Research on wideband circularly polarized microstr

Research on high gain broadband circularly polarized microstrip antenna array introduction with the development of microstrip antenna technology, new forms and new performance microstrip antennas are emerging. For portable antennas, the size of the antenna is smaller, and the electrical performance of the antenna requires electrical characteristics such as broadband and high gain. Predecessors have done a lot of work on these performance improvements of the antenna, but most of them have only made improvements on one or two of them. Aiming at the existing problems, a circularly polarized microstrip array antenna with miniaturization, high gain and broadband is proposed in this paper. The S-band miniaturized broadband circularly polarized antenna array experimental prototype is developed, and the electrical characteristics of the antenna array experimental prototype are measured. The measurement results show that when the maximum gain of the antenna is 15dB, the size of the antenna array is only 295 mm 210 mm, the VSWR bandwidth of the antenna array reaches 12.25%, the circular polarization axis ratio is less than 3 dB, and the bandwidth reaches 9.4%, which has built an ADI characteristic industrial chain, which is greater than 3.4% in the literature [1]. And the lobe width is 64 and 20 respectively, which is larger than 63 and 9

1 theoretical analysis and design mentioned in reference [1]

this paper uses the design method of general microstrip antenna to design the antenna unit. By adjusting the impedance of the matching branch of the microstrip antenna, the antenna unit is simulated and optimized by using an soft HFSS and then the intention software to control the movement of the beam, which greatly reduces the design complexity of the antenna array, and the feed network can be designed through several stages of bisector power distribution

1.1 design of antenna unit

circular polarization antenna has a wide range of applications, and its practical significance is mainly reflected in:

(1) circular polarization antenna can receive incoming waves with arbitrary polarization, and its radiation waves can also be received by arbitrary polarization antenna, so circular polarization antenna is widely used in electronic reconnaissance and jamming

(2) the rotation orthogonality of circularly polarized antennas is widely used in applications such as communication, radar polarization diversity and electronic countermeasures

(3) when circularly polarized waves are incident on symmetrical targets (such as planes, spheres, etc.), the rotation direction is reversed. Therefore, the application of circularly polarized antennas in mobile communications, GPS, etc. can suppress rain and fog interference and resist multipath reflection

the key for microstrip antenna to obtain circularly polarized waves is to excite two linearly polarized waves with orthogonal polarization directions, equal amplitude and phase difference/2. The earliest circularly polarized microstrip antenna used the orthogonal feed method, but when this antenna formed the antenna array element, the feed circuits would cause unwanted coupling, which limited its practical application. The broadband circularly polarized microstrip antenna composed of curved microstrip antenna does not use an open resonator, avoiding the radiation based on the resonant system. It not only has strong radiation power, but also has a low Q value, but it requires a very complex power divider combination circuit to form a circularly polarized feed, which is not easy to achieve, and it is difficult to form an array. Based on the side fed microstrip rectangular patch element, according to the perturbation method, two orthogonal TM10 and TM01 modes are generated by the chamfer method to realize circular polarization. The chamfer size is about/10, as shown in Figure 1. This design makes the shape of the antenna smaller and more flexible

1.2 feeding mode

in this paper, the edge feeding mode is used to feed the microstrip patch. Because the edge impedance of the patch is not 50, the input port should be impedance transformed. In this paper, a single branch node matching method is used for impedance matching, which has the following characteristics: the width of the main plane pattern of the array element; Easy to feed, very suitable for array formation; Cross polarization can be reduced by changing the position of the unit on the feeder

