|
The feeding network of microstrip antennas is generally divided into two forms: parallel feeding and series feeding. The parallel feeding network is generally composed of a multi-stage simple one-to-two power divider. The transmission line is relatively long. In the millimeter wave band, the transmission loss of the microstrip transmission line is relatively large, and there are many irregular points such as corner cuts and bends in the power division network. These irregular points will generate a certain amount of radiation, interfere with the antenna radiation pattern, and increase the antenna side lobe. In contrast, the series feeding network has a shorter transmission line and fewer irregular points, which makes it easier to reduce transmission loss and reduce the impact of feed line radiation on the antenna radiation pattern. There are generally two types of series feeding antennas. The first type is to transform the impedance of the antenna unit [1] to obtain the current distribution required for the low sidelobe radiation pattern. The second type is to transform the transmission line [2] to obtain the current distribution required for the low sidelobe radiation pattern. In the first method, when the number of units is large, the impedance transformation line is too thin and the processing accuracy cannot be guaranteed. In the second method, the microstrip line is relatively thick, which is easy to process and the accuracy is easy to guarantee.
2 Design of new microstrip antenna
Since low side lobes are required, this antenna adopts the second design method of series-fed antenna. In the literature [2] , the author proposed a design method for corner-fed microstrip standing wave array antenna, which calculates the size of each branch of the power divider according to the ideal lossless condition; the wavelength of the microstrip line medium with different characteristic impedance is different, which leads to the current phase error and current amplitude error of each branch of the power divider. Under the influence of factors such as the current phase error and current amplitude error, the antenna side lobe generally does not meet the design requirements during simulation design and needs to be continuously debugged. Since this method cannot obtain the specific data of the current amplitude and phase distribution, it requires a lot of simulation debugging, which consumes a lot of time and causes a waste of resources. For this reason, this paper proposes a new antenna array design method combining a microstrip standing wave power divider with a microstrip side-fed antenna unit. The advantages of this method are: the microstrip power divider and the antenna unit can be designed separately, the specific data of the current amplitude and phase distribution of the power divider are obtained, the design scheme is simplified, the debugging difficulty during simulation is reduced, the design time is reduced, the output current amplitude and phase of the power divider are more accurately controlled, and the low side lobe design requirements of the antenna are successfully achieved. According to the current amplitude distribution required by the antenna, the required initial size data of the power divider can be easily calculated according to the calculation formula in reference [2] , and the actual size of the antenna can be obtained after correction in the simulation design.
2.1 Unit design
Use HFSS simulation to design the microstrip side-feed unit. The microstrip antenna radiates through the two edges of the feeding direction, and the unit impedance gradually increases from the middle to the edge. By slotting, the feed line is extended into the antenna to obtain the required impedance and good matching. Figure 1 is a diagram of the microstrip radiation unit structure.
Figure 1 Microstrip radiating unit
2.2 Power divider design
The design principle of the power divider is: the two adjacent power branches are composed of two quarter-wavelength dielectric impedance transformation sections ( the first section has a characteristic impedance value of ZC1 and the second section has a characteristic impedance value of ZC2) and a half-wavelength dielectric main transmission line ( characteristic impedance value is ZC) . By adjusting the ratio of the characteristic impedance values ZC1 and ZC2 of the two quarter-transformation sections , the ratio of the current values at the output ports of the two power branches can be adjusted. By analogy, it is very convenient to obtain the current distribution required for the low side lobe of the antenna. As shown in Figure 2 : the basic structure of a microstrip standing wave power divider. As shown in Figure 3 : an 8- branch microstrip standing wave power divider.
Figure 2 Basic structure
Figure 3 8- branch standing wave power divider
This power division network is a series-parallel network. As shown in Figure 2 , the first conversion section and all subsequent parts are in parallel with power branch 1 ; the circuit from the first conversion section to power branch 2 is in series with power branch 2. For the specific calculation formula, refer to reference [2] .
According to the above design method, a 16 × 6 millimeter wave band microstrip antenna is designed , and the antenna structure is shown in Figure 4 .
Figure 4 Antenna structure
3 Test Results
3.1 Standing wave coefficient
The measured standing wave of the 16 × 6 microstrip antenna was biased towards high frequency. After debugging, satisfactory results were obtained, as shown in Figure 5. The reasons are as follows: dielectric constant error, processing accuracy error, and due to the high operating frequency, although the error is not large, the antenna standing wave has shifted to high frequency.
Figure 5 Antenna standing wave test results
3.2 Center frequency pattern
The measured directional patterns of the E- plane and H -plane antennas at the center frequency of the 16 × 6 microstrip antenna are shown in Figure 6. The antenna sidelobes are all below -20dB , meeting the design requirements. Due to space limitations, the directional patterns of other frequency points are not shown one by one. In the design frequency range, the directional patterns of the 16 × 6 microstrip antenna meet the design requirements and reach below -20dB .
Figure 6 Center frequency pattern test results
4 Conclusion
This antenna meets the design requirements very well, achieves low sidelobe, has low difficulty in design, simulation and debugging, and can be used in microstrip low sidelobe antenna projects.
| 
提升卡
变色卡
千斤顶