The mobile communication industry is experiencing rapid growth worldwide, and in China, the arrival of the 3G era has been marked by the successive issuance of 3G licenses. However, the construction of 3G networks remains a complex and long-term task, especially when it comes to indoor communication infrastructure. According to data collected by Hong Kong SUNDAY, more than half of the traffic in 3G environments occurs indoors. Similarly, NTTDoCoMo's latest statistics indicate that indoor traffic accounts for approximately 70% of total usage. This highlights the critical need for efficient and reliable indoor coverage solutions.
Antennas play a vital role in mobile communication systems as they directly impact network performance and signal quality. In many commercial buildings and hotels, omnidirectional antennas are commonly installed in the middle of corridors. However, users are typically located on both sides of these corridors, leading to high penetration loss and spatial signal degradation. Traditional omnidirectional antennas often fail to provide even coverage, resulting in signal dead zones and dropped calls. Increasing antenna density does not effectively resolve these issues. Additionally, with the growing number of mobile users, the demand for more communication channels continues to rise, pushing the need for advanced antenna technologies capable of supporting wider frequency bands.
To address these challenges, this paper introduces a new loop antenna design based on the theory of loop antennas. The proposed antenna uses a graded metal ball to excite the ring structure, achieving high-gain bidirectional radiation. The measured impedance bandwidth reaches 100%, making it an ultra-wideband antenna suitable for 2G/3G systems. This design offers an efficient and practical solution for improving signal coverage in deep indoor areas along corridors.
The main design goals of the antenna include a working frequency range of 800 MHz to 2400 MHz, a voltage standing wave ratio (VSWR) less than 1.5, low-frequency gain greater than 5 dBi, and high-frequency gain exceeding 8 dBi. The antenna also features a bidirectional radiation pattern with a 3 dB beamwidth of over 160°. Loop antennas are well-suited for such applications due to their directional characteristics and wide bandwidth potential.
To enhance the antenna’s performance while maintaining its structural integrity, a metal ring with specific dimensions is used instead of traditional thin wires. This approach improves the antenna’s mechanical strength and reduces feeding complexity. A central metal sphere is employed to achieve balanced feeding and impedance matching, eliminating the need for additional components like baluns or matching networks. This simplifies the overall design and supports miniaturization efforts.
The antenna structure consists of an outer radius R2 = 60 mm, inner radius R1 = 30 mm, and a ring thickness t0 = 2 mm. A small metal sphere with radius R0 is placed at the center, and the cone angle θ determines the shape of the ball cone. The coaxial cable used for feeding has a semi-rigid 50 Ω structure, with the upper part designed to support the ring and sphere. This configuration ensures stability and efficient signal transmission.
Through simulation and analysis, the antenna’s radiation patterns were evaluated under different configurations of R1 and R2. These results demonstrate how variations in the ring dimensions affect the antenna’s performance, providing insights into optimizing the design for real-world applications. The proposed loop antenna represents a significant advancement in indoor 3G communication systems, offering improved coverage, reliability, and efficiency.
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