Bluetooth is a radio technology that supports short-range communication (usually within 10 m) of the device. It can exchange wireless information between many devices including mobile phones, PDAs, wireless headsets, laptops, etc. The working frequency band is industrial and scientific research. The medical (2.4 to 2.383 GHz) global communication free frequency band has been widely used in mobile communication devices. The antenna is an important device for transmitting electromagnetic waves in a Bluetooth wireless system, and it is currently not integrated into a semiconductor chip. In Bluetooth products, the size and performance of the Bluetooth antenna determines the size and performance of the entire Bluetooth module. With the development of mobile communication, personal mobile devices tend to be smaller and lighter, and in order to adapt to this development, the size of the Bluetooth antenna has strict requirements. The monopole antenna is too large in size and is not suitable for use in mobile communication devices. Although the conventional PIFA antenna has been reduced in size by half, it is still too large in terms of a relatively fast and miniaturized mobile communication product. According to the working principle of traditional printed inverted F antenna, this paper designs a folded PIFA antenna with a size of only 16 mm & TImes; 4.5 mm. The design is simple, the manufacturing cost is low, and the work efficiency is high. It is suitable for Bluetooth system.
1 Analysis of traditional printed inverted F antennaThe printed inverted F antenna is an antenna developed at the end of the last century. At present, its theoretical analysis has matured, and its application range has become increasingly widespread. He has the unique advantages of simple structure, light weight, conformal shape, low manufacturing cost, high radiation efficiency, and easy realization of multi-band operation. Therefore, in recent years, the printed inverted F-type antenna has been extensively researched and developed.
The structure of the printed inverted F-type antenna is as shown in Fig. 1. The terminal open transmission line of length L is connected in parallel with the terminal short-circuit transmission line of length S. When the transmission line conductor line width d "H, the characteristic impedance Z0 of the transmission line can be expressed. for:
According to the long-line theory, the input reactances of the terminal short-circuit and the open-ended transmission line are respectively:
When the loss is neglected, the input resistance of the antenna is equal to its radiation resistance. The radiation resistance of the inverted-F antenna given in the literature can be obtained as:
When the length of the horizontal portion of the antenna is L=λ/4, it can be known from the equation that the input reactance of the antenna is 0, and the antenna is in a resonant state, that is:
At this time, the input resistance RPIFA of the antenna is a pure resistance, which is independent of the spacing S, and is only related to the height H of the antenna, and increases as the height H of the antenna increases.
As the resonating part of the antenna, the length L of the horizontal portion of the antenna has the most direct influence on the input impedance of the antenna. When it increases, the input resistance of the antenna decreases, and the antenna is inductive, and vice versa. By adjusting L, the input impedance of the antenna can be made purely resistive, and then the antenna height H can be adjusted so that the input resistance of the antenna is close to 50 Ω, that is, the impedance matching can be completed without any additional circuit.
2 Improved printed inverted F-type Bluetooth antennaAlthough the traditional printed inverted-F antenna has high performance, its volume is basically proportional to the operating frequency, so it is still too large in the increasingly miniaturized and ultra-thin mobile phone terminals.
This article takes a Bluetooth-enabled GSM mobile phone as an example, and designs a small-sized, good-performance Bluetooth antenna through ANSOFT's HFSS 10.0. The size of the mobile phone motherboard is 103 mm & TImes; 41.5 mm. Due to the structural requirements of the mobile phone, the Bluetooth chip is located at the lower right of the motherboard, which limits the antenna to the lower right corner of the mobile phone. Because the board size is very limited, the size of the Bluetooth antenna is only 16 mm & TImes; 5 mm, so it is difficult to use the traditional PIFA antenna. Considering that monopole antennas are currently commonly used to reduce the physical size of the monopole antenna, this form can be applied to a conventional printed inverted-F antenna, as shown in FIG.
Where: d = 0.5 mm, H = 4 mm, S = 2.3 mm, M = 2 mm, N = 3 mm, L = 5 mm, h = 1 mm. The phone motherboard uses FR-4 material with a relative dielectric constant of 4.2 and a 50 Ω microstrip line for the antenna feed point. Taking into account the impact of the phone's casing on the antenna's operating frequency, we increased the antenna's operating frequency during simulation, choosing 2.5 GHz.
The antenna is fabricated according to the structure and size of the above software simulation, as shown in FIG. For the convenience of measurement, the inner conductor of a coaxial cable with a characteristic impedance of 50 Ω is soldered to the 50 Ω microstrip line connected to the antenna feed point, and the outer conductor is connected to the ground of the mobile phone motherboard, and the coaxial cable is The SMA connector is soldered at one end and connected to the vector network analyzer MS4622B. The test results are shown in Figure 6. The return loss 10 dB bandwidth is about 120 MHz, which fully covers the ISM band in which Bluetooth operates: 2.400 to 2.48 GHz.
In order to evaluate the performance of the antenna in the indoor environment, this paper does the following experiment: respectively, using a verified manufacturer's dielectric resonant antenna and the PIFA antenna of this paper to test the packet error rate of the transmitted data through the Bluetooth tester. Experiments show that the performance of the two is equivalent. In practical applications, the GSM mobile phone using the PIFA antenna and the NOKIA Bluetooth headset HS-11W can reach a distance of more than 15 m, which meets the requirements of general terminal test specifications.
4 ConclusionAccording to the above analysis and test results, the improved PIFA antenna performance meets the manufacturer's requirements. At the same time, the antenna design is simple and flexible, and the size of S can be determined after selecting L and H according to the actual size of the circuit board. The solution is low in cost, high in efficiency, compact in structure and convenient in feeding, and is completely suitable for a Bluetooth application environment.
1. This product is used to convert the car power supply of mobile phone chargers
2. This product has over-current short-circuit overload protection
3. Insert this product into the cigarette lighter socket of the car
4. Confirm whether the power indicator light of this product is on
5. Insert the mobile phone charger into the socket of this product
6. This product is used in mobile phones, PHS, mobile phone line chargers, travel chargers, universal chargers, etc.
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