Effects of placing coin-cell battery in tuned design
To understand the effects of the coin-cell battery on the antenna matching we took a BGM121 test board and placed a coin-cell in the back with a piece of blu-tak to keep it in place.
Figure 1 - BGM121 test board with coin-cell battery attached
The antenna matching can be seen in the following figures, first for the test board without battery and then after attaching the coin-cell battery.
Figure 2 - Original antenna matching for the BGM121 test board
Figure 3 - Antenna matching for BGM121 test board after attaching the coin-cell battery
Markers 1, 2 and 3 correspond to the beginning, middle, and end of the Bluetooth band. As it is visible in Figure 3 the presence of a metallic body (coin-cell) shifts the antenna tuning outside of the Bluetooth band. This is mostly due to the fact that the coin-cell is covering part of the copper clearance area and therefore reducing the antenna current loop.
Re-tuning the antenna
To counteract this frequency shift we can extend the clearance area simply by scrapping away the ground layer as shown in Figure 4. Due to the the underlying uncertainty on how much it needs to be extended we’ll just extend it about 50% more as it is then easier to re-add a metal plane in order to fine-tune the clearance area.
Figure 4 - Extending the copper clearance area
The antenna matching of the modified board is show in the next figure.
Figure5 - Antenna matching after extending the copper clearance area
The matching is still off-band but this time it is towards the lower frequencies which can be compensated by reducing the copper clearance. To reduce the copper clearance we simply cover it with a piece of metal (a shield from one of our modules in this specific case) until we get the correct matching as shown in the next figure.
Figure 6 - Reducing the copper clearance area to fine tune antenna matching
The antenna matching is now back into the Bluetooth band and very close to the original design.
Figure 7 - Antenna matching after reducing size of the copper clearance area
The presence of metal very near to the BGM12x can have a negative impact on the antenna matching. However, it is easy to compensate the effect by adjusting the size of the copper clearance area to bring the antenna matching back into the Bluetooth band and ensure the best RF performance for your design.