Data Sheet

Silicon Labs
Page 37 of 45
ANY metal in close proximity of the antenna will prevent the antenna from radiating freely. It is recommended
not to place any metal or other conductive objects closer than 20 mm to the antenna except in the directions
of the ground planes of the module itself.
For optimal performance, place the antenna end of the module outside any metal surfaces and objects in the
application, preferably on the device corner. The larger the angle in which no metallic object obstructs the
antenna radiation, the better the antenna will work.
The ANT pad on the antenna end of the WF121-A can be connected to the ground or left unsoldered.
15.4 Thermal considerations
The WF121 module may at continuous full power transmit consume up to 1.3 W of DC power, most of which
is drawn by the power amplifier. Most of this will be dissipated as heat. In any application where high ambient
temperatures and constant transmissions for more than a few seconds can occur, it is important that a
sufficient cooling surface is provided to dissipate the heat.
The thermal pad in the bottom of the module must be connected to the application board ground
planes by soldering. The application board should provide a number of vias under and around the pad to
conduct the produced heat to the board ground planes, and preferably to a copper surface on the other side of
the board in order to dissipate the heat into air.
The module internal thermal resistance should in most cases be negligible compared to the thermal resistance
from the module into air, and common equations for surface area required for cooling can be used to estimate
the temperature rise of the module. Only copper planes on the circuit board surfaces with a solid thermal
connection to the module ground pad will dissipate heat. For an application with high transmit duty cycles
(low bit rate, high throughput, long bursts or constant streaming) the maximum allowed ambient temperature
should be reduced due to inherent heating of the module, especially with small fully plastic enclosed
applications where heat transfer to ambient air is low due to low thermal conductivity of plastic.
The module measured on the evaluation board exhibits a temperature rise of about 25
o
C above ambient
temperature when continuously transmitting IEEE 802.11b at full power with minimal off-times and no collision
detection (a worst case scenario regarding power dissipation). An insufficiently cooled module will rapidly heat
beyond operating range in ambient room temperature.
15.5 EMC considerations
Following recommendations helps to avoid EMC problems arising in the design. Note that each design is
unique and the following list do not consider all basic design rules such as avoiding capacitive coupling
between signal lines. Following list is aimed to avoid EMC problems caused by RF part of the module.
• Do not remove copper from the PCB more than needed. For proper operation the antenna requires a
solid ground plane with as much surface area as possible. Use ground filling as much as possible.
Connect all grounds together with multiple vias. Do not leave small floating unconnected copper areas
or areas connected by just one via, these will act as additional antennas and raise the risk of
unwanted radiations.
• Do not place a ground plane underneath the antenna. The grounding areas under the module should
be designed as shown in Figure 4.
• When using overlapping ground areas use conductive vias separated max. 3 mm apart at the edge of
the ground areas. This prevents RF from penetrating inside the PCB. Use ground vias extensively all
over the PCB. All the traces in (and on) the PCB are potential antennas. Especially board edges
should have grounds connected together at short intervals to avoid resonances.
• Avoid current loops. Keep the traces with sensitive, high current or fast signals short, and mind the
return current path, having a short signal path is not much use if the associated ground path between
the ends of the signal trace is long. Remember, ground is also a signal trace. The ground will conduct
the same current as the signal path and at the same frequency, power and sensitivity.