SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11 Design-in Page 84 of 157
However, real antennas do not have a perfect 50 load on all the supported frequency bands. So to reduce
as much as possible any performance degradation due to antenna mismatching, the RF termination must
provide optimal return loss (or VSWR) figures over all the operating frequencies, as summarized in Table 7.
If an external antenna is used, the antenna connector represents the RF termination on the PCB:
Use suitable a 50 connector providing a clean PCB-to-RF-cable transition.
Strictly follow the connector manufacturer’s recommended layout, for example:
o SMA Pin-Through-Hole connectors require a GND keep-out (i.e. clearance, a void area) on all the
layers around the central pin up to the annular pads of the four GND posts, as shown in Figure 34
o U.FL surface mounted connectors require no conductive traces (i.e. clearance, a void area) in the
area below the connector between the GND land pads.
Cut out the GND layer under the RF connector and close to any buried vias, to remove stray capacitance
and thus keep the RF line at 50 , e.g. the active pad of UFL connector needs to have a GND keep-out
(i.e. clearance, a void area) at least on the first inner layer to reduce parasitic capacitance to ground.
If an integrated antenna is used, the integrated antenna represents the RF terminations. The following
guidelines should be followed:
Use an antenna designed by an antenna manufacturer providing the best possible return loss (or VSWR).
Provide a ground plane large enough according to the relative integrated antenna requirements. The
ground plane of the application PCB can be reduced down to a minimum size that must be similar to
one quarter of wavelength of the minimum frequency that needs to be radiated. As numerical example,
Frequency = 750 MHz Wavelength = 40 cm Minimum GND plane size = 10 cm
It is highly recommended to strictly follow the detailed and specific guidelines provided by the antenna
manufacturer regarding correct installation and deployment of the antenna system, including the PCB
layout and matching circuitry.
Further to the custom PCB and product restrictions, the antenna may require a tuning to comply with
all the applicable required certification schemes. It is recommended to consult the antenna manufacturer
for the design-in guidelines for the antenna matching relative to the custom application.
Additionally, these recommendations regarding the antenna system placement must be followed:
Do not place the antenna within a closed metal case.
Do not place the antenna in close vicinity to the end user since the emitted radiation in human tissue
is restricted by regulatory requirements.
Place the antenna as far as possible from VCC supply line and related parts (refer to Figure 28), from
high speed digital lines (as USB) and from any possible noise source.
Place the antenna far from sensitive analog systems or employ countermeasures to reduce EMC or EMI
issues.
Be aware of interaction between co-located RF systems since the LTE transmitted power may interact
or disturb the performance of companion systems.
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