Getting Started with EZ-BT WICED Modules
Document Number: 002-23400 Rev. **
51
8.3
Enclosure Effects on Antenna Performance
Antennas used in consumer products are sensitive to the PCB RF ground size
, the product’s plastic casing, and metallic
enclosures. This section describes the effect of each of these environmental factors on RF performance.
8 . 3 . 1
An t e n n a N e a r - F i e l d a n d F a r - F i e l d
Every antenna contains two regions surrounding it: the near-field and the far-field.
The near-field is the region where the radiated field has not yet formed. In this region, the electric and magnetic fields
are not orthogonal to each other. This region is very close to the antenna. The near-field region has two regions: the
reactive near-field region
and the
radiating near-field region
. The transition to a far-field region happens in the radiating
near-field region.
The radiation field is formed after the transition to the far-field region. In this region, the relative angular variation of the
field does not depend on the distance. This means that if you plot the angular radiation field at a distance from the
antenna in the far-field region, their shapes remain the same. Only with distance, the field strength decreases. However,
the shape of the radiation pattern remains the same with respect to the angular variation. This region is called the far-
field region. An object in the far field does not affect the radiation pattern much. However, any obstruction in the near-
field can completely change the radiation pattern. If the obstruction is metal, the effect on the radiation pattern is much
more pronounced.
shows the regions for a dipole antenna.
Figure 32. Near and Far Field
For a module based on a 2.4-GHz chip antenna, the near-field extends up to 4 mm.
8 . 3 . 2
E f f e c t o f N o n m e t a l l i c E n c l o s u r e
Any plastic enclosure changes the resonating frequency of the antenna. The antenna can be modeled as an LC
resonator whose resonant frequency decreases when either L (inductance) or C (capacitance) increases. A larger RF
ground plane and plastic casing increase the effective capacitance and thus reduce the resonant frequency. See the
application note
for more details on the effect of an enclosure.
details a module antenna in a plastic enclosure. The clearance from the antenna to the plastic enclosure can
be as little as 2 mm. However, clearance of this amount can affect the tuning of the antenna. This can be resolved by
retuning the antenna; however, for a module solution, it is not recommended to attempt retuning of the antenna. To
minimize effects on the module antenna, it is recommended to have a minimum clearance of 5 mm.
Dipole Antenna
With length D
