The built-in antenna often adopts several forms, including shrapnel form, chip patch antenna and FPC antenna. The form of patch antenna is uniform and has fixed size. The position and size of the pad are also fixed according to the specific specification of the antenna. In addition, according to the specific type of antenna, there are relevant design guidelines such as requirements for clearance around the antenna and suggestions on equipment size.
If the shrapnel form is used, we recommend that the customer use the PIFA antenna as the wireless antenna. According to our experience, the success rate and performance of the PIFA antenna are better. Antenna RF feed pad shall be 2*3mm. No copper is distributed on all layers of PCB under the area of pad with periphery more than 0.8mm. If it is a PIFA antenna, add a 2 * 3mm ground pad, and the distance between the two pads is 2mm.
The wireless antenna is usually located on the top of the equipment. From the top of the PCB, cut off the ground of all layers in this area by 2 ~ 3mm, but the pad part of the layer belonging to the antenna ground pad shall be retained.
The topological structure of the wireless antenna matching circuit is four pieces in parallel from the antenna to the test port or power amplifier. Do not lay the ground below the matching circuit and around the matching network within 1.5 mm. It is better to place the matching network close to the feed pad (but not too close).
The signal transmission line from the Wi-Fi module to the antenna matching circuit is a microstrip line with a characteristic impedance of 50 ohm. In order to avoid the loss on the microstrip line, the module should be as close to the wireless antenna as possible. The size of the microstrip line must be determined according to the specific PCB. No crossed lines are allowed to pass between the microstrip line and the ground.
Try to make the ground of the outer area complete and not damaged (the part in the non shield cover), which is particularly important for the area near the antenna. The wireless antenna current must be isolated from the noise current. If the grounding area near the antenna is damaged to be incomplete, a filled ground plane must be generated in the relevant area below it and sewed with a ground through hole to make it a complete ground. The wiring in this area must ensure that the antenna current only flows through the surface plane, and the noise current must be limited to the complete ground plane inside.
When using a pre produced wireless antenna, it should be noted that its characteristics depend on the connected ground plane. The specifications indicated by the manufacturer can be achieved only when the size and shape of the grounding plane are consistent with the manufacturer's evaluation board. In other cases, the user needs to measure the impedance of the pre-produced antenna under the actual application conditions and match it to the required characteristic impedance.