Novel loop antenna design for 3G mobile communication indoor signal coverage problem

At present, the mobile communication business is developing rapidly in the world. In China, with the successive issuance of 3G licenses, the 3G era has also arrived. However, the construction of 3G networks is a huge and long-term task, especially the construction of indoor communication networks is more complicated and important. According to the collection results of business data in Hong Kong SUNDAY, the traffic volume in 3G rooms accounts for the total traffic volume. More than half of it. The latest statistics from NTTDoCoMo show that indoor business accounts for about 70% of total business volume.

Antennas are the eyes and mouthpieces of mobile communication systems and are directly related to network efficiency and communication quality. According to the survey, most of the indoor antenna systems of commercial office buildings and hotels are covered by the form of omnidirectional antennas installed in the middle of the corridor. Most of the users are distributed on both sides of the corridor and have a certain depth position. The penetration loss is high and the space loss is large. The traditional omnidirectional antenna cannot meet the coverage requirement, resulting in uneven distribution and instability of the 3G signal, and there is a problem of dead zone and dropped line, and the problem cannot be increased by increasing the density of the antenna. To solve. At the same time, with the increase of mobile users, the number of mobile communication channels can no longer meet the needs, and the user system will face an upgrade, so it is necessary to develop antennas with wider frequency bands to meet the application requirements. Therefore, the development of 2G/3G shared broadband high-gain bidirectional distributed antenna has important theoretical and practical significance. Therefore, based on the theory of loop antenna, this paper proposes a ring antenna excited by a graded metal ball to achieve high-gain bidirectional radiation, and the measured impedance bandwidth can reach 100%, realizing the ultra-wideband characteristics of the antenna. In order to solve the problem of signal coverage in the deep area on both sides of the indoor corridor of 2G/3G system, an efficient and feasible antenna selection scheme is provided.

The design goals of this paper antenna are: working frequency 800MHz～2400MHz, voltage standing wave ratio less than 1.5, low frequency band gain greater than 5dBi, high frequency band gain greater than 8dBi, antenna relative bandwidth 100%, belonging to ultra-wideband antenna. At the same time, with bidirectional radiation characteristics, the 3dB lobe width is greater than 160° in the bidirectional radiation section. For bidirectional radiation, a typical antenna form is a loop antenna [2-3].

In order to widen the frequency band of the antenna and take into account the weight factor of the antenna, the form of the metal ring piece is used to ensure the weight and strength of the antenna, and the difficulty of feeding the antenna is also increased. The loop antenna is generally excited by a balanced double line in the form of a single point feed. Since the frequency is higher, it cannot be used continuously. Another type of feed is coaxial feed, but since the coaxial is an unbalanced structure, it is needed. Balanced and unbalanced converters (baluns) are added, and regardless of the type of feed, an impedance matching network is required to match the antenna to the feeder, which increases the complexity of the antenna and makes it difficult to achieve miniaturization. This paper proposes a form of excitation with small metal balls in the center of the ring, which achieves balanced feeding and impedance matching of the antenna, saves the use of balun and matching network, and reduces the complexity of the antenna. Its structure is shown in Figure 1.

Referring to Figure 1, R1 and R2 are the inner and outer radii of the metal ring respectively, the ring thickness t0, R0 is the radius of the small metal sphere, the cone angle of the ball cone is θ, and the radius of the cross section of the coaxial inner conductor is a, The radius of the disk base is d, the material of the ring piece is copper, and the material of the ball cone is aluminum. The 50 Ω semi-steel coaxial cable is used for feeding. The upper half of the coaxial cable is thicker, mainly for fixing the ring piece and supporting the sphere.

According to the relationship between the total length of the wound conductor and the wavelength of the free space, the loop antenna can be divided into an electrical small loop antenna (2πr ""λ0) and an electric large loop antenna (2πr≈λ0), where r is the radius of a single loop. The electrical small loop antenna is the most widely used, but mainly used for receiving antennas, and the operating frequency band is relatively narrow. The electric large loop antenna belongs to a resonant antenna. If the load is connected to the appropriate part of the antenna, the traveling current of the conductor can form a load loop antenna, which has a wide frequency band characteristic. For a single-turn electric large-loop antenna, the current on the ring can theoretically be expanded to the following Fourier series for analysis [3]:

Where I0 is the ring current, m=I0NS, N is the number of turns, and S is the ring area. When the size of the ring is close to resonance (2πr/λ0=1, 2, 3), the main function of the Fourier series expression of the current is that n is an integer term, for example, close to the first resonance point 2πr/ Λ0=1, the current on the ring is approximately I(θ)=2I1cosφ. The ring of 2πr/λ0≈1 is usually called the resonance ring, and its input reactance is zero, and the input impedance is R≈100Ω, which is easy to match with the transmission line.

The loop antenna designed in this paper mainly includes two aspects of work. On the one hand, in order to improve the radiation bandwidth of the antenna, a metal ring with a certain width and thickness is used instead of the metal wire; on the other hand, it is to simplify the matching network and ensure bidirectional symmetry. The radiation characteristics are fed in the form of metal spheres excited in the ring.

First, the inner and outer radii of the ring piece are selected according to the working frequency of the antenna: R2=60mm; R1=30mm, the ring piece thickness t0=2mm, and the influence of R1 and R2 on the antenna radiation pattern is shown in Fig. 2 and Fig. 3.