The BGM121/BGM123 Blue Gecko Bluetooth® SiP Module family is targeted for applications where ultra-small size, reliable high RF performance, low-power consumption and easy application development are key requirements.

At 6.5 x 6.5 x 1.4 mm the BGM121/BGM123 module fits applications where size is a constraint. BGM121/BGM123 also integrates a high performance, ultra robust antenna, which requires minimal PCB, plastic and metal clearance. The total PCB area required by BGM121/BGM123 is only 51 mm2.

The BGM12x SiP module has an internal chip antenna which is properly tuned with a GND loop, i.e. copper clearance area, on the carrier board. For the best possible antenna performance Silicon Labs recommends to precisely follow the layout suggestions around the SiP module including the copper clearance area, as shown in the following figures.

In order to have the internal antenna tuned the GND pins 53 and 54 of BGM12x shouldn’t directly be tied together. They need to be connected to each other through the GND loop which is routed along the circumference of the copper clearance area. This GND loop makes the antenna resonate and its size determines the resonant frequency, so its layout parameters should exactly be followed as shown in the figure above. The SiP module should be placed at the board edge of the carrier PCB.

If these layout recommendations above are precisely followed on the carrier board of the SiP module then the BGM12x has the following unique features:

1. It remains always tuned on any application board as well. Its antenna tuning is not sensitive to the size of the carrier board. However, the minimum recommended board design width is 10mm (to ensure enough place for the module together with the copper clearance area and GND loop).
2. It can always be tuned on any application board if needed. For instance, the BGM12x SiP module antenna gets de-tuned if any metal object, e.g. coin cell battery, is mounted or attached onto the carrier board beneath the antenna area of the SiP module on the opposite layer – this de-tuning effect due to close metal proximity can generally be seen with any antenna. However, this de-tuning effect can be compensated out and the BGM12x SiP module can get tuned again by adjusting the antenna matching, i.e. modifying the antenna clearnace area on the carrier board.
3. The module antenna is not sensitive to any dielectric material in its close proximity. It is nearly immune against any dielectric (e.g. plastics, conformal coating) in touch with the module as well.
4. In an optimal board design it has as good performance (in terms of antenna efficiency) as what could be achieved with any external antenna (-1 to -2 dB antenna efficiency).

The impedance of the BGM12x SiP module’s built-in antenna can be fine-tuned by adjusting the copper clearance area (i.e. GND loop) on the carrier board. The figure below shows the return loss (S11) – where the resonant frequency can also be observed from the notches – of the built-in antenna with different copper clearance width. This tuning method is a unique feature for BGM12x, so the module antenna can get tuned on any custom design layout and thus exceptional RF performance can always be achieved with this SiP module.

The generally recommended copper clearance area dimensions are 5.0 x 2.2 mm on the top layer - when there isn't any metal object in the close proximity of the antenna and copper clearance area. If any metal object, e.g. coin cell battery, is mounted or attached onto the carrier board beneath the antenna area of the SiP module on the opposite layer, then the suggested GND loop dimensions are 6.2 x 2.2 mm - this assumes that the front edge of the coin cell battery is in line with the front edge of the PCB.  

A more detailed guidance on the antenna tuning of the BGM12x SiP module can also be seen under the below KB article here:

https://www.silabs.com/community/wireless/bluetooth/knowledge-base.entry.html/2017/01/10/antenna_tuning_forb-R5av

The optimal board design mentioned above in point 4 refers to the generally recommended GND plane size of monopole-type antennas. Since the GND plane itself is also the part of the monopole-type antennas it is important to ensure big enough GND reference for the antenna. The smaller GND reference is ensured for any monopole-type antenna the weaker antenna performance can be expected. If the GND plane size goes below quater-wavelength then the antenna performance drops quickly.

To have the best antenna performance with BGM12x Silicon Labs recommends to use 40...50 mm wide carrier board. If the module antenna is kept tuned by following the layout recommendations, especially around the copper clearance area and GND loop, then the following curve describes the effects of the GND plane on the antenna efficiency.

RF performance when using BGM12x SiP module on a very small carrier board:

The minimum recommended board design width is 10 mm. The antenna efficiency in that case is around -12 dB. Under these conditions, the tested practical RF range with the BGM12x SiP module in office environment is about 15 meters.

The following figure shows the antenna efficiency versus board size curves for different antennas. As the curves show below the considerable antenna efficiency degradation is a general feature of any monopole-type antenna.

Training & demonstration videos:

BGM121/123 Hardware Design Guidelines: https://www.youtube.com/watch?v=5DCn7AzIyig

BGM121/123 Antenna Robustness: https://www.youtube.com/watch?v=LA2j4AXqd7Q