Design of waveguide slot antenna based on vehicle-mounted radar system

Waveguide slot antennas have made great progress since the middle of the last century and are widely used in various fields such as ground, shipborne, airborne, and navigation. Slot array antennas are widely used because they can easily control the amplitude distribution within the antenna aperture plane, have high aperture utilization, are small in size, and are easy to achieve low or very low side lobes. In the research of waveguide slot antennas, many scholars have conducted a lot of basic research on the theory and experiment of slot antennas, so the theory of waveguide slot antennas is becoming more and more mature. This paper designs a small waveguide slot antenna based on the application of vehicle-mounted radar system . The antenna is required to have a wide beam in the horizontal plane and can cover a relatively wide range, thereby more effectively improving the battlefield survivability of the vehicle. The performance indicators that the antenna needs to meet are as follows: a. Gain: greater than 11dB; b. 3dB beam width: 20° for E plane and 110° for H plane; c. Sidelobe level: less than -13dB; d. Standing wave ratio: less than 2.

In order to simplify the design, this design adopts the waveguide wide-wall oblique slot resonant array method, and the number of cut slots is 4, which achieves the effect required by the index.

1 Theoretical Analysis

1.1 Model of Serial Slot Array

From the field distribution in the waveguide, it can be seen that: when an oblique slit is opened in the center of the wide side of the waveguide, the narrow slit does not cut the current line in the longitudinal direction; in the transverse direction of the slit, due to the disturbance of the electric field, the total electric field jumps on both sides of the slit, that is, the voltage jumps, which is equivalent to connecting an impedance in series on the transmission line. For the center-fed resonant linear array model, it is assumed that there are IV-claw oblique slits on the waveguide wall, and the distance between the slits and the centers is λg/2. In order to obtain in-phase excitation, adjacent slits are placed crosswise and tilted, and the short-circuit plate at the end of the waveguide is λg/2 away from the terminal slit, so that the center of the slit is at the maximum voltage or current position. The linear array model is shown in Figure 1.

Its equivalent circuit is shown in Figure 2.

The figures show normalized equivalent resistances.

1.2 Analysis of gap characteristic parameters

In the selection of antenna operating frequency, the operating frequency of this radar system is 10.5GHz, so the operating frequency of the antenna is 10.5GHz. For a linear array in which each unit is located on a straight line with equal spacing, the array factor can be expressed as:

Where An is the excitation amplitude, θ is the angle between the observation direction and the straight line, and d is the array element spacing. Since each unit of the resonant array is in phase, that is, φn=O, the above formula can be simplified to:

When u=2mπ, m=O, ±1,…, S takes the maximum value, and m=0 is the main lobe. In order to achieve low side lobes and widen the main lobe, the center feed is used to decrease the amplitude from the center to the edge of the array, and the slots are weighted according to the Taylor line source distribution, with a symmetrical distribution on both sides. The zero point position of the radiation pattern is determined by the following formula:

Expand the latter term as a polynomial, and the coefficients of each power of Z are the corresponding excitation amplitudes.

From Figure 2, when the waveguide is center-fed and in resonance (its imaginary impedance is zero), for Taylor distribution, we have:

Substituting the excitation amplitude of each gap obtained previously can obtain the corresponding normalized resistance value. In this design, N is 4.

AF Stevenson used the Lorentz reciprocity theorem and the power balance equation in the waveguide to obtain the normalized equivalent resistance expression of the series gap:

Where β is the angle between the centerline of the slot and the centerline of the wide side of the waveguide, α is the length of the wide wall, and b is the length of the narrow wall. Substituting the previously obtained rn into the equation and solving it, we can get the corresponding slot deflection angle.

1.3 Factors Affecting Antenna Performance

When using the results calculated above to design the antenna, the mutual coupling between the slots must also be considered; if the mutual coupling is not considered, the amplitude distribution and phase distribution of the antenna aperture will deteriorate, and the input matching of the antenna will also deteriorate. In recent years, with the rapid development of computer-aided technology, the near-field diagnostic method of obtaining near-field data through simulation has received more and more attention when designing relatively small slot arrays. When the number of slots is 4, based on the parameters obtained above, combined with the parameter scanning function in the CST software, accurate electrical parameters can be quickly found, greatly improving the efficiency of the design.

Compared with longitudinal slots, the cross-polarization radiation of series slots is higher due to their angle deflection, which will lead to an increase in the sidelobe level and a decrease in gain. The simulation results also confirm this, which is what we do not want to see in the design, and measures need to be taken to suppress cross-polarization radiation. In this design, a small waveguide port is added above each slot, and the propagation direction of the small waveguide is perpendicular to the plane where the slot is located. Without increasing its propagation direction length, the cross-polarization level is suppressed by controlling the wide side size of the small waveguide so that its cutoff wavelength is smaller than the cutoff wavelength of the slot propagation mode in the cross-polarization direction. In order to further reduce the cross-polarization level, the main lobe waveform is also adjusted. Referring to the simulation results, a metal sheet can be inserted in the middle of the small waveguide port to further reduce its wide side size. The simulation results show that this method can effectively reduce the impact of cross-polarization.

2 Modeling and Simulation

In the process of designing the waveguide slot antenna in this paper, the numerical simulation in the design is completed in the CST time domain solver environment.

2.1 Antenna model establishment

The size of the radiation waveguide is 22.86×10.16mm, the waveguide wall thickness on one side of the slot is 1mm, the slot width is 2mm, and the two ends of the waveguide are ideal short-circuit surfaces; the cutoff waveguide is 16×8mm. The model is established, and its framework diagram is shown in Figure 3:

The black mark is the center feeding point of the coaxial line; the square material above the radiation port is the antenna cover; the gaps are numbered 1 to 4 from left to right.

2.2 Simulation Results Analysis

In the simulation, the slot length l and the inclination angle β are set as variables, and the initial value of l is λ/2. The parameter scanning function of CST is used to scan the slot length and inclination angle. By setting a reasonable step size, the scanning progress can be accelerated and the calculation time can be reduced. Since this design uses coaxial line center feeding, the impedance matching problem needs to be considered, otherwise reflection will occur at the connection with the waveguide, affecting the performance of the antenna. According to the principle of λ/4 impedance transformation, matching is performed by changing the length of the conductor probe in the coaxial line in the simulation. When the input impedance of the coaxial line is 50 Ω, the desired effect is considered to be achieved by observing the port mode. After simulation, the length of the conductor probe in the coaxial line is 8.5mm. On this basis, the parameters of the slot are simulated as follows:

It can be clearly seen from the simulation results that the standing wave ratio at the center frequency has achieved a very ideal effect. The bandwidth below the standing wave ratio of 2 is about 400MHz, and the result meets the design requirements; the beam width of its H-plane directional pattern (i.e. the horizontal directional pattern after the antenna is installed) has reached the requirements of wide-angle detection; the E-plane directional pattern has also reached the index requirements, but the shortcoming is that its sidelobe level is not very ideal, which is mainly due to the use of fewer slots to meet the needs of beam width, but its loss is within an acceptable range. In general, all indicators have reached the goals at the beginning of the design.

3 Conclusion

Based on the actual situation of the application target, this paper designs a small four-element linear array antenna using the waveguide wide side center slant slit. Through simulation analysis, its various performance parameters have reached the specified index requirements. And because of its small size, good stability, and ability to meet the needs of practical applications, in actual production, due to processing technology and other reasons, certain errors will be caused, and the processing error needs to be strictly controlled.