## 4 Lunar Antenna Types
### Reflector Antennas
#### 4.2.1 Inflatable Impulse-Radiating Antenna (Inf-IRA)  
Figure 9 provides the concept and general schematic of the inf-IRA that can be used in lunar penetrating radar. It is designed to be highly portable and easily transportable, with the ability to be deflated and rolled up into a compact size for convenience. However, once air is pumped into the antenna, it rapidly inflates to a paraboloid shape, similar to that of a balloon.  
The antenna comprises three primary components: a parabolic reflector, feed arms, and matching resistors. What distinguishes this antenna from traditional impulse-radiating antennas is that it features only a single feed arm, resulting in an input impedance of 400 \(\Omega\)[52]. Consequently, the implementation of a balun becomes necessary. To facilitate the construction of a balun, it is necessary to decrease the input impedance of the antenna. In this work, the implementation of asymptotic conical dipole (ACD) feed arms is employed to reduce the input impedance from 400 \(\Omega\) to around 200 \(\Omega\). Figure 10 illustrates the depiction of the traditional IRA and the intended ACD arm of the fed-IRA. To prevent energy from being reflected back into the antenna system, 100-ohm impedance matching resistors are placed between the ends of the 0.3 m diameter ACD feed arms and the reflector. It is important to note that for lower-frequency applications, proportional scaling of the antenna is necessary to achieve optimal performance, which includes increasing the antenna's diameter accordingly [57; 58; 59].  
Figure 10: Side view of two antennas: conventional impulse-radiating antenna and asymptotic conical dipole fed impulse-radiating antenna [57].  
Figure 9: Inf-IRA concept and overall diagram [57].  
When fully inflated with air, the edge of the reflector may become wrinkled, so margins are included along the rim to prevent inaccuracy. Eight stays are placed on the interior to prevent deviation from the parabolic form and they are fastened to the reflector vertically with flexible Kapton tape. The ACD feed arms are printed on a flexible thin Kapton substrate, which also serves as a stay to maintain the reflector's parabolic shape. Once the air is removed and the Mylar surface is deflated, the thin Kapton substrate remains flat, allowing the inf-IRA to be folded up for portability. The inflated and curled-up morphologies of a constructed inf-IRA are shown in Figure 11[57]. The antenna is simulated using FEKO.  
In terms of functionality, this antenna functions as an impulse-radiating antenna and has the ability to cover frequencies ranging from 0.5 GHz to 4 GHz, enabling broadside operation that encompasses VHF and S band frequencies. Within its operational range, the antenna can achieve a gain ranging from approximately 2 to 17 dBi. For applications such as lunar penetrating radar, the need for the high-resolution detection of geological structures is evident. In this context, the advantages of utilizing ultra-wideband antennas that can operate across a wide frequency spectrum, from very low to high frequencies, are apparent. Additionally, employing high-directivity antennas is essential to minimize energy wastage. While these options necessitate the use of large antennas, it is important to note that larger antennas can escalate launch costs. In this regard, inflatable structures hold particular significance due to their compact packaging and minimal mass, offering a potential solution to address this challenge.