User's Guide

7.3 Module Antenna
To minimize impedance detuning and degradation of the radiation pattern, reduce as much shielding of the selected antenna as possi-
ble.
The ground plane usually forms an important part of the antenna as there is significant current running along the ground plane (i.e., the
GND plane is also part of any monopole-type antenna, in general). For optimal performance when using an integral antenna, ensure
that the PCB size is at least 55 mm (± 3mm) in width and follow the layout recommendations for the different host PCB sizes as de-
tailed in this section. These layout drawing solutions are patented by Silicon Labs.
For optimal performance of the WFM200 module antenna, follow these generic guidelines:
• Place the module at the edge of the carrier PCB where the centers of pins 1, 47, and 49 are 1.8 mm away from the metal edge of
the PCB.
• Do not place any metal (traces, components, etc.) in the antenna clearance area. This keep-out applies on each layer. The form and
dimensions of the antenna clearance area are shown in Section 10 of this document.
• Connect all ground pads directly to a solid ground plane and place the ground stitching vias close to the ground pads. The recom-
mended GND via stitching placement for WFM200 is shown in Figure 7.3 Grounding and Via Stitching of WFM200 Part on page
17.
• Using size, thickness, or permittivity for the main board other than optimum requires tuning and testing to re-optimize antenna loop
trace capacitor value (C_ANT). The default capacitor value is C_ANT = 0.9 pF in an SMD0201 package.
• Additionally, a series-matching capacitor/inductor between the 2G4ANT_IN and RF_1 ports can also be utilized for fine impedance
tuning (C_SER). The default configuration of C_SER is a 0 Ω jumper as shown in Figure 7.1 WFM200 Portion of Reference Design
Board Layout on page 13 above.
•
The area occupied by WFM200, its required loop trace, and keep-out is 8.5 mm x 13.5 mm = 115 mm
2
. The rest of the required area
can have components and traces which are surrounded by GND fill, especially 1 mm wide around the perimeter of the placement
areas, which amounts to 120 mm
2
.
• There can be no cable connections within the required area during TX operation except for the UFL connector for the second RF I/O
port in the location shown just north of WFM200 and away from the edge (refer to the figure below). The co-ax cable must exit the
northern placement area over the allowed GND bridge or routing area between the WFM200 required board portion and the rest of
the main board portion.
• Any digital component added to the loop side and supply side could degrade WFM200 sensitivity because of signal harmonics falling
into Wi-Fi band. To get lower than 0.5 dB desensitization, the radiated level of CW interference measured at pin 2G4ANT_IN should
be less than -108 dBm. To get lower than 3 dB desensitization, the level of AWGN-like interference measured at pin 2G4ANT_IN in
channel bandwidth should be less than -94 dBm.
Figure 7.3. Grounding and Via Stitching of WFM200 Part
UG395: WFM200 Hardware Design User's Guide
Layout Recommendations
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