# Low-Profile Dual-Band Reflector Antenna for High-Frequency Applications
## 1 Introduction  
The millimeter wave (MMW) and terahertz (THz) bands are widely used in wireless communications due to their high data rate, wide transmission bandwidth, and high security. Owing to the massive increase in application scenarios, spectrum resources in different frequency bands have been explored and utilized, e.g., the 220 GHz antenna for sensing [1] and 0.3 THz and 0.5 THz antennas for high-speed wireless data transmission [2; 3; 4]. With the increasing exploitation of spectrum resources, the number of required antennas is also increasing, leading to higher costs. Therefore, a single antenna that can work in different frequency bands can reduce the number of antennas and save on costs. Moreover, in order to guarantee the wireless link budget, it is important to enhance the antenna directivity. With the development of wireless communication systems, characteristics such as high gain, high efficiency, low profile, and low cost are required for MMW/THz communication systems. In recent decades, researchers have developed various types of antennas to accommodate these requirements. For instance, substrate-integrated waveguides (SIWs) [5; 6; 7] are employed to achieve high gain. However, these array antennas exhibit low-efficiency performance due to dielectric loss when the array scale increases to 32 \(\times\) 32. In addition, waveguide slot array antennas in [8; 9] are among the solutions for MMW/THz antennas. However, the processing cost is increased due to their intricate structure. To accommodate high-gain and high-efficiency requirements, reflector antennas have been extensively investigated in communication systems. In recent decades, various methods of designing dual-band reflector antennas have been proposed [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Ultra-wideband feed antennas, capable of covering two different bands, have been proposed, e.g., quadridge horn [14], sinuous feed antenna [15], andEleven feed antenna [16]. However, these antennas have some shortcomings. For example, the phase center of the quadridge horn and the polarization direction of the sinuous feed change based on frequency, which can cause deterioration in the dual-band performance of the reflector antenna. Similarly, the radiation patterns of Eleven feed antenna change significantly based on frequency. Therefore, it cannot be guaranteed that the efficiency of the reflector antenna will maintain an acceptable level throughout the entire operating bandwidth [17]. Additionally, a dual-band feed antenna can be used to illuminate the main reflector antenna to achieve dual-band functionality [18; 19; 20; 21; 22]. However, when this approach is applied in MMW/THz bands, dual-band feeds may confront processing challenges due to the complicated structures of the feed antenna [18; 19]. To obtain a high aperture efficiency of the reflector antenna, a high focal-to-diameter (F/D) [20; 21; 22] ratio is required. Generally, the profile of the reflector antenna is relatively high, which contradicts the low-profile requirement. Essentially, the two methods, i.e., utilizing ultra-wideband feed antennas and using dual-band feed antennas for dual-band operation, focus on the designs of feed antennas. To obtain dual-band capability through the subreflector (SR) of the antenna, a frequency-selective SR (FSSR), which can separate the higher- and lower-band feeds, is introduced [23; 24; 25; 26; 27]. Generally, the FSSR is designed to reflect signals in one frequency band and propagate signals in the other frequency band. However, the unavoidable deterioration of radiation occurs due to the imperfect reflection and transmission of the FSSR [26; 27], and the use of the FSSR also leads to a high profile. To achieve a lower profile, dual reflectors are employed in the design, instead of a single reflector. For example, a subreflectarray (SRA) is employed in [28] as the SR which can work in two different frequency bands.