“In recent years, with the development of modern microwave communications, the development of broadband circularly polarized microstrip antennas has attracted more and more attention from researchers, and various forms of broadband circularly polarized microstrip antennas have emerged one after another. Left-handed materials are more widely used in the field of broadband and miniaturized microwave devices in the form of periodic loading structures based on lumped capacitance and inductance. On the basis of relevant literature, a broadband 90° power division phase shifter with a center frequency of 1.8 GHz is designed, and the microstrip patch is fed through the L-shaped probe structure, thereby improving the antenna’s performance. Circular polarization bandwidth.
In recent years, with the development of modern microwave communications, the development of broadband circularly polarized microstrip antennas has attracted more and more attention from researchers, and various forms of broadband circularly polarized microstrip antennas have emerged one after another. Left-handed materials are more widely used in the field of broadband and miniaturized microwave devices in the form of periodic loading structures based on lumped capacitance and inductance. On the basis of relevant literature, a broadband 90° power division phase shifter with a center frequency of 1.8 GHz is designed, and the microstrip patch is fed through the L-shaped probe structure, thereby improving the antenna’s performance. Circular polarization bandwidth.
1 Antenna structure
A schematic diagram of the structure of the antenna is shown in Figure 1. The antenna divides the input energy into two signals with the same amplitude and 90° phase difference through the Wilkinson power divider and phase shifter. These two signals are fed to the circular radiating patch through probe coupling. This structure can introduce a larger capacitive reactance between the metal rod and the antenna metal sheet, thereby compensating for the high inductive reactance brought in by the probe itself, and further increasing the height between the antenna and the bottom plate. In order to increase the bandwidth as much as possible, the mixed air dielectric layer introduced in this design is a very effective method. This method can not only easily obtain an air layer with a dielectric constant of 1, but also can easily print feeds on a common dielectric layer. electrical network. In order to expand the circularly polarized bandwidth of the antenna, a broadband circularly polarized antenna structure is used in this paper. The antenna consists of three parts. The input characteristic impedance of the feeding network layer based on the dielectric plate is 50Ω, and the dielectric plate is a square with side length W. ; The length of the L-shaped metal rod with the radius Rs in the air layer is L1, the height is H1, and the distance beyond the edge of the antenna is S1; the third part is the metal sheet used for radiation, its diameter is D, and the distance from the ground is D. The height is H.
The main mode of the circular patch is the TM11 mode, and according to the above antenna structure, the field energy of the TM11 mode is concentrated in the air layer. If the resonant frequency of the excitation unit is f, the excitation mode is the TMll mode. Then, when the patch shape is circular and the excitation plate radius is a, there are:
According to the above formula, by selecting the dielectric constant and thickness of the substrate, the initial circular patch size of the desired frequency point can be obtained. In this design, the dielectric plate Arlon Diclad 880 ™ with a substrate thickness of 0.8 mm and a dielectric constant of 2.2 is selected as the substrate of the feeding network, the dielectric constant of the air layer is 1, and the center frequency is 1.8 GHz. When determining the height of the disc, in order to expand the bandwidth, the distance between the patch and the substrate can be increased, but as the height between them increases, the pattern will no longer have good radiation characteristics, and the increase in the bandwidth will It is no longer obvious. Generally, the height can be selected between 0.1 and 0.15λ. At the same time, in order to make the L-shaped metal rod play a better role in feeding, it is more appropriate to take H=20 mm (0.11λ). . By formula (1), the diameter of the initial circular patch can be obtained as 104 mm. The values of other parameters are: W=180 mm, L1=36 mm, H1=11 mm, Sl=14 mm, Rs=1 mm. Through the optimization simulation of HFSS software, the optimal patch diameter D can be obtained as 76.5 mm.
2 Design of broadband power division feed
The structure of the Wlnkinson power divider is shown in Figure 2. The power divider can be seen as a three-port network. Port 1 is the input end, ports 2 and 3 are the output end, and the two ends are isolated from each other. The characteristic impedance of the two branch lines of the power divider is . By adding CRLH-TLs and conventional transmission lines to the two outputs respectively, the two ports can have a phase difference of 90°.
Assuming that the 3 ports are common transmission lines, and the phase at the center frequency f0 = 1.8 GHz is -54°, the line length calculated by Agilent’s ADS software is 18.3 mm. Due to the linearity of ordinary transmission lines, the phases at f1=1.5 GHz and f2=2.1 GHz can be easily obtained as φR(f1)=-45° and φR(f)=-63°, respectively.
Phases at frequencies f1 and f2 after adding CRLH-TLs to port 2
According to formula (6), the structure of RH-TLs in CRLH-TLs can be obtained, and then the values of L and C can be obtained from formulas (5), (7) and matching impedance. Taking N=2, the length of RH-TLs is 3.1-mm, the LH part is L=11.5 nH, and C=4.6 pF. This way, ADS can be used to build the circuit and optimize the simulation.
3 Simulation results
Figure 3 shows the S11, S21, S31, S22, S33 and S23 parameter curves obtained by simulation, and Figure 4 shows the phase comparison of the two ports after adding the left-hand structure.
From the S-parameter curve shown in Figure 3 and the phase comparison diagram shown in Figure 4, it can be clearly found that after adding the CRLH-TLs structure, the transmission characteristics of the two ports can still meet the requirements, and within the range of 1.28 to 2.53 GHz The port phase difference meets 90°±5°, which is incomparable to ordinary microstrip lines.
According to the software simulation results when the feed network is not added, the bandwidth of the designed dual-feed circularly polarized antenna structure when S11 is less than -10 dB is 30% (1.46-1.94 GHz), and when the gain is greater than 5 dB The bandwidth is 62.2% (1.25 to 2.37 GHz). Obviously, the bandwidth of the new antenna has been greatly improved compared to the circularly polarized antenna of the ordinary dielectric substrate.
In this paper, the left-handed microstrip line and the traditional right-handed microstrip line are cascaded on the Wilkinson power divider, respectively, and used as the feed network of the broadband antenna; thus a new broadband circularly polarized patch antenna is designed. All performance indicators are outstanding, and all indicators are significantly improved compared with the traditional double-fed circularly polarized antenna.