# Design of Multifunctional Mesosphere-Ionosphere Sounding System and Preliminary Results
## 2 System Description
### Miniaturized Antenna  
The antenna of MAI system should be adapted to the mesospheric and ionospheric sounding mode simultaneously, which means that the antennas need to guarantee the performance at MF band while ensuring that the serious beam splitting and directivity change would not occur at HF band. Apparently, the large-sized standing-wave antenna array employed in conventional MF radar is not suitable for the MAI system. Therefore, two schemes are adopted in this paper for the functions of MF radar and the ionospheric sounding. Moreover, some simulations are also carried out based on computer simulation technology (CST). For the mesospheric sounding, MAI system adopts the mode of one transmitting and six receiving. A 75 m three-wire linear polarized dipole antenna is employed to match the MF band. The physical erection image is also can be seen as Figure 8. The frequency of 1.98 MHz is chosen as the working point for the mesospheric sounding. The simulation results of the variation of the transmitting characteristics with the erection height at 1.98 MHz are shown in Table 3. And the impedance is 50 \(\Omega\). When the erection height is within 6 m, the directivity gain increases with height slightly, but when it exceeds 4 m, the VSWR deteriorates rapidly. Obviously, the height of 3 m is the most suitable choice. It has the best VSWR to guarantee the radiation characteristic, and the lower erection height also reduces the construction difficulty. Figure 8: Erection image of 75 m three-wire transmitting antenna. It is a linear polarized dipole with the feed point in the center. Figure 7: The structure of the DDC module. Digital mixing and filtering are implemented in FPGA, which generate the baseband signals and reduce the data rate. For the MF echoes' receiving, MAI system employs two-wire dipole progressive-wave Barker & Williamson broadband folded dipole antennas ([PERSON], 1993), the length of which is 54.9 m, compared with Wuhan MF radar, it is reduced by 20 m. When MAI system works in SA mode at 1.98 MHz, an equilateral triangle array with a side length of 180 m is employed for receiving. At each vertex of the array, two antennas are crossed horizontally in an orthogonal polarization state. The specific receiving antenna section shape is shown in Figure 9. The nearest one is 200 m away from the transmitting antenna. Figure 9a is the array structure and Figure 9b is the erection form of each vertex. In this way, although there is only one transmitting channel, the separate reception of the ordinary wave (O) and the extraordinary wave (X) can still be achieved. Figure 9c shows the schematic erection form of a single receiving antenna and Figure 9d shows the structure image. The erection height is indicated by \(h\). Due to gravity, the antenna cannot be completely straightened, so the spacing between poles is only 50 m.  
Further, Table 4 shows the variation of the receiving characteristics with the elevation height of the receiving antennas. In the range of 4-25 m, the directivity gain has little change. However, when the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & **H = 2 m** & **H = 3 m** & **H = 4 m** & **H = 5 m** & **H = 6 m** \\ \hline Directivity Gain (dB) & 8.5 & 8.51 & 8.52 & 8.54 & 8.55 \\ Beamwidth (\({}^{\circ}\)) & 67.17 & 67.28 & 67.19 & 67.22 & 67.27 \\ VSWR. & 1.15 & 1.12 & 1.21 & 1.28 & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The variation of the transmitting antenna characteristics with the erection height. Figure 9: The antenna erection form: (**a**) The array structure, with a positive triangle structure; (**b**) The erection form of each vertex. The two antennas are erected in an orthogonal way; (**c**) The schematic erection form of a single receiving antenna with the feed point in the center. Due to the influence of gravity, the antenna cannot be straightened completely. The distance between the two poles is 50 m; (**d**) The structure image of the Barker & Williamson Broadband Folded Dipole Antennas.