Design of inverted F-type antenna based on Bluetooth technology (Part 2)

　　Figure 3 shows a principle block diagram of a network analyzer. When measuring an inverted-F antenna, the instrument first sends a frequency sweep signal and sends the signal to the device under test through the output port. After the signal passes through the device, it is sent back to the network analyzer.

　　Since the input impedance of the device under test and the output impedance of the network analyzer cannot be perfectly matched, part of the signal will inevitably be transmitted. In this way, the network analyzer compares the output and input signals to obtain the transmission indicators of the device under test, such as gain, insertion loss, distribution loss, etc.; and compares the output and reflected signals to obtain the reflection indicators of the device under test (such as reflection loss, etc.).

　　When testing the inverted-F antenna, the high-frequency wire should be soldered to the antenna test point on the PCB first, and then connected to the network analyzer and recalibrated (the frequencies of the three mark points are 2.402 GHz, 2.440 GHz and 2.480 GHz respectively). After that, the antenna matching components should be soldered, and then the Smith chart of the network analyzer should be used to test the antenna parameters. The Smith chart should be used for software calculation to modify the antenna length and matching component parameter values, and finally the impedance, frequency, bandwidth and standing wave ratio of the antenna network should be optimized.

　　For the convenience of measurement, the inner conductor of a coaxial cable with a characteristic impedance of 50 Ω can be welded to the 50 Ω microstrip line connected to the antenna feed point, and the outer conductor can be connected to the ground of the Bluetooth module. At the same time, an SMA connector is welded to the other end of the coaxial cable and connected to the Agilent network analyzer N3382A. In this way, the Smith chart measured by the network analyzer shows that the antenna impedance after the matching network is very close to 50 Ω. The bandwidth at a return loss of 10 dB is about 90 MHz, which can fully cover the ISM frequency band (2.400～2.483 GHz) where Bluetooth works, and its standing wave ratio is less than 2, which can meet the requirements of Bluetooth radio technology. Figure 4 shows the test results of the inverted F-type Bluetooth antenna. Figure 5 shows the connection diagram of its antenna matching network.

　　3 Conclusion

　　The Bluetooth inverted F-shaped antenna designed by the method in this paper is compact, efficient, easy to produce, and inexpensive. It can fully meet the bandwidth requirements of Bluetooth antennas within the 2.402~2.480GH working frequency band specified by the Bluetooth protocol. At the same time, the antenna is flexible in design, easy to process, and can form an independent RF transceiver with the RF chip, which is very suitable for use in short-range wireless communication systems.