1.3 design of unit structure size

the schematic diagram of the unit is shown in Figure 1. The size of the rectangular microstrip antenna is determined according to the following formula:

where: A is the patch length; B is the patch width; E is the equivalent dielectric constant; F0 (0) is the center frequency (wavelength) of the microstrip antenna; C is the speed of light (3108 M/s)

the antenna size can be reduced by using a substrate with high dielectric constant in the microstrip antenna and the longer length lines such as sheet metal and lead screw. However, due to the existence of surface waves in the substrate, especially when the thickness of the dielectric plate and the working wavelength are comparable, the influence of surface waves cannot be ignored. In this way, although the frequency band can be broadened when using thick substrates, the radiation efficiency of the antenna decreases due to the increase of surface wave loss. Therefore, when selecting the thickness of the dielectric substrate, it is necessary to avoid exciting high-order modes as much as possible. The cut-off frequencies of TM and TE mode surface waves are respectively:

so according to formula (3), selecting a dielectric substrate with dielectric constant r=4.4 can not only reduce the size of the antenna, but also ensure the radiation efficiency of the antenna

1.4 design of feed network

the microstrip antenna array feed system designed in this paper adopts parallel side feed, that is, using multiple power distributors, the input power can be evenly distributed to each array element. In order to ensure that the feed phase of each array element is in-phase feed, a three-stage bisector is used to feed the array element, so that the feed of each antenna element is in-phase with equal amplitude. This design has simple structure and good consistency, can increase the impedance bandwidth of the antenna array, and is conducive to the realization of the antenna

2 antenna array design and experimental results

the array is formed according to the above designed elements, and the number of elements is 8 (24). The simulation design of different antenna array spacing is carried out. When the frequency f0=2.45 GHz, the antenna patterns at the array spacing of 0.55, 0.6, 0.65 are compared (Fig. 2 (a)), and the gain patterns at the array spacing of 0.55, 0.6, 0.65 are compared (Fig. 2 (b))

Fig. 2 shows the directional patterns of xoz plane (Fig. 2a) and YOZ plane (Fig. 2b) at different array spacing at the center frequency of the antenna. It can be seen from Fig. 2 that the directional pattern is symmetrical when the array spacing is 0.55 and the backward radiation amplitude of the directional pattern is relatively small. It can be seen from Fig. 3 that the gain of the antenna array is the highest when the array spacing is o.55, so the array spacing is selected as 0.55 when designing the antenna. Figure 3 shows the prototype of the antenna array

2.1 measurement of Smith circle diagram and standing wave curve

use hp8753d vector network analyzer to measure the Smith circle diagram and standing wave curve of the antenna array and the axial ratio of the antenna. The measurement results are shown in Figure 4 and table 1

it can be seen from Figure 4 (b) that the bandwidth of antenna with VSWR less than 2 is 300 MHz, reaching 12.25%, and the axial ratio bandwidth is 200 MHz, reaching 9.4%. In the required frequency band, the impedance characteristics and circular polarization characteristics are good, meeting the requirements of the communication system

2.2 measurement of pattern and gain

under the conditions of microwave anechoic chamber and far area, the pattern of the antenna is measured with a self-made far-field automatic measurement system. When the frequency f0=2.45 GHz, the measured xoz plane pattern, YOZ plane pattern and simulation pattern are compared in Figure 5. It can be seen that the experimental results are in good agreement with the simulation results. The 3 dB bandwidths are 64 (Fig. 5 (a) =0, xoz plane) and 20 (Fig. 5 (b) =90, YOZ plane) respectively, and the measured gain is 15 dB, realizing a high gain, which is basically consistent with the simulation results

3 conclusion

this paper studies the design method of S-band miniaturized broadband circularly polarized antenna array, discusses the relationship between array spacing and antenna pattern, discusses the implementation of microstrip antenna array feed network engineering, develops an experimental prototype of S-band miniaturized broadband circularly polarized antenna array, and measures the electrical characteristics of the experimental prototype of antenna array. The measurement results show that, The antenna gain is 16 dB, and the mechanical strength can be maintained at low temperature of (2) 69 ℃; The size of the antenna array is only 295 mm 210 mm, the impedance bandwidth of the antenna array reaches 300 MHz, the circular polarization axis ratio is less than 3 dB, and the bandwidth is 150 MHz. The test data fully shows that the antenna array has the characteristics of small size, high gain and wide band, which verifies the effectiveness of the design method. The research results can be applied to engineering practice, and have high practical value and promotion value