and dispersive behaviors [26; 27]. GGW requires the fabrication of two periodic structures of metallic pins acting as PMC sidewalls. These periodic structures slightly increase the complexity and fabrication cost, and the width of the waveguide with respect to classic waveguide technologies. However, these drawbacks are not enough compared to its benefits, since unlike classic waveguide technologies it does not need an electrical contact between the bottom and top plates, thus reducing the alignment problems for low- and high-frequency bands. Poor electrical contact between metallic blocks in classic waveguide technologies, with a cut in the E-plane, can produce high losses, thus deteriorating the \(Q\)-factor in low- and high-frequency applications (e.g., filters). Recently, glide-symmetric holey electromagnetic bandgap (EBG) structures have also been proposed instead of periodic structures of metallic pins to perform artificial PMC sidewalls for cost-effective gap waveguide technology [28; 29]. Several bandpass filters have been successfully implemented in GGW technology [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. Their designs are based on resonant cavities [30; 31; 32; 33; 34; 35; 36], coupled resonators [37; 38; 39; 40; 41; 42], evanescent wave coupled resonators [43] and periodic corrugations acting as resonators [44]. Due to their configurations, these bandpass filters are narrowband type. Their frequency responses have an _FBW_ lower than 4%. A wideband bandpass filter with _FBW_ = 22.2% has also been proposed, introducing quasi-periodic transverse corrugations inside the GGW [45]. The resulting structures have been manufactured using computer numerical control (CNC) milling or 3D printing techniques and are generally small. The aim of this paper is to design compact bandpass filters in GGW technology with large fractional bandwidths (\(FBW>10\%\)) using a robust technique against manufacturing tolerance, thus allowing their low-cost implementation with an emerging three-dimensional (3D) printing technique. To achieve compactness and large \(FBWs\), simple metallic pins have been employed, and a rigorous synthesis method based on stepped impedances has been applied to obtain the desired frequency response.