1 Introduction
The rapid development of wireless broadband communications requires new broadband antennas capable of transmitting high bit rates. The millimeter band is an important band for short-range high bit rate wireless communication. Therefore, in recent years, small-scale and high-performance ultra-wideband antennas in the millimeter wave band have attracted a large number of researchers to carry out research work in this area.
Another important trend in antenna design is the integration of the antenna's RF front-end circuitry. In the past few years, low temperature co-fired ceramic technology (LTCC) has been used extensively in RF front-end circuits. However, LTCC is not an ideal material for antenna integration due to its relatively high dielectric constant resulting in a narrow impedance bandwidth and significant surface waves. Recently, liquid crystal polymer (LCP) materials have been proposed for the integration and packaging of microwave and millimeter wave RF front-end circuits. As a new material, LCP has lower loss than LTCC, and is very suitable for manufacturing microwave and millimeter wave devices, so it has a good application prospect. The advantages are as follows: low loss (loss tangent value 0.002-0.004 at 60 GHz), flexibility, and sealing (water absorption less than 0.004%) [1]. Based on the above advantages, LCP can be used to manufacture high frequency devices.
2, ultra-wideband slot antenna design
2.1 Structural design of single-slot antenna
A tapered slot antenna is an important type of ultra-wideband antenna that shows some advantages such as wideband and high gain. The basic design principles of such antennas. In conventional tapered slot antennas, the ground plane is unused and the energy is radiated to both sides of the tapered slot. In designing an integrated antenna, it is generally necessary to mount the antenna on a metal floor, but the metal plate can seriously affect the performance of the antenna, such as reducing the working bandwidth. Therefore, the design of a tapered slot antenna with a floor is an important task and a serious challenge.
Figure 1 is a side view of the LCP process structure. The board consists of 8 layers of metal and 7 layers of dielectric, thickness h1 = 50μm, h2 = 18μm, and the dielectric constant of the dielectric layer is 2.9.
Figure 1 LCP process structure
We design an ultra-wideband slot antenna with a floor covering of millimeter-wave low-band based on a multi-layer LCP board. The three-dimensional structure is shown in Figure 2. Energy is fed into the antenna through the uppermost microstrip line, and the linear tapered slot is placed on the third layer of metal, coupling energy from the feed line to the radiating slot through the microstrip-slot line, see Figure 4. For the microstrip feeder, the third layer of metal is equivalent to the floor, so the radiant groove and the ground plate are connected together by a metal post. The second, fourth, fifth, sixth, and seventh metal etches form an air gap, consisting of two cuboids, shaped like a "convex", as shown in Figure 5. The dimensions of the two cuboids are L9 × W7 × h1, L10 × W8 × h1, respectively. The main role of the air gap is to increase the thickness of the dielectric layer and broaden the bandwidth. In addition, since the metal layer is only partially etched, the mechanical strength can be enhanced. The optimized structural dimensions are shown in Table 1.
Figure 2 three-dimensional structure
Figure 3 The first layer of microstrip line
Figure 4 The third layer of radiation slots
Figure 5 Air gaps of layers 2, 4, 5, 6, and 7.
Table 1 Size of single layer tapered groove
2.1.2 Simulation results of the antenna
This antenna model was simulated using Ansoft HFSS and CST software. The simulation results are shown in Figure 6. The bandwidth with a reflection coefficient of less than -10 dB is only from 40 GHz to 52 GHz. The gains of the two resonance points of 42 GHz and 47 GHz are 2.1 dBi and 3.0 dBi, respectively. The simulation results of the two softwares show that it is difficult to meet the design requirements.
Figure 6 S11 diagram of a single layer tapered groove
2.2 Structural design of double-slot antenna
The simulation results of the single-layer slot antenna cannot meet the bandwidth requirements of the actual application. In order to further broaden the bandwidth, considering that such an antenna is mainly radiated by a linear gradient slit, on the basis of the original, the fifth layer of metal is etched into another tapered groove of different size, as shown in FIG. See Table 2 for structural dimensions.
Figure 7 The fifth layer of tapered grooves
Table 2 Fifth layer tapered groove size
2.2.2 Simulation results of the antenna
Similarly, we simulated the antenna model using Ansoft HFSS and CST software, as shown in Figure 9. It can be seen from the S11 diagram that the double-layered tapered groove greatly broadens the bandwidth, and the reflection coefficient is less than -10 dB from 33 GHz to 60 GHz, covering the millimeter wave low frequency band. The gains at 39 GHz, 42.6 GHz, and 52.7 GHz at the three resonance points are 2.1 dBi, 3.0 dBi, and 3.2 dBi, respectively, and the pattern is as follows.
Figure 8 S11 diagram of double-layered tapered groove
(a) f = 39 GHz (b) f = 42.6 GHz (c) f = 52.7 GHz
Figure 9 Antenna pattern (phi=0o)
As can be seen from Figure 9, the tapered slot antenna exhibits significant multi-frequency characteristics. The position of the resonance point is mainly determined by the length of the gradient slit. When the length of the gradient gap becomes longer, the resonance point of the same frequency band becomes larger. Moreover, the opening angle of the grading gap has an effect on the return loss value of the resonance point. Through the pattern, it is found that such an antenna has a very stable pattern. As the frequency increases, the directivity of the antenna gradually increases, and the beam width is narrowed but the beam pointing is always unchanged.
3, conclusion
Based on the LCP circuit process, this paper proposes a millimeter-band ultra-wideband tapered slot antenna. In order to further broaden the bandwidth, a design combining two tapered slots was proposed for the first time. The structure with metal floor can effectively suppress the backward radiation of the antenna. The design results show that the antenna can work at 33GHz-60GHz, and the pattern is basically the same within the entire working bandwidth. Since the antenna is elliptically polarized, it can work in complex environments. The study shows that the LCP circuit process is suitable for developing low-cost, lightweight and high-performance millimeter-wave antennas.
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