**1. Introduction**

High-quality transmission lines become mandatory to fulfill the requirements of the nowadays applications. Indeed, the constraints applied to microwave and millimeter-wave circuits are continuously reviewed and increased: low cost, low consumption, high density, high operational frequency, … At frequencies higher than 30 GHz, interferences, radiation and material losses prevent the use of conventional microstrip and coplanar transmission lines. Within this context, the substrate integrated waveguide (SIW) topology, introduced in [1], represents a promising perspective. This technology uses two rows of vertical metallic wires or metallized via holes inserted throughout the substrate entire height to form a shielded line of rectangular section, reducing subsequently radiation and conductor losses. The presence of walls also prevents spurious crosstalk interferences between devices in a same circuit [2–4].

The two virtual walls have to be carefully placed to allow the appearance of the same propagation modes as in a macroscopic rectangular waveguide. It is possible to integrate various components in a same substrate, including passive or active devices, as well as antennas. Different variations of the concept emerged for several functionalities (filter, coupler, antenna power supply, … ) with applications up to 180 GHz, and in different types of substrates (PCB, paper, polymers or alumina) [5, 6].

Over the last decades also, the interactions between nanowires embedded in porous templates and microwave signals have been intensively studied and exploited to create planar monolithic devices. Both the permeability and permittivity of the template filled with nanowires can be modulated in order to create various microwave and millimeter-wave components. The advantages of microwave devices based on nanowires compared to classical components are manifold: wide range of operating frequencies, temperature stability, monolithic integration into a single substrate and possible miniaturization. Moreover, compared to classical ferrites, ferromagnetic nanowire arrays display higher saturation magnetization and ferromagnetic resonance (FMR) frequencies as well as high operation frequencies at remanence due to their large aspect ratio.

Thanks to those properties, noise suppressors [7], absorbers [8, 9], inductors [10], or filters based on electromagnetic bandgap effect [11] have been developed, together with non-reciprocal devices, such as phase shifters [12], isolators [13] and circulators [14]. More recently slow-wave transmission lines were designed exploiting the high permittivity of metallic nanowire arrays [15], while a filter was designed using the double ferromagnetic resonance effect in magnetic nanowires [16]. The reported devices use microstrip or coplanar waveguide topologies combined with different ferromagnetic materials for the nanowires.

Given the respective advantages of nanowires and SIW [17–19], using nanowires to conceive Substrate Integrated Waveguide (SIW) devices is an interesting alternative. The basic idea underlying this concept is to use metallic nanowire arrays electrodeposited into a porous template to form the waveguide walls. With two copper layers deposited onto the template's both faces, a simple rectangular waveguide is created, denoted NWSIW for nanowire-based substrate integrated waveguide. To achieve different microwave functionalities, various nanowire arrays can be added inside the cavity of the NWSIW, combining different heights, shapes and materials (magnetic or not).

Concepts presented in this chapter are widely inspired by the work of Van Kerckhoven et al. [17–22]. The chapter is organized as follows; Section 2 introduces the Nanowired-based Substrate Integrated Waveguide (NWSIW) topology and compares it with the classical SIW and metallic rectangular waveguide (MRW) geometries. Section 3 details the design features of the NWSIW, while Section 4 discusses fabrication techniques. Section 5 presents some experimental realizations of microwave devices based on NWSIW topology, while Section 6 discusses challenges and perspectives of this new technology.
