Fractal dual-polarized antenna for the direction backtracking antenna array

1 Introduction

With the rapid development of communication systems, there is an urgent need for a new type of antenna with low cost, high gain, and automatic beam tracking capability. The retrodrillecture (RetrodirecTIve Antenna) based on the phase conjugate technique can automatically transmit the incoming signal of the incoming wave (incident signal or processed incident signal) to the incoming wave direction without the prior knowledge and complexity of the incoming wave direction. Digital signal processing algorithm. Compared with the traditional antenna technology and the conventional adaptive antenna technology, the direction backtracking antenna technology has the characteristics of fast tracking and better cost performance, which makes it widely available in the fields of communication, radio frequency identification, conformal antenna, power transmission, etc. Application prospects.

In order to reduce the number of antenna elements in the direction of backtracking and to achieve transceiver isolation, the direction backtracking antenna array often uses dual-polarized antennas. The dual-polarized antenna itself has the characteristics of frequency multiplexing, transceiver integration, polarization diversity, improved system sensitivity, and polarization agility. In the direction backtracking antenna array system, the dual-polarized antenna is integrated and transmitted, and a pair of orthogonal polarization ports are used as the receiving end and the transmitting end respectively, so as to realize the transmission and reception isolation, so that the direction backtracking antenna plays a better role in the communication system. The role.

The dual-polarized antenna feed structure has various forms, and the basic forms commonly used are: (1) Coplanar feed form, which is divided into two basic forms: angular feed and edge feed. Coplanar feeding is poorly isolated, generally only used in specific occasions or when engineering requirements are not high; (2) Coplanar and caliber coupling combined with feeding form. This kind of feeding mode can achieve high isolation and high polarization purity. However, since the feeding is on different dielectric layers, it is not easy to be used in the design of the antenna array; (3) the coupling mode of the aperture coupling. High-isolation dual-polarized antennas are implemented using open C-shaped or H-shaped slots, and many workers use this type of feed to achieve dual polarization.

In order to improve the working bandwidth and port isolation of the antenna, this paper adopts the aperture coupling type of H-shaped slot and capacitor-loaded H-shaped slot. The radiating patch adopts the second-level Minkowski fractal structure to realize a high isolation fractal double. The design of the polarized microstrip antenna is used as a direction backtracking antenna array unit (operating frequency range 4.4 GHz ~ 5 GHz). The antenna was electromagnetically simulated using an electromagnetic simulation software Ansoft HFSS 11.1, and the actual object was processed using an Agilent N5230A vector network analyzer. The test results show that the relative impedance bandwidth of VSWR<2 is 17.6%, the V port is up to 20.5%, the dual port isolation is greater than 35 dB, the antenna gain is greater than 8 dB, and the front-to-back ratio is greater than 20 dB. The simulation and test results satisfy the direction backtracking. Antenna system requirements for antenna units.

2 antenna design

The fractal-caliber coupled dual-polarized microstrip antenna structure is shown in Fig. 1, wherein Fig. 1(a) is a side view of the antenna, and Fig. 1(b) is a top view of the antenna. The antenna is mainly composed of two dielectric plates. The upper dielectric plate is made of polytetrafluoroethylene material, and the radiation patch is etched on the lower surface of the upper dielectric plate, so that the upper dielectric plate can be protected by the function of the radome. The lower dielectric plate is made of FR4 material, the grounding plate and the feeding line are respectively located on the upper and lower sides of the lower dielectric plate, and the H-shaped and loaded H-shaped grooves are opened on the grounding plate and perpendicular to each other.

The air layer between the two dielectric plates is equivalent to increasing the thickness of the upper dielectric plate to the sum of the original thickness and the air gap, thereby reducing the average relative dielectric constant of the dielectric plate, that is, reducing the Q of the microstrip antenna. The value, in order to achieve the purpose of increasing the bandwidth, and also helps to increase the gain of the antenna.

In order to reduce the antenna size, the antenna radiating patch adopts a two-stage Minkowski fractal structure, and the fractal structure changes the current distribution of the radiating patch, so that the current is distributed along the tortuous conductor surface instead of the simple geometric surface, which is equivalent to increasing the electrical length. The resonance frequency is lowered, so the antenna size can be reduced.

