Analysis of planar back cavity antenna with top suppression and horizontal omnidirectional radiation

In some wireless sensor network systems that need to search and locate ground and near-ground targets, such as ground penetrating radar systems for border detection and recognition, Vehicle AnTI-Collision systems, etc., It is required that the antenna be highly radiated within a certain angle with the ground. The top of the antenna is not in the detection service area. The radiation in these areas causes waste of energy. External signals also interfere with the system and need to be suppressed. Antennas in these systems should have top suppression and horizontal omnidirectional radiation characteristics. At the same time, from the perspective of security and stealth, it is also required that the structure of these antennas be designed to be conformal and planar.

On the other hand, the conventional planar microstrip antenna is difficult to realize ultra-wideband. On the other hand, its maximum radiation direction is at the top of the antenna, that is, the surface of the vertical and radiating elements, and is generally a standard 8-shaped radiation pattern, which is obviously not suitable for application in these fields. . Double-cone or single-cone antennas are widely used in fields such as ground penetrating radar. However, the radiation characteristics of this type of antenna require that it must be exposed to the ground in order to ensure effective detection and identification of surrounding objects. It shows that its stealth performance is poor.

This paper presents an ultra-wideband planar cavity antenna that can work in 2.8-4.5GHz. According to the principle that the multi-mode mixing in the back cavity can broaden the bandwidth of the antenna, the antenna is fed by a 50 ohm coaxial outer conductor and a rectangular back cavity. Crossed microstrip patches can act to suppress top radiation and enhance horizontal radiation. The antenna was modeled and simulated using the three-dimensional full-wave finite element simulation software Ansoft HFSS. According to its powerful parameter optimization function, the length of the inner conductor of the coaxial line, the length of the cross-patch patch, the width of the cross-patch patch, and the size of the cavity are overall optimized. The results show that the designed antenna operates at 2.8-4.5GHz, the antenna has a -10dB (VSWR) impedance bandwidth of 48%, the apex gain is less than -25dB, and the maximum gain occurs at 45-55 degrees from the horizontal plane. The gain in the horizontal direction is greater than -0.5dB. And has a good level of omnidirectional radiation characteristics.

The antenna structure proposed in this paper is shown in Figure 1. The bottom of the antenna is fed with a 50 ohm coaxial cable to feed the antenna. The diameters of the inner and outer conductors on the coaxial line are 3 mm and 10 mm, respectively. An RT/Duraid 5880 medium with a dielectric constant of 2.2 is filled between the inner and outer conductors of the coaxial cable. . Compared with the traditional rectangular plate, the microstrip slice of the criss-crossing structure not only plays a role in suppressing the top radiation, but also enhances horizontal electromagnetic radiation due to the electromagnetic coupling between the cross plates. For the convenience of parameter optimization, we set the length and width of the microstrip of the criss-crossing structure to L1 and L2, respectively, and the vertical distance g from the inner conductor to the microstrip patch in the coaxial line.

According to the theory of the eigenmode of the cavity, it can be calculated that the length, width and height of the cuboid back cavity can be set to 70mm, 70mm and 18mm, respectively, in the range of 3.5GHz.

Fig. 1 Antenna 3D model

A three-dimensional antenna model as shown in FIG. 1 is established in full-wave three-dimensional finite element software. Here we use Ansoft HFSS for its calculation and performance analysis. With the software's powerful parameter optimization function, we can obtain the geometric parameters that meet the requirements. For example, when only a single parameter is optimized, the other parameters of the antenna can be kept unchanged, and only the influence of the change of this parameter on the antenna performance (such as return loss) can be studied, so that the optimal value of the single parameter can be determined. For multi-parameter optimization, in addition to the above method, various optimization algorithms integrated by Ansoft HFSS can optimize the overall parameters.

The return loss vs. frequency curve of the antenna obtained through the Ansoft HFSS optimization processing is shown in Figure 2. From the figure we can see that the return loss of this antenna in the 2.8-4.5GHz band is less than -10dB.

Figure 2 Antenna return loss as a function of frequency

In order to study the radiation characteristics of the antenna over the entire frequency band, we give the antenna's XYZ, 3.6 GHz, and 4.5 GHz xoz and yoz planes and xoy planes, as shown in Figure 3.

(a) Patterns of the xoz face (solid line) and yoz face (dashed line) at 2.8 GHz

(b) XYGHz (solid line) and yoz face (dashed line) patterns at 3.6GHz

(c) Patterns of the xoz (solid) and yoz (dotted) planes at 4.5 GHz

(d) 2.8 GHz xoy surface pattern

(e) 3.6 GHz xoy surface pattern

(f) 4.5 GHz xoy surface pattern

Figure 3 Directional pattern of the antenna at three frequency points xoz, yoz, and xoy

It can be seen from Fig. 3 that the apex gain of the antenna is less than -20 dB at the three points of high school and low frequency. At the same time, the maximum gain at the three frequency points also appears between 45 and 55 degrees from the horizontal plane. In the horizontal direction of the antenna, that is, in the xoy plane, the gains at the three frequency points are all greater than -0.5 dB. In addition, we can see from Figure 3 (d-f), the gain of the antenna in the horizontal direction fluctuations within 1dB, indicating that there is a good level of omnidirectional radiation characteristics.

In this paper, a new type of broadband planar back cavity antenna is designed. The coaxial line is connected with the cavity to generate multiple modes to achieve broadband impedance matching. The cross-center microstrip at the center of the surface is used to suppress the top radiation and enhancement level through electromagnetic coupling. Direction of radiation. The antenna has a simple structure, a small volume, and an omnidirectional radiation characteristic, and can be widely used in wireless sensor networks such as ground penetrating radar and automobile collision avoidance.