## 4 Lunar Antenna Types
### Planar Antennas
#### 4.1.1 Patch Antennas  
NASA's Glenn Research Center (GRC) has investigated numerous miniature antenna designs that could be employed for communicating on the surfaces of planets and moons. Among the designs explored, two have shown potential: the CMMA and the fHCFA. Both of these antennas were designed and tested in the research laboratory. While both of these antennas incorporate certain non-planar elements, such as the vertical metallic enclosure in the case of the CMAA antenna and the third-order Hilbert curve that folds in upon itself into four layers incorporated into the fHCFA antenna, their fundamental structure still relies on a patch and microstrip configuration. Therefore, these antennas have been classified within the planar category despite these non-planar components. GRC has developed and simulated the CMMA for lunar and space communication applications using IE3D simulation software ([[http://www.zeland.com/](http://www.zeland.com/)]([http://www.zeland.com/](http://www.zeland.com/))) (accessed on 4 October 2023). The CMMA exhibits a unique configuration characterized by a three-lobed patch enclosed by a vertical metallic barrier and a grounding boundary near the input location, as illustrated in  
Figure 3: Classification of lunar antenna types based on the existing literature. Figure 4a,b. The selected substrate for the antenna is Duroid 5880, which has a thickness of 1.57 mm and a relative permittivity (\(\epsilon_{\mathrm{r}}\)) of 2.2. This antenna boasts an exceptionally compact form factor, measuring 12 mm \(\times\) 12 mm with a height of 11 mm, making it one of the smallest antennas in the documented group. Its size corresponds to \(\lambda/12\), where \(\lambda\) represents the wavelength in free space at its minimum operational frequency, rendering it well-suited for integration into small devices. The PMMA antenna operates at a frequency of 2.05 GHz and boasts the widest fractional bandwidth among the three reported patch antennas with a remarkable 6.3%. Notably, this antenna delivers an impressive directivity of 6 dBi, surpassing the performance of the other two patch antennas in the study. This enhanced gain, coupled with its extensive bandwidth and directivity, positions the PMMA antenna as a promising candidate for various applications. In contrast to conventional small antennas, which are often omnidirectional, the PMMA's attributes make it suitable for deployment in a wide range of scenarios where tiny, high-performance antennas are typically utilized [44]. NASA's GRC center has also developed the fHCFA using Zeland's IE3D electromagnetic simulator. This marks NASA's second antenna designed for lunar communication. The fHCFA boasts the smallest dimensions within its antenna group, consisting of four layers with measurements of 5 mm \(\times\) 5 mm \(\times\) 4.5 mm (length \(\times\) width \(\times\) height), resulting in a \(\lambda/30\) design. The antenna structure was etched onto Duroid 5880 with \(\epsilon_{\mathrm{r}}\) = 2.2 at a thickness of 0.127 mm. It was affixed to an aluminum ground plane using multiple layers of adhesive, with 1 mm thick HTP-6 foam spacers (\(\epsilon_{\mathrm{r}}\) = 1.07) separating the layers. A probe feed was employed to excite the antenna, as depicted in Figure 5a,b. These antennas were designed for operation across multiple frequencies, with particular interest in two different operating frequencies: 2.3 GHz and 16.8 GHz. These frequency bands, specifically the S band and Ku band, are well-suited to lunar surface network communications and planetary exploratory missions that rely on local area networks (S band) and satellites (Ku band) for establishing communication links. Remarkably, this antenna's  
Figure 4: (a) PMMA 3-directional view. (b) PMMA prototype [44]. Figure 5: (a) fHCFA with dimensions layout. (b) Fabricated fHCFA [44]. multi-band capability eliminates the need for electrical or mechanical switching, allowing it to perform the tasks of two separate antennas while maintaining a reduced size.