Novatel HS 3001, Integration Manual

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Figure 4-4 Modem Tabs - Ideal Ground Structure
Antenna And RF Signal Trace
The PCB trace that feeds the RF output port must be designed for a 50 ohm characteristic
impedance. The RF trace should be coplanar, or routed into internal layers to keep the top
layer continuous around and underneath the cellular module. Provide ample ground vias
around the RF contacts, the RF trace, and launch pad. If possible, keep I/O and power traces
away from the RF port. This includes traces running parallel or orthogonal to it. Do not use
thermal relief on the antenna output port ground pads. Pay close attention to the size of the
pad and thickness of the dielectric beneath the signal pad and trace. Most PCB manufacturers
can adjust the trace width to maintain 50 ohms impedance if the traces are identified and
instructions are included on the FAB drawing. This service is typically provided at no or
minimal additional cost.
For minimum RF emissions due to the fundamental frequency of operation, the cellular
module works best with an antenna load that has a VSWR of 1.5:1 or better. The antenna
should not have gain at the harmonic frequencies, otherwise, the conducted harmonics could
get amplified to a point where the product no longer passes type approval. However, for
applications where antenna quality is less than ideal, we recommend that you have a low pass
filter (Pi structure with N=3) in the RF path to the antenna. This is a secondary plan should
there be a need to lower harmonic levels at frequencies above the PCS band. The pad
structure may also be used to match the antenna load impedance, if required. If it is not
needed, you may use a low-reactance capacitor to bridge the Pi structure.