User's Manual

In applications where the module must be placed in a metallic enclosure, an external
antenna must be used. The external antenna must match to 50 Ohm.
A PCB antenna can be made as a copper track where the ground plane is removed on the
back side. The rest of the PCB board should have a ground plane as large as possible,
preferably as large (in one dimension) as the antenna itself, to make it act as a reflector
mirror to the antenna. A quarter-wavelength antenna on a PCB must be shorter than the
wire antenna due to the influence of the dielectric material of the PCB. The length reduction
depends on the PCB thickness and material, as well as how close to the edge of the board
the antenna is placed. Typical reduction is to 75-90 % but specific results may vary.
The length of a quarter-wave antenna is given in the table below.
Frequency
(MHz)
Length of whip antenna
(cm)
Length of PCB track
(cm)
2450 2.9 2.2 2.7
If, for space reasons, the track is made even shorter than the resonating quarter of
wavelength, the antenna should be matched to 50 ohms using a series inductor and a shunt
capacitor.
17.0 PCB Layout Recommendations