## 4 Lunar Antenna Types
### Reflector Antennas
#### 4.2.2 All-Metal Reflector Antennas  
All-metal antennas are advantageous in space because they can tolerate low temperatures, intense radiation, and vacuum settings. Another significant advantage is the elimination of all dielectric materials in the antenna structure, which prevents electrostatic charge accumulation and related noise. Moreover, a short backfire design may be simply made and is lightweight, without the need for complex deployment procedures [60]. In recognition of its high gain, high efficiency, minimal side lobes, great front-to-back ratio, small form, and sturdy construction, the short backfire antenna (SBFA), initially reported in 1965, has been widely employed in space, marine, and terrestrial applications. This antenna was used in, for example, the communication link between the NASA ground station and the Apollo spacecraft's two moon capsules. The fundamental low-profile design of the SBFA has experienced very modest alterations over the past 50 years, consisting of a cylindrical cavity housing a feed element situated between a ground plane and a smaller sub-reflector [61]. In [60], construction is illustrated of an X-band short backfire antenna, specifically designed to meet the requirements of communication and navigation applications in the harsh environments of space and the lunar surface. Additionally, it enables direct communication with Earth from both the lunar surface and the lunar orbit space. As per  
Figure 11: (**a**) Inflated and (**b**) deflated shape of fabricated inflatable impulse-radiating antennas [57]. Space Frequency Coordination Group (SFCG) recommendations, the bands 7190-7235 MHz and 8450-8500 MHz are suggested as targets for these links, and this design demonstrates suitable performance and tunability within these bands [62; 63]. Moreover, due to its significant gain, a backfire element can be employed to replace numerous patch elements in an array antenna. Backfire antennas exhibit characteristics similar to those of large reflector antennas, such as high gain, equal E-plane and H-plane beamwidths, and low side lobes. However, they can be scaled down to smaller sizes, making them advantageous for space-constrained applications. The schematic image of the short backfire is presented in Figure 12a and the cross-sectional image of the antenna element is illustrated in Figure 12b, with the dimensions in Table 6, where \(\lambda_{\rm o}\) is the free-space wavelength corresponding to the design frequency (8.425 GHz). The proposed antenna has a cylindrical primary reflector and a circular sub-reflector disc. The antenna is linearly polarized and has high gain due to its rectangular waveguide feed incorporated into the main reflector [60]. Multiple designs of the above-mentioned basic antenna construction have also been created and discussed in [60]. First, a concave reflector was constructed by tilting the back face of the primary reflector forward by 15 degrees. Secondly, hexagonal implementation was performed. When arraying, the hexagonal form allows for the best packing efficiency. These antennas produce a gain ranging from 13.4 to 15.6 dB in the cylindrical design and from 12.5 to 14.2 dB in the hexagonal design over the X-band frequencies. The \(-\)10 dB return loss bandwidth for the cylindrical main reflector is 8.92 to 9.4 GHz, while it is 8.0 to 8.42 GHz for the hexagonal main reflector [60; 64; 65], enabling a narrowband operation compared to inf-IRA. Tables 7-9 compare reflector lunar antennas in terms of application, performance, and simulation parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline C\({}_{1}\) & 2\(\lambda_{\rm o}\) \\ \hline C\({}_{2}\) & 0.6\(\lambda_{\rm o}\)–0.7\(\lambda_{\rm o}\) \\ \hline L & 0.5\(\lambda_{\rm o}\) \\ \hline E\({}_{1}\) & 0.6\(\lambda_{\rm o}\) \\ \hline E\({}_{2}\) & 0.25\(\lambda_{\rm o}\) \\ \hline \(\lambda_{\rm o}\) & 35.61 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 6: All-metal single-element short backfire antenna dimensions.