1) dipole antenna
The appearance of dipole antenna is usually cylindrical or thin, and there is an adapter at the bottom of the antenna as an energy feeding device, which is connected with the adapter outside the RF front-end circuit of Bluetooth module. Another way to connect the antenna is to use a rotatable adapter. The advantage of this method is that the antenna can rotate at any angle according to the use requirements, which improves the transmission effect. The disadvantage is that the rotatable adapter is high in cost. The length of even antenna is related to its working frequency. Generally, a half wavelength or a quarter wavelength is used as the length of the antenna. In addition, the Bluetooth antenna can be designed as SMD(Surface-Mounted Device) element that can be soldered on the circuit board by using the plane design method, or the antenna can be directly designed on the PCB circuit board with a simple microstrip line structure, so as to obtain a low-cost hidden antenna, which is helpful to the diversified design of product appearance.
2) Fabry-Perot antenna
The Fabry-Perot antenna is named because its side structure is similar to the inverted English letter F. Its working length is only a quarter of the working wavelength, and its structure already contains the grounding metal surface, which can reduce the sensitivity to the grounding metal surface in the module and is very suitable for Bluetooth module equipment. On the other hand, the Fabry-Perot antenna only needs to use metal conductor with proper feed and short-circuit the antenna to the ground plane, so its manufacturing cost is low and it can be directly welded with PCB. The metal conductor of the Fabry-Perot antenna can be linear or flaky. If it is flaky, it can be designed that SMD elements are welded on the circuit board to hide the antenna. In order to prevent the short circuit between the metal sheet and the grounded metal surface, an insulating medium is usually added between the metal sheet and the grounded metal surface. If an insulating material with high dielectric constant is used, the size of the Bluetooth antenna can be reduced.
3) Ceramic antenna
Ceramic antenna is another miniaturized antenna suitable for Bluetooth devices. The types of ceramic antennas can be divided into block ceramic antennas and multilayer ceramic antennas. The former uses high temperature (above 1000 degrees Celsius) to sinter the whole ceramic body at one time, and then prints the metal part of the antenna on the surface of the ceramic block. In the latter case, the multilayer ceramics are aligned by low-temperature co-firing and then sintered at 800~900 degrees, so that the metal conductor of the antenna can be printed on each ceramic dielectric layer according to the design requirements, thus effectively reducing the required size of the antenna and achieving the purpose of hiding the antenna design layout. Because the dielectric constant of ceramic itself is higher than that of PCB, using ceramic as antenna medium can effectively reduce the antenna size; In terms of dielectric loss, ceramic dielectric is also smaller than PCB, which is very suitable for Bluetooth module with low power consumption. In addition, when the Bluetooth module must use LTCC technology to minimize the module size, the LTCC Bluetooth antenna can be easily integrated with the Bluetooth module in LTCC multilayer ceramic media, which will be the best choice for miniaturized Bluetooth modules.