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## 4 Ku-Band Antennas Design Techniques
### SiW Technology Based Antennas  
SIW (Substrate Integrated Waveguide) technology consists of integrating a waveguide into a dielectric substrate, presenting characteristics that are similar to those of a waveguide filled with dielectric material with the same height, permittivity, and equivalent width.  
In traditional rectangular waveguides, waves are confined within the waveguide and, given the dielectric, the air losses affecting waves in these guides are minimal, but with disadvantages, such as the difficulty of integration with other means, such as weight and size. In a rectangular waveguide based on SIW, the waves are confined within the conductive walls and travel through the dielectric material of which it is filled. This type of technology is compatible with existing PCB and LTCC [36; 37] manufacturing techniques and, in turn, allows the design and construction of guides in the microwave and millimetre-wave range with high-quality factor, low cost, and easy integration with planar circuits, such as power dividers, filters, and antennas [19; 38].  
Planar antennas can be easily integrated on the SIW surface, in order to place complex shaped-slots to obtain multi-band operation and circular polarization. In [39], two radiant split ring slots are placed on a SIW cavity-backed. Figure 7 dr\(<\)ws this kind of antenna. The cavity can be excited by using a coaxial connector producing two orthogonal linearly polarized radiations, whose linear superposition achieves an RHCP diagram. The cavity is made on FR-4 substrate, and its size is 16 \(\times\) 16 mm to operate at 11 GHz:  
* impedance bandwidth of 10%,
* 3 dB AR bandwidth of 1.5%,  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Feature** & **Value** \\ \hline operation band & Ku \\ inclination & 45\({}^{\circ}\) \\ sub-array size & 2 × 2 \\ impedance bandwidth & 25.1\% \\
1 dB AR bandwidth & 15.6\% \\ peak gain & 15.1 dBi \\ \hline \hline \end{tabular}
\end{table}
Table 20: Features of the dielectric resonator antenna (DRA) antenna.  
peak gain of 4.4 dBi.  
Traditional slot arrays can also be implemented while using SIW. Their behaviour is similar to waveguide antennas. Slots can form a travelling-wave or resonant array, depending on whether the array is either ended by a matching load or by a short-circuit respectively. An example of resonant slot array on SIW is presented in [40], where it manufactured SIW on Rogers-RT3010, with four longitudinal slots, 11 mm length to resonate at 5 GHz. The antennas are placed orthogonal to each other at the edge of a nano-satellite and they are fed through a 90\({}^{\circ}\) hybrid to generate the desired circular polarization.  
As an alternative, they propose the design of a Half-Mode Substrate Integrated Waveguide (HMSIW) [41]. The proposed HMSIW structure is similar to the SIW one, but, in HMSIW, there is only one row of vias to construct the required electric wall, as shown in Figure 8, while the other side is kept open-ended to constitute a virtual magnetic wall. Furthermore, the same behaviour than a conventional SIW is obtained by using a half-width structure. The authors construct an HMSIW smaller new nano-cube antenna. Table 21 compares the most relevant features of the SIW and HMSIW antennas.  
Figure 8: SIW antenna for nano-cubes.  
Figure 7: SIW Cavity-backed CP Antenna.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Feature** & **Proposed Antenna** & **Conventional Antenna** \\ \hline AR Bandwidth & 100 MHz & 220 MHz \\ Impedance bandwidth & 70 MHz & 200 MHz \\ Dimensions & \(10\times 10\) cm & \(5\times 47.7\) cm \\ Gain (dBi) & 5 dBi & 5.5 dBi \\ Radiation efficiency & 58\% & 93\% \\ \hline \hline \end{tabular}
\end{table}
Table 21: Features of the SIW antenna vs. HMSIW antenna.