Figure 4

Figure 4 Input impedance as a function of r: real (a) and imaginary (b).

Figure 5

Figure 5 Measured and simulated reference antenna |S11|.

Table 1

The curved portion of the feedline provides current to the radiators. Further simulation shows the input impedance also depends on the radius r (see Figure 4). By optimizing the values of lg and r, making Re (Zin) close to 50 Ω and Im (Zin) near 0 Ω, |S11| < 10 dB can be achieved over a wide frequency range from 3.8 to 7.5 GHz (see Figure 5). Measurements of a prototype antenna built on an FR4 substrate (εr = 4.4) agree well with the simulation. Optimized antenna parameters are listed in Table 1.

AMC-Backed Antenna

Although the reference antenna has a wide impedance bandwidth, the reduced area of the ground causes bidirectional radiation, resulting in low directivity in the forward radiation. The multilayer AMC structure acts as a reflector to improve radiation performance (see Figure 6a). The distance between the reference antenna and the top layer of the multilayer AMC structure is denoted as h2. Antenna impedance bandwidth is primarily affected by h2 (see Figure 6b). A value of h2 = 9 mm is chosen so that the in-phase reflection band corresponds with the operating band to the greatest extent. This is the frequency range from 4.5 to 7.5 GHz.

Figure 6

Figure 6 Lateral view of the antenna and multilayer AMC structure (a) and simulated |S11| as a function of h2 (b).

Figure 7

Figure 7 Simulated and measured antenna gain (a) and simulated E-plane radiation patterns at 6 GHz (b) with different reflectors.

To assess the influence of the multilayer AMC structure on antenna radiation performance more intuitively, gain is simulated with three different reflectors, including a PEC, monolayer AMC and multilayer AMC for comparison (see Figure 7a). For the reference antenna, the gain in the forward direction is only about 3 dBi. With the PEC, gain is significantly improved.

Figure 8

Figure 8 Photograph of the multilayer AMC antenna (a), the antenna mounted in an anechoic chamber for test (b).

With the monolayer AMC structure, the simulated gain is improved by approximately 1 to 2 dB compared to the PEC reflector. This is attributed to the in-phase reflection characteristics of the AMC structure. In addition, an array-like effect caused by the cells of the AMC structure, which can be considered as sub-radiators, also helps to improve antenna system gain.

With the multilayer AMC structure, gain is further increased. The incremental improvement is up to 3 dB with a maximum gain of 12 dBi. This is mainly attributed to a flatter in-phase reflection characteristic. Additionally, for the monolayer AMC structure, the thickness of the substrate is 13.6 mm, while for the multilayer AMC structure, the thicknesses of the five plates are 1 mm, 1 mm, 0.8 mm, 0.8 mm and 0.8 mm, respectively. This results in lower substrate loss and higher gain. Gain of the multilayer AMC antenna is measured as well (see Figure 7a), demonstrating close agreement with the simulation.

Simulated E-plane radiation patterns of the antenna with different reflectors at 6 GHz (see Figure 7b) show that higher directivity with a narrower beamwidth is achieved with the multilayer AMC structure.

Figure 9

Figure 9 Simulated and measured |S11|.

Figure 10

Figure 10 Measured radiation patterns at 5 (a) and 6 (b) GHz.

MEASUREMENTS

Performance of the prototype multilayer AMC-backed antenna (see Figure 8), including |S11|, gain and radiation patterns, is measured in a far-field anechoic chamber. Gain is shown in Figure 7a, |S11| is shown in Figure 9 and measured radiation patterns are shown in Figure 10. Measurements agree closely with the simulations.

CONCLUSION

A wideband and high gain antenna employs a five-layer AMC structure. The AMC acting as a ground plane is constructed by printing periodic patches on substrates with different dielectric constants to effectively broaden the frequency range of in-phase reflection. Attributing to this in-phase reflection characteristic, antenna gain from 4.5 to 7.5 GHz is significantly improved, achieving a maximum gain of 12 dBi. This wideband high gain antenna has potential communication systems applications.

ACKNOWLEDGMENT

This work was supported by the Science and Technology Research Project of Henan Science and Technology Department (Grant No: 242102210067), the Key R&D Project of Henan Province-Research and Application of Key Technology (Grant No: 241111212500) and Henan Province Key R&D and Promotion Special (Technology Tackling Key) Project (No.232102210181, No.252102521071).

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