“In recent years, with the development of modern microwave communications, the development of broadband circularly polarized microstrip antennas has received more and more attention from researchers, and various forms of broadband circularly polarized microstrip antennas have emerged one after another. The left-handed material is more widely used in the field of broadband and miniaturized microwave devices in the form of a periodic loading structure based on lumped capacitance and inductance. On the basis of relevant literature, a wideband 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 received more and more attention from researchers, and various forms of broadband circularly polarized microstrip antennas have emerged one after another. The left-handed material is more widely used in the field of broadband and miniaturized microwave devices in the form of a periodic loading structure based on lumped capacitance and inductance. On the basis of relevant literature, a wideband 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
The schematic diagram of the antenna structure is shown in Figure 1. The antenna divides the input energy into two signals with the same amplitude and a phase difference of 90° through a Wilkinson power division and phase shifter. These two signals are coupled and fed to the circular radiating patch through the probe. This structure can introduce greater capacitive reactance between the metal rod and the antenna metal sheet, thereby compensating for the high inductance brought by the probe itself, and further increasing the height between the antenna and the base 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 print and feed on a common dielectric layer. Electric network. In order to expand the circular polarization bandwidth of the antenna, this article uses a broadband circular polarization antenna structure, which consists of three parts, in which the input characteristic impedance of the feed network layer based on the dielectric plate is 50Ω, and the dielectric plate is a square with a side length of W ; The length of the L-shaped metal rod with a radius of 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 for radiation, its diameter is D, and it is above the ground The height is H.
The main mode of the circular patch is the TMll mode. According to the above-mentioned antenna structure, the field energy of the TMll mode is concentrated in the air layer. If the resonance frequency of the excitation unit is f, the excitation mode is TMll mode. Then, when the patch shape is circular and the radius of the excitation plate is a, then:
According to the above formula, select the dielectric constant and thickness of the substrate, and the initial circular patch size at the required frequency point can be obtained. In this design, a 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 feed 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. However, as the height between them increases, the pattern will no longer have good radiation characteristics, and the increase in 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 feeding effect, it is more appropriate to take H=20 mm (0.11λ) here. . By formula (1), the diameter of the initial circular patch is 104 mm. The values of other parameters are: W=180 mm, L1=36 mm, H1=ll mm, Sl=14 mm, Rs=l mm. Through the optimized simulation of HFSS software, the best patch diameter D is 76.5 mm.
2 Broadband power division feed design
The structure of the Wlnkinson power divider is shown in Figure 2. The power divider can be regarded 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 splitter is. Adding CRLH-TLs and traditional transmission lines to the two output terminals respectively can make the two ports have a phase difference of 90°.
Assuming that the 3 ports are ordinary transmission lines, the phase at the center frequency f0=1. 8 GHz is -54°, then the line length calculated by Agilent’s ADS software is 18.3 mm. Due to the linear characteristics of ordinary transmission lines, f1=1. 5 GHz and f2=2.1 can be easily obtained. The phases at 1 GHz are φR(f1)=-45° and φR(f)=-63°, respectively.
After 2 ports are added to CRLH-TLs, the phases at frequencies f1 and f2
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 can be obtained as 3.1-mm, LH part L=11.5 nH, C=4.6 pF. In this way, ADS can be used to build circuits and optimize simulations.
3 Simulation results
Figure 3 shows the simulated S11, S21, S31, S22, S33 and S23 parameter curves, 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 is obvious that after adding the CRLH-TLs structure, the transmission characteristics of the two ports can still meet the requirements, and in the range of 1.28 ~ 2.53GHz The phase difference of the port meets 90°±5°, which is unmatched by ordinary microstrip lines.
According to the software simulation results when the feeder network is not added, the bandwidth of the dual-feed circularly polarized antenna structure of this design when S11 is less than -lO dB is 30% (1.46 ~ 1.94 GHz), and when the gain is more than 5 dB The bandwidth is 62.2% (1.25 ~ 2.37 GHz). Obviously, the bandwidth of the new antenna has been greatly improved compared to the circularly polarized antenna on the common dielectric substrate.
In this paper, the left-handed microstrip line and the traditional right-handed microstrip line are respectively cascaded on the Wilkinson power divider and used as the feed network of the broadband antenna; thus, a new type of broadband circularly polarized patch antenna is designed. All performance indicators are outstanding, and all indicators are significantly improved compared to traditional double-fed circularly polarized antennas.