# Quad-Band Circular Polarized Antenna for GNSS, 5G and WIFI-6E Applications
## 1 Introduction  
The circular polarization (CP) antenna is an important component in many communication systems. The investigation into this component has been a hot topic for many years [1, 2]. Electromagnetic waves usually encounter reflection and refraction during propagation, which often leads to a change in polarization direction or state [3]. In addition, when travelling through the ionic and rain layers, depolarization effects may occur, which is very harmful to communication links. It is well recognized that a circularly polarized wave is less affected by the Faraday rotation [4, 5] in comparison to a linearly polarized one. Moreover, it is superior in countering the depolarization effect. Therefore, circularly polarized antennas are usually used in global navigation satellite systems (GNSS). They also have important applications in various wireless systems, such as radar and mobile communication systems [6]. With the swift development of wireless communication, many functions are integrated into a single terminal device, and multi-band antennas are in active demand. Antennas working over several frequency bands with circularly polarized properties are, therefore, highly preferred. The first requirement may be the coverage of several navigation systems simultaneously; the second requirement is that other wireless communications, such as5G and WIFE, can be integrated in a single terminal device. In this connection, further investigation into multi-band circularly polarized antennas is required. For an arbitrary electromagnetic wave, its electric field \(\overset{\rightharpoonup}{E}\) can be divided into two components, i.e., \(\overset{\rightharpoonup}{E}=E_{x}\mathrm{e}^{\mathrm{i}\varphi_{x}}\hat{e}_{x} +E_{y}\mathrm{e}^{\mathrm{i}\varphi_{y}}\hat{e}_{y}\). If the amplitudes of the two components are equal (\(E_{x}=E_{y}\)) and the phase difference meets \(\varphi_{y}-\varphi_{x}=\pm 90^{\circ}\), the resultant field appears as a circularly polarized wave [7]. There are many techniques to realize CP antennas, such as cutting slots [8; 9; 10], adding branches [11; 12], using a feed network with a phase difference of \(90^{\circ}\)[13; 14], etc. In Ref. [14], a polarization reconfigurable antenna was developed for GNSS. The variation in operation frequency is tuned by PIN-diode switches, which are not multi-band operation antennas. A compact antenna was proposed in Ref. [15], through using two T-coupling lines with different lengths as feeds, to achieve circular polarization. It can fully cover the GPS L1 band. Stacking radiating patches is another common method to generate multi-band circular polarization [16; 17; 18; 19]. The main drawbacks of the stacking technique are the difficulty in fabrication and relatively larger thickness, which is not preferred in many mobile terminals. Multi-band circular polarization can also be achieved by etching slots on the ground layer [20; 21; 22; 23]. Most of these designs can provide dual-band operation. In Ref. [24], a circularly polarized antenna is developed based on a dielectric resonator. This antenna can excite six different mixed radiation modes and generate four frequency bands for BDS, GPS, WLAN, and WiMAX. However, the axial-ratio (AR) bandwidth is narrow, while the antenna size is relatively large. In Ref. [25], a three-band circularly polarized antenna was proposed. By using a linear-to-circular polarization converter on the bottom layer, circular polarization waves can be radiated. The antenna provides three AR bandwidths of 3.53%, 1.73%, and 13.62%, which are very suitable for satellite applications. However, it does not have extra bands for other wireless communications. In Ref. [26], a tri-band, wearable circularly polarized antenna was proposed, which can be used in WLAN, C band, and X/Ku band. The AR bandwidths of each band are 10.10% (4.7-5.2 GHz), 4.95% (5.9-6.2 GHz), and 10.44% (11.8-13.1 GHz), respectively. At present, the navigation systems include GPS, BDS, GALILEO, and GLONASS [27; 28].