Shielding; Ground Plane Isolation; Antenna And Cabling - Sierra Wireless MC5720 Hardware Integration Manual

P C I E x p r e s s M i n i C a r d H a r d wa r e I n t e g r a t i o n G u i d e
the noise cannot be reduced, isolating the mounting holes from the ground may
actually help.

Shielding

The module is fully shielded to protect against EMI and to ensure FCC Part 15
(or equivalent) compliance. To maintain the shield effectiveness, the modem
shields must not be removed.

Ground plane isolation

The coaxial cable connecting the module to the antenna carries the ground
connection. To avoid a ground loop, the ground plane for the antenna and the
ground plane used by the modem must be electrically isolated from each other.
If your integration uses the device's case as part of the ground connection, then,
to avoid creating a ground loop, the external antenna connection must be isolated
from the case.

Antenna and cabling

In selecting the antenna and cable, it is critical to RF performance to match
antenna gain and cable loss.
For proper matching, the antenna along with any associated circuitry should have
a nominal impedance of 50 Ω with a return loss ≤ -10 dB across each frequency
band of operation. Overall system antenna gain, with cable loss, should be
≥ 0 dBi and ≤ +6 dBi. Keep in mind that the system gain value impacts radiated
power and the FCC MPE/SAR (or equivalent) test results.
Many antenna vendors produce dual-band antennas that would work with the
EM. You may instead choose to develop a custom-designed antenna. This
requires a skilled RF engineer to ensure that the RF performance is maintained.
Antenna location may also impact RF performance. The modem itself is shielded
to prevent interference in most applications, but this does not mean that antenna
placement can be ignored.
Any connecting cables between the modem and the antenna (if required) must
have 50 Ω impedance. Mismatching the impedance of the EM will result in a
significant reduction in RF performance.
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