## 4 Lunar Antenna Types
### Wire Antennas
#### 4.4.1 Dipole Antennas  
Dipole antennas, also known as doublets, are widely used in various fields, including lunar communication, because of their omnidirectional properties, low mass, and low power requirements. A dipole antenna is characterized by a structure resembling a rod or metallic wire, separated by a certain distance. The most basic dipole configuration consists of straight wires or rods connected end to end. Typically, the dimensions of a dipole antenna are proportional to the wavelength, with the half-wave dipole antenna being the most prevalent design, where each rod has a length equivalent to one-quarter of the wavelength. The half-wave dipole has a radiation resistance of 73 \(\Omega\)[67; 68].  
Dipoles were used in the radio observatory on the lunar surface (ROSS), which is an innovative concept for a near-side low radio frequency imaging interferometric array aimed at researching particle acceleration in the sun and inner heliosphere. One of its objectives is to limit the density of the lunar ionosphere by investigating a lower radio frequency limit for solar radio emissions and determining the population of low-energy electrons in astrophysical sources. The design comprises three antenna arms arranged in a Y-configuration, with a central electronics package (CEP) situated at the center. The Y-shaped configuration allows for the creation of high dynamic range images in a short time and facilitates easy deployment. The antenna arms consist of linear strips of polyimide film, like Kapton, where 16 dipole antennas are deposited by a conductor, such as silver. Each one of the dipole antennas is a single polarization dipole. The strips can be rolled up for easy transport and deployed by unrolling. Transmission cables transport the radio signals from the scientific antennas to the CEP through the arms. The CEP holds the receivers for the dipole antennas, command and data handling hardware, and the downlink antenna, positioned outside the package [69; 70].  
The interferometric array was meticulously designed to operate within the frequency range of 1 MHz to 10 MHz [69; 70]. To accomplish this, dipole antennas were strategically positioned on the polyimide film of each array arm. Notably, the dipole antenna operates at the higher end of the array, specifically at 10 MHz. However, at the lower end (1 MHz), the antenna becomes electrically short, resulting in a small feed point resistance and high capacitance, sharing similar characteristics with the previously mentioned low-frequency antennas. Within this frequency range, the antenna's performance exhibits significant drawbacks, including poor efficiency, radiation resistance, and fractional bandwidth. Despitethese shared characteristics, we opted not to classify these antennas within the previously mentioned low-frequency antenna category for two specific reasons. Firstly, these antennas are fundamentally dipole antennas, and, secondly, their resonant frequency is 10 MHz, which falls outside the conventional definition of the low-frequency spectrum.  
The second dipole antenna that is used in lunar communication is the small-sized active dipole antenna. It was noted that studies have produced many ultra-long-wavelength antennas that are a prototype of ultra-long-wavelength radio telescopes for future moon radio observatories. In [71], the authors conducted a study on the complex geometry of an active dipole antenna that was situated above a partially conductive ground. This study involved numerical simulations of the antenna prototype as well as measurements of various parameters. The active dipole antenna is presented in Figure 13[71]. The antenna is designed to operate at 1-70 MHz. However, other characteristics in terms of performance or structure are not available.