A Circularly-polarized Patch Antenna for BDS Applications

The BDS (Beidou) Navigation Satellite System has been a powerful tool in real-time navigation and positioning. In the transitional stage between BDS-2 and BDS-3, it would be a top priority to ensure the antenna is compatible with the two systems. In our investigations, a circularly-polarized (CP) patch antenna is proposed for receiving B1I signals. This antenna is a newly designed slit-loaded square patch structure. The T-shape structure with F-shape slots is deployed on each edge of the antenna, and a single feed is employed to excite right-hand CP radiation. In simulations, all the parameters of the antenna are optimized through Ansys HFSS. In experiments, the processed antenna is further testified, where the measured bandwidth is compared with the simulated one to verify the effectiveness of the antenna.


Introduction
GNSS (Global navigation satellite system) based navigation technologies have recently drawn increasing attention in transportation, engineering surveys [1], environmental monitoring, etc.The Beidou Navigation Satellite System (BDS), as a newly developed navigation system, is capable of providing real-time positioning with short message communication, playing an important role in aeronautics and astronautics.BDS is currently iterated from BDS-2 to BDS-3, during which B1I signals with identical bands will be applied for communication compatibility.
Navigation antennas assembled in the terminal are generally required to be circularly polarized for effective positioning.With recent implementations focused on geometrical configurations, the navigation antennas now can be summarized as dipole antenna [2], spiral antenna, quadrifilar helix antenna [3][4], and micro-strip antenna [5] [6], respectively.
In order to receive B1I signals, the bandwidth of the antenna under -10 dB would be strictly required from 1.559 GHz to 1.563 GHz.One possibility is to optimize configurations of quadrifilar antennae, where parasitic or passive elements [7] are employed to enhance antenna performance.Nevertheless, the corresponding geometrical parameters are not available for assembly on the monitored devices, hence, it is necessary to meet application requirements.Another approach for B1I signals is to use a multi-layer antenna [8][9].Central feedings are further investigated with a specially designed feed model, such as bridge-network-based feeding [10] or waveguide feeding.Despite the high performance, low radiation gain will still occur in practical measurements.Considering both capability and geometry of the antenna, a micro-strip antenna would be a good candidate to be potentially applied to B1I signals.
In our approach, a circularly-polarized (CP) patch antenna is proposed for B1I signals of BDS.The antenna is developed based on a slit-loaded square patch.T-shaped structures with F-shaped slots are employed on top of the patch, and a single probe feed is implemented as an exciter for right-hand CP radiation.The bandwidth of S11 and the axial ratio is achieved as 1.53-1.60GHz and 1.556-1.569GHz, respectively.The parameters of the antenna are optimized through analysis on Ansys HFSS.The antenna is also fabricated and tested by a vector network analyzer to verify its performance and reliability.

Preliminary design of the designed antenna
Figure 1 displays the structure of the given antenna.The designed antenna is based on a slit-loaded square patch which is a fabricated copper radiation layer on the top of an FR-4 dielectric substrate (permittivity 4.4) and a copper ground on the bottom.Both the patch and the ground layer are made of copper.The widths of the square patch and the substrate are defined as WT and g, respectively.Parameter WT can be preliminarily defined as Equation ( 1), where c refers to the velocity of light, f is the working frequency of the antenna, and ε r is the permittivity of FR4.In this work, WT is calculated as 58.5 mm, and the thickness of substrate h is set as 2 mm.One T-shape structure is added to each edge of the square patch.In addition, four F-shape slots are also etched in the square patch.In the patch center, a narrow slot with dimensions of p×q is embedded along the x-axis.In this patch, a single probe feed is implemented at (x 0 , y 0 ) and serves as an exciter for right-hand CP radiation.In T-shape structures, the roots along each axis exhibit identical configurations, where the length of the roots of the T-shapes is set as da.And for the F-shape slots, the slot width is set as St.

Optimization of parameters
The antenna is further analyzed and optimized by using Ansys HFSS. Figure 2 and Figure 3 give the retrieved axial ratios and S parameters with different p and dw, respectively.It is clearly noticed that the center frequency of AR moves left belongs to the raise of p but moves right with the increase in dw, while that of S 11 moves right with the increase of p and dw.The optimal selection of p and dw are selected as 3.9 mm and 1.5 mm after a series of iterations.The parameters of the antenna are optimized in simulations and summarized in Table 1.  Figure 4 shows the characteristics of the antenna after parameter optimizations.Figure 4(a) corresponds to the S parameter of the antenna, the bandwidth for S 11 < -10 dB is 1.53-1.60GHz. Figure 4(b) gives the AR map, where bandwidth below 3 dB is in a range from 1.556 GHz to 1.569 GHz.The radiation patterns of this given antenna are retrieved as given in Figure 4(c).It can be observed that the given antenna is capable to emit more right-hand CP waves in the upper hemisphere for B1I signals.

Results and Discussion
To solidify our investigations, the antenna is fabricated and testified by using the vector network analyzer.Figure 5 displays the fabricated antenna soldered on the SMA interface.Figure 6 shows the measured results of bandwidth for S 11 < -10 dB in free space, which is approximately in a range from 1.486 GHz to 1.529 GHz.It can be observed that the variation of the measured S 11 is basically in agreement with the simulated one, but the frequency shift of 0.04 GHz can also not be ignored.This is caused by various fabrication errors like welding errors or copper thickness deviations, etc., which potentially contribute to imperfections of bandwidth.

Conclusions
A circularly-polarized antenna is designed for the transitional stage between BDS-2 and BDS-3 in this design.Based on T-shaped structures with F-shaped slots, the effective bandwidth of S 11 and axial ratio can be derived as 1.53-1.60GHz and 1.556-1.569GHz accordingly.The antenna is preliminarily optimized by Ansys HFSS, then fabricated and further testified with comparisons to the theoretical one.More investigations will be carried out focusing on multiband circularly-polarized antenna applied for the third generation of BDS.

Figure 2 .
Figure 2. Simulated AR with different antenna parameters (a) Under different values of p (b) Under different values of dw.

Figure 3 .
Figure 3. Simulated S 11 with different antenna parameters (a) Under different values of p (b) Under different values of dw.

Figure 5 .
Figure 5. Photograph of the fabricated antenna.

Figure 6 .
Figure 6.Comparisons of the simulated and experimental results of bandwidth of the antenna.

Table 1 .
Parameters as well as the values of the antenna.