# A Multi-Frequency Omnidirectional Antenna Based on a Ring-Shaped Structure
## 1 Introduction  
In recent years, with the rapid development of wireless communication technology, wireless communication equipment is widely used in satellite navigation [1; 2; 3], wireless interconnection [4; 5; 6; 7], high-speed information transmission, and other communication fields [8; 9; 10; 11; 12]. The antenna is an important component used to send and receive information in a wireless communication system [13; 14]. The use of single-frequency antennas has been unable to meet the needs of modern multi-frequency communication. Moreover, when multiple antennas work at the same time in a limited space, there will be coupling effects between each other, resulting in the failure of the communication system to work normally. Modern wireless communication systems require antennas to be multifunctional, easy to integrate, low-cost, and miniaturized [15; 16; 17; 18]. Multi-frequency antennas can work in multiple communication frequency bands at the same time. Moreover, using a single antenna to achieve multi-frequency effects, in a sense, greatly reduces the area of the antenna, saves the design cost, and meets the needs of industrial communication equipment [19; 20; 21]. Therefore, how to realize the multi-frequency radiation characteristics of antennae under other good conditions has become a hot research topic at present. In recent years, various approaches have been developed to design and produce multi-frequency antennas. The main methods include the multilayer method [22; 23; 24], slotmethod [25; 26], branch method [27; 28], the reconfigurable method [29; 30; 31], the metamaterial loading method [32; 33; 34; 35; 36], and Dipole multifrequency antenna [37; 38; 39; 40]. For example, in 2010, [PERSON] et al. [22] proposed a dual-frequency microstrip antenna using low-temperature co-fired ceramic (LTCC) technology by adopting a multi-layer method, which can be used for GPS positioning. In 2015, [PERSON] et al. [25] proposed a multi-frequency antenna with three operating frequency points by etching three slots of different lengths on a triangular radiation patch. In 2018, [PERSON] [28] designed an antenna composed of multiple branches. Changing the length of the corresponding branch effectively modulates the resonance frequency and impedance matching of the antenna. This antenna is an excellent choice for LTE, WLAN, WiMAX, and other narrowband systems. In 2018, [PERSON] et al. [31] proposed a new antenna that uses two PIN diodes to modify the working state of the diode without changing the physical size of the antenna, which can obtain the reconfiguration of the antenna in four different frequency bands. In 2017, [PERSON] [33] designed a metamaterial structural unit as the radiation patch of the antenna to obtain dual-frequency performance. In 2019, [PERSON] et al. [34] achieved dual-frequency operation by loading a transmission line-inspired structure onto the antenna. In 2020, a four-port four-frequency antenna available at high-low frequencies was presented by [PERSON] et al. [37]. In 2021, [PERSON], [PERSON] et al. [38] designed a multi-frequency microstrip antenna that works at seven frequency bands. In 2022, [PERSON] et al. [39] demonstrated a four-band compact dual-element MIMO (multi-input multi-output) antenna system consisting of a compact inverted F antenna with a zigzag arm. One side of the T-type monopole antenna is extended and twisted, and the other side is grounded, forming the IFA structure. In 2023, [PERSON] et al. [40] designed a dual-port MIMO antenna that provided broadband of 25.5-30 GHz and a peak gain of 8.75 dBi. However, the above-proposed antennas could not meet the requirements of industrial communication equipment for multi-frequency communication in terms of multi-band, size, and gain for most of the operating frequency bands of multi-frequency antennas mentioned above. In this paper, we introduce a new multi-frequency antenna structure, which can realize the function of the multi-frequency antenna at six frequency points.