# Compact UHF Circularly Polarized Multi-Band Quadrifilar Antenna for CubeSat
## 2 Antenna System Design
### Electromagnetic Modeling of the Proposed Structure  
Ansys HFSS software is used to simulate the proposed antenna system; then, the design concept could be confirmed. In the next section, two different cases are presented in order to understand the effect of the 3U satellite structure on antenna performance. The case of \"without structure\" refers to the antenna alone, as shown in Figure 1. Then, the case of \"with structure\" refers to the antenna mounted on the 3U CubeSat metal frame. In this case, the satellite structure is added and analyzed to verify its influence on the antenna features. As shown in Figure 4b, a simplified CubeSat structure with an external conductive frame is considered. It can be seen that four frame corners extend 10 mm around the antenna, which could affect the impedance matching and the radiation pattern of the proposed antenna. It should be noted that the maximal dimension of the 3U satellite is 34 cm, which is exactly the wavelength at 870 MHz. The simulated results are shown in Figure 5 without and with a satellite structure. The reflection coefficient on port 1 and the transmitted power to ports 2 and 3 (matched isolated port) are considered in order to analyze the global performance of the structure. The most important parameter is \(S_{11}\) in order to provide an optimal input impedance to the power amplifier. The transmission to ports 2 and 3 should be minimized to maximize the antenna's total efficiency. A \(-\)10 dB criterion is targeted for \(S_{11}\), \(S_{21}\), and  
Figure 4: (**a**) Layout of the feeding circuit; (**b**) 3D view of the antenna model integrated within the 3U structure. \(S_{31}\) on the entire frequency band ranging from 860 to 930 MHz. According to Figure 5a, the \(S_{11}\) curves of these cases are quite different. These dissimilarities could be explained by the influence of a large metallic part placed under the antenna. However, they are shown to respect the S-parameter criteria for the required frequency band. The gain achieved on the broad side versus the frequency is presented in Figure 5b. Without and with the CubeSat structure, the right-hand circular polarization is achieved with an axial ratio lower than 3 dB (a 3 dB axial ratio is equivalent to a 15.35 dB difference between the RHCP and LHCP gains [38]). Without the structure, the simulated realized gain is higher than 1.6 dBic on the 860-930 MHz band. However, when the antenna is mounted on the CubeSat structure, the simulated realized gain is lower, which is only more than 0 dBic in the whole band. Several reasons can explain this effect. First, the reflection coefficient of the antenna-only case is better than the complete system. Second, the total directivity with the structure is slightly lower because of vertical currents flowing on the CubeSat chassis and increasing the antenna beamwidth. Without the CubeSat structure, the realized gain radiation patterns at 870 MHz and 920 MHz are presented in Figure 6. A 120\({}^{\circ}\) beamwidth of RHCP is obtained in both bands, with 2.5 dBic as gain. Regarding the LHCP radiation pattern, the proposed antenna obtained a gain of \(-\)2.5 dBic and is radiated in the opposite direction. When the antenna system is mounted on the metal satellite frame, Figure 7 shows a wider beamwidth radiation pattern. However, a lower RHCP gain is obtained, and back LHCP radiation is also reduced on both bands. The circular polarization feature is also reported in Figure 8. According to these results, the proposal has a 3 dB AR beamwidth of 120\({}^{\circ}\) toward the z-axis. Moreover, due to the effect of the satellite structure, the obtained axial ratio is more efficient than the antenna-only case. Figure 5: (a) S parameter simulation of the antenna with and without structure; (b) gain radiation simulation of the antenna with and without structure. Figure 8: Axial ratio versus theta simulation of the antenna without and with structure.