Abstract
This work presents a Butler matrix based four-directional switched beamforming antenna system realized in a two-layer hybrid stackup substrate for 28-GHz mm-Wave 5G wireless applications. The hybrid stackup substrate is composed of two layers with different electrical and thermal properties. It is formed by attaching two layers by using prepreg, in which the circuit components are placed in both outer planes and the ground layers are placed in the middle. The upper layer that is used as antenna substrate has εr = 2.17, tanδ = 0.0009 and h = 0.254 mm. The lower layer that is used as a Butler matrix substrate has εr = 6.15, tanδ = 0.0028 and h = 0.254 mm. By realizing the antenna array on the lower-εr layer while the Butler matrix on the higher-εr layer, the Butler matrix dimension is significantly reduced without sacrificing the array antenna performance, leading to significant overall antenna system size reduction. The two-layer substrate approach also significantly suppresses parasitic radiation leaking from the Butler matrix toward the antenna side, allowing overall radiation pattern improvement. The fabricated beamforming antenna is composed of 1 × 4 patch antenna array and a 4 × 4 Butler matrix. The measured return loss is lower than −8 dB at all ports in 28-GHz. It demonstrates the switched beam steering toward four distinct angles of—16°, +36°, −39°, and +7°, with the sidelobe levels of −12, −11.7, −6, and −13.8 dB, respectively. Antenna gain is found to be about 10 dBi. Due to the two-layer hybrid stackup substrate, the total antenna system is realized only in 1.7λ × 2.1λ, which shows the smallest form factor compared to similar other works.
Highlights
The fifth-generation (5G) millimeter-wave frequency range, referred to as the frequency range 2 (FR2), includes the band between 24.25 and 52.6 GHz
For the 5G mm-Wave wireless applications, beamforming antenna system is essentially needed to cope with high free-space path loss, severe multi-path fading, and possible interference management for improving signal-to-noise ratio [1,2]
The switched beamforming antenna system is more preferred when precise and continuous beam steering function is not necessarily required, whereas implementation cost and producibility are of Electronics 2019, 8, 1232; doi:10.3390/electronics8111232
Summary
The fifth-generation (5G) millimeter-wave (mm-Wave) frequency range, referred to as the frequency range 2 (FR2), includes the band between 24.25 and 52.6 GHz. The multi-layer substrate design approach can alleviate the spurious radiation issue more effectively because the Butler matrix and the antenna elements are placed in different layers Such previous multi-layer designs have employed further miniaturization techniques, such as the suspended stripline [8], dual transmission line structure in a multi-layer structure [9], forward-wave directional coupler [10], modularized multi-folded structure [11], slot-directional hybrid coupler [12], and dual-layer substrate integrated waveguide (SIW) based crossover elimination structure [13,14]. We propose to adopt a two-layer hybrid stackup substrate for realizing a 28-GHz band Butler matrix based switched beamforming antenna system.
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