Sierra Wireless EM75 User Manual

Product Technical Specification

Rev 1  Dec.17

10

41111980

The module is fully shielded to protect against EMI and must not be removed.

When selecting the antenna and cable, it is critical to RF performance to match 
antenna gain and cable loss.

When matching antennas and cabling:
•

The antenna (and associated circuitry) should have a nominal impedance of 
50

 with a return loss of better than 10 dB across each frequency band of 

operation.

The system gain value affects both radiated power and regulatory (FCC, IC, 
CE, etc.) test results.

Consider the following points when designing custom antennas:
•

A skilled RF engineer should do the development to ensure that the RF 
performance is maintained.

If both UMTS and CDMA modules will be installed in the same platform, you 
may want to develop separate antennas for maximum performance.

When deciding where to put the antennas:
•

Antenna location may affect RF performance. Although the module is 
shielded to prevent interference in most applications, the placement of the 
antenna is still very important—if the host device is insufficiently shielded, 
high levels of broadband or spurious noise can degrade the module’s perfor-
mance.

Connecting cables between the module and the antenna must have 50

 

impedance. If the impedance of the module is mismatched, RF performance 
is reduced significantly.

Antenna cables should be routed, if possible, away from noise sources 
(switching power supplies, LCD assemblies, etc.). If the cables are near the 
noise sources, the noise may be coupled into the RF cable and into the 
antenna.

Use the AT command 

!RXDEN=0

 to disable receive diversity or 

!RXDEN=1

 to 

enable receive diversity.