# Worldwide Coverage Mobile Systems for
## 5 Results  
As aforementioned, travelling-wave arrays can also be used to increase the bandwidth, among other features. Because the SIW behaviour is very close to waveguides, more than one mode can propagate. These features are exploited by the authors to design a dual-band circular polarized antenna slot array in the Ku-band. Selected bands, which are independently tunable, are 10.7-11.7 GHz for RX side and 14-14.5 GHz for TX link. Because \(H_{x}\) and \(H_{y}\) are phase-shifted 90\({}^{\circ}\), circular polarization can be achieved by using a set of slots: transversal and longitudinal, at the same place. In the downlink (RX frequencies), \(TE_{10}\) propagates and the design includes slots placed to enable surface currents to produce radiation. Figure 9 shows the radiation pattern on different planes. In the up-link (TX-band), the \(TE_{20}\) mode excites another set of two slots. The magnetic field and the surface currents do not perturb each other, as shown in Figure 10. In consequence, the set of slots can be independently tuned. Figure 10 shows the magnetic field while Figure 11 depicts the surface currents. The two sets of slots can be seen as a single LHCP Radiator block. Co-polar and Cross-polar diagrams are shown in Figure 9, and the main features are summarized in Table 22.  
The dual-band feeder is designed using a three-layer stack-up. A transition from microstrip-to-SIW is placed on the top face with a bandpass filter centred at RX-band, while at the bottom face we place a 180\({}^{\circ}\) phase shifter to excite \(TE_{20}\) mode into SIW at TX-band. Figure 12 shows the resulting structure. Once the single radiator block element is designed, we can construct an array by uniformly distributing basic elements, where a matched-end is provided for both propagation modes by using a proper termination as shown in Figure 13.  
A dual-band antenna with circular polarization working at Ku-band can be aimed to act as transceiver in the ground-segment for novel LEO constellations such as OneWeb, SpaceX and other Cubesat constellations. For this purpose, slots in Figure 11 are replicated 20 times along the Y-axis to enhance directivity, as shown in Figure 14. Slots working at TE\({}_{10}\) are 5.5 mm-long, whereas TE\({}_{20}\) mode works with 4.5 mm-slots. To make sense of the satellite application, each couple of slots is placed at different distances to obtain the same beam pointing for both bands (12.5 mm for RX and 11.8 mm for TX). Figure 11: Surface currents. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Feature** & **RX-Band** & **TX-Band** \\ \hline Bandwidth & 9\% & 3.5\% \\ Dimensions & 10 \(\times\) 10 cm & 5 \(\times\) 7.7 cm \\ Directivity & 5.6 dBi & 6 dBi \\ Decoupling losses & 0.4 dB & 0.2 dB \\ Slot length & 6 mm & 5 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 22: Features of the dual band circular polarized antenna. Figure 12: Dual-mode feeder. If the structure of Figure 14 were replicated again ten times along X-axis, allowing electronic steering for satellite-tracking, directivity would increase above 25 dBi, which can be considered as a good value for LEO-satellites links. Regarding OneWeb constellation [7], portable transceivers for user-ground-segment are suggested to be 30 cm-dish antennas. This novel constellation deploys 720 satellites in 18 1200 km-orbits and, for the user-link, provides 16 beams with 250 MHz-bandwidth each for downlink and 1.5-20 MHz for uplink. Although the proposed SIW antenna is smaller and not as directive as the reflector one, due to the circular polarization, SIW provides 3 dB to the satellite-link-budget, whose receiver antenna for user link holds a \(G/T=-1\) dB. Assuming a 50W-HPA for the user terminal, a comparison between the suggested dish-antenna and the proposed one is shown in Table 23.