## 4 Lunar Antenna Types
### Planar Antennas
#### 4.1.2 Slot Antennas  
The tapered slot antenna (TSA) is a microstrip antenna with a coplanar structure and a tapered slot printed on a dielectric substrate. In support of the feeding apparatus, the tapered slot's narrow extremity is affixed to the anterior portion of a receiver or the output phase of a transmitter, while its wider extremity either captures or emits radio waves in the end-fire orientation. The slot antenna offers a broad frequency bandwidth, a symmetrical radiation pattern, and end-fire radiation characteristics. The slot taper can take various geometrical forms, including linear taper (LT), exponential taper (ET or Vivaldi), constant width taper (CWT), and others, each suited to specific applications. An array of TSAs may be arranged parallel to one another and utilized for airborne radar, beam-shaping antennas, imaging systems, and phased arrays [46, 47, 48]. The Vivaldi antenna is a TSA that is recognized as the most advanced antenna for a variety of applications including wireless communications, biomedical engineering, and military applications [49].  
A Vivaldi antenna was used in the Chinese mission due to its compatible design and broader frequency bandwidth for lunar communication. In 2020, China launched the Chang'e-5 mission, which includes the LRPR on its lander. The LRPR's role is to analyze the lunar regolith's properties and structure to support drilling and sampling activities [11, 12]. To accomplish this goal, an ultra-wideband multiple-input multiple-output (MIMO) array was created, with the ability to conduct two-dimensional scans of the area beneath the array. This is achieved by toggling antennas between transmitting and receiving modes using a switch matrix. The MIMO array design encompasses 12 strategically positioned small quarter-elliptical slotted Vivaldi antennas (QESAs) mounted on the CE-5 lander. These antennas were constructed using a ceramic-filled polyimide laminate material with a relative permittivity of 3.8, loss tangent of 0.006, and thickness of 1 mm. A visual representation of the QESA can be found in Figure 6, and its detailed structure specifications are provided in Table 2[50].  
In order to attain extensive frequency compatibility for the antenna, the slot line and microstrip terminals were designed as a wideband short stub and open stub, respectively. The 12 elements are divided into three groups: A, B, and C. Groups A and B are positioned on the lander's lower surface while Group C is located on the lander's side, as depicted in Figure 7. Operating across a frequency range of 1.0 to 4.75 GHz, this antenna can deliver a  
Figure 6: Quarter elliptical slotted antenna (QESA) [50].  
realized gain ranging from 2 to 8 dBi. Its remarkable broadband capability enables it to achieve an impressive fractional bandwidth of 130%, marking it as the highest bandwidth reported in its class. However, lunar mission antennas face specific challenges. During lunar landing, the engine's plume can subject the antenna to temperatures as high as 250 \({}^{\circ}\)C. Simultaneously, the plume stirs up charged lunar dust, which, if it adheres to the antenna elements in significant quantities, can adversely affect antenna performance. To safeguard against these conditions, a radome was employed to cover the antennas, capable of withstanding temperatures of up to 350 \({}^{\circ}\)C. These radomes were constructed from polyimide composites to ensure robust protection for the [PERSON] array [50; 51; 52; 53; 54].