**5.1 NWSIW transmission line**

**Figure 11** shows the transmission measured in a NWSIW transmission line having a width W = 6 mm, and built on a 100 μm-thick AAO template. The location of the three first cut-off frequencies corresponding to TE 10, TE20 and TE30 modes are visible; their values are in good correspondence with Eq. (5). For a proper operation of NWSIW devices, they should be designed for the 8–18 GHz frequency range, i.e. between the first and second cut-off frequencies.

#### **Figure 11.**

*Transmission measured on a NWSIW built in a 1OO μm-thick AAO template. Inset shows picture of the device having width W = 6 mm.*

Insert of **Figure 11** shows a picture of the fabricated NWSIW line. Material for MNW is Cu and areas filled by MNW are visible as lighter strips on upper and lower side of the brown rectangle forming the top metallization of the NWSIW, and having a length equal to 12 mm. On left- and right-hand sides of the picture taper microstrip transitions aiming at improving impedance matching are visible.

#### **5.2 NWSIW isolator**

**Figure 12** shows a practical realization of a NWSIW isolator basing on the operation mode of a ferrite-slab resonant mode isolator in classical MRW technology [24, 25]. Here in order to mimick the ferrite slab, a narrow wall made of ferromagnetic NW (alloy of Nickel and iron, noted NiFe, red color) is grown close to a wall of the NWSIW (blue color), made of Cu NW grown in AAO, as shown in the inset of **Figure 12**. It has to be noted that NiFe NW forming the slab are not grown over the full height of AAO template in order to avoid the formation of a short-circuit due to contacts between conductive NiFe NW and top metallization of the waveguide, since a short-circuit would prevent the transmission of the signal.

As expected a nonreciprocal transmission is observed in the NWSIW, due to the presence of ferromagnetic material located at a specific position in the waveguide. Transmission S21 in forward direction is more than 10 dB lower than transmission S12 in reverse direction. We can conclude that the device ensures an isolation level superior to 10 dB [21], which is close to the state-of-the art, as shown in **Table 2**.

Compared to isolators based on ferrite slabs in MRW (resonant-mode isolator), the planar devices are much thinner and usually do not require an external magnetic field to bias the ferrite. The best result is obtained by Cheng et al. [26]: their planar ferrite resonance isolator is 635 μm-thick and works without DC magnetic field bias. Hemour et al. [27] propose an SIW isolator based on ferrite, with lower performances reported in **Table 2**.

*Challenges and Perspectives for SIW Hybrid Structures Combining Nanowires and Porous… DOI: http://dx.doi.org/10.5772/intechopen.105148*

#### **Figure 12.**

*Measurement of NWSIW isolator. Inset shows the topology: Cu NW (blue) form lateral walls, while NiFe NW (red) form a ferromagnetic slab. Electroplated copper metal layers (orange) form bottom and top walls of NWSIW.*


#### **Table 2.**

*Comparison of performances of planar SIW isolators.*

The first demonstration of an isolator based on ferromagnetic nanowires (NW) was reported in [28]. Cobalt NW are included in nanoporous polycarbonate (PC) thin substrate, and 10 dB isolation is obtained, however with 15 dB insertion losses and requiring a 9 kOe DC magnetic bias. Next two other devices based on NW in AAO porous template are reported in the literature. Kuarn [29] proposed a coplanar waveguide (CPW) technology including Nickel (Ni) MNW in AAO. Reported isolation level is 6 dB/cm at 23 GHz under 5.6 kOe DC magnetic bias. The best performances for nanowire-based planar isolators were obtained by Carignan et al. [13], who measured an isolation of 14 dB/cm without DC bias on a microstrip topology including CoFeB in AAO.

Two realizations of UCLouvain combining NW in AAO and planar SIW into an NWSIW isolator are reported in the literature and do not require external DC bias. The first one [18] shows an isolation level of 7 dB/cm, while the second one [21] shown in **Figure 12** has an isolation of 11 dB/cm. These two last realizations are among the thinnest according to **Table 2**.