To improve dual port isolation, the antenna uses H-shaped high isolation coupling. Since the two H-shaped slots perpendicular to each other occupy a larger area and increase the size of the antenna, the V-port adopts an H-shaped slot, and the H-port is improved on the basis of the H-shaped slot as a capacitively loaded H-shaped slot, that is, an H-shaped The slotted arms are bent, thus increasing the length of the slotted arm and increasing port isolation. By adjusting the size of the two feed slots and the slot of the radiating patch, the impedance characteristics of the input port can be improved, and the broadband resonance of the radiating patch can be realized.

(a)

(b)

Figure 1 (a) Antenna side view (b) Top view

3 Results and analysis

In this paper, Ansoft's HFSS software is used to perform electromagnetic simulation calculation on the fractal-caliber coupled dual-polarized antenna. The size of the simulated debug antenna is: upper dielectric constant εr1 = 2.65, lower dielectric constant εr2 = 4.4; dielectric plate size W = 40 mm, Thickness h = 1 mm; patch size pa = pb = 16.8 mm, H-groove construction dimensions: xa = 7 mm, xb = 0.5 mm, xc = 6 mm, xd = 0.2 mm; loaded H-slot size: ya = 5 mm, yb = 0.8 mm, yc = 6 mm, yd = 0.2 mm, tc = 1 mm; feeder open section size: vt = 2 mm, ht = 3.5 mm.

According to the basic size of the above antenna, a physical antenna is processed. Figure 2 (a) is a side view of the antenna, Figure 2 (b) is a two-stage Minkowski fractal radiation patch.

(a)

(b)

Figure 2 (a) Antenna physical side view (b) Fractal radiation patch

Figure 3 shows the simulation and test results of the two-port standing wave. It can be seen from Fig. 3 that the frequency range of the port H standing wave less than 2 is 4.38 GHz ~ 5.168 GHz, the relative impedance bandwidth is 16.8%, and the frequency range of the port V standing wave is less than 2 is 4.27 GHz ~ 5.203 GHz, and the relative impedance bandwidth is 19.7%; the measured relative impedance bandwidths of port H and port V are 17.6% and 20.5%, respectively. It shows that the simulation and test results are basically consistent, but the test curve is slightly offset due to the non-uniformity of the dielectric constant distribution and the test error.

Figure 3 antenna dual port VSWR simulation and test chart

The simulation and test results of antenna port isolation are shown in Figure 4. In the operating frequency range, the simulation result of dual port isolation is S21 ≤ -31.5 dB, and the test result is S21 ≤ -35 ​​dB, indicating that the two ports have higher Isolation can achieve engineering requirements for transceiver isolation.

Figure 4 Antenna isolation S

Figure 5 shows the measured results of the antenna gain. It can be seen that the antenna gain is greater than 8 dB in the operating band, which satisfies the gain requirement of the direction back matrix unit.

Figure 5 Antenna gain

Figure 6 shows the resulting simulation and measurement at f = 4.7 GHz. Comparing the simulation and test results, it can be seen that the E-plane and H-plane pattern curves measured by the two ports are in good agreement with the simulation curve, and the antenna front-to-back ratio is greater than 20 dB. However, the H-plane curve actually measured by port V has some ripples due to the influence of the test environment and test conditions.

(a)

(b)

Figure 6 (a) Port H simulation and test pattern

(b) Port V simulation and test pattern

4 Conclusion

In this paper, a direction back-tracking antenna array unit is designed, which adopts a caliber-coupled dual-polarized antenna. The two-stage Minkowski fractal radiation patch reduces the antenna size, and the improved H-shaped coupling slot is a capacitively loaded H-shaped slot, which improves the two-port isolation. degree. The measured results are in good agreement with the simulation results. The measured results show that the impedance bandwidths of the antenna ports H and V are 17.6% and 20.5%, respectively. The antenna gain is above 8 dB, the front-to-back ratio is greater than 20 dB, and the polarization port isolation is greater than 35 dB. The antenna can be used as a direction backtracking antenna array unit to realize transceiver isolation and reduce the number of backtracking antenna array elements, which lays a foundation for further research on backtracking antenna array.