SARA-R4 series - System Integration Manual
UBX-16029218 - R06
Design-in
Page 49 of 102
2.4
Antenna interface
SARA-R4 series modules provide an RF interface for connecting the external antenna: the
ANT
pin represents the
RF input/output for RF signals transmission and reception.
The
ANT
pin has a nominal characteristic impedance of 50
and must be connected to the physical antenna
through a 50
transmission line to allow proper transmission / reception of RF signals.
2.4.1
Antenna RF interface (ANT)
2.4.1.1
General guidelines for antenna selection and design
The antenna is the most critical component to be evaluated. Designers must take care of the antenna from all
perspective at the very start of the design phase when the physical dimensions of the application board are
under analysis/decision, since the RF compliance of the device integrating SARA-R4 series modules with all the
applicable required certification schemes depends on antenna’s radiating performance.
Cellular antennas are typically available as:
External antennas (e.g. linear monopole):
o
External antennas basically do not imply physical restriction to the design of the PCB where the SARA-R4
series module is mounted.
o
The radiation performance mainly depends on the antennas. It is required to select antennas with
optimal radiating performance in the operating bands.
o
RF cables should be carefully selected to have minimum insertion losses. Additional insertion loss will be
introduced by low quality or long cable. Large insertion loss reduces both transmit and receive radiation
performance.
o
A high quality 50
RF connector provides proper PCB-to-RF-cable transition. It is recommended to
strictly follow the layout and cable termination guidelines provided by the connector manufacturer.
Integrated antennas (e.g. PCB antennas such as patches or ceramic SMT elements):
o
Internal integrated antennas imply physical restriction to the design of the PCB:
Integrated antenna excites RF currents on its counterpoise, typically the PCB ground plane of the device
that becomes part of the antenna: its dimension defines the minimum frequency that can be radiated.
Therefore, the ground plane can be reduced down to a minimum size that should be similar to the
quarter of the wavelength of the minimum frequency that has to be radiated, given that the orientation
of the ground plane relative to the antenna element must be considered.
As numerical example, the physical restriction to the PCB design can be considered as following:
Frequency = 750 MHz
Wavelength = 40 cm
Minimum GND plane size = 10 cm
o
Radiation performance depends on the whole PCB and antenna system design, including product
mechanical design and usage. Antennas should be selected with optimal radiating performance in the
operating bands according to the mechanical specifications of the PCB and the whole product.
o
It is recommended to select a custom antenna designed by an antennas’ manufacturer if the required
ground plane dimensions are very small (e.g. less than 6.5 cm long and 4 cm wide). The antenna design
process should begin at the start of the whole product design process
o
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 PCB layout
and matching circuitry
o
Further to the custom PCB and product restrictions, antennas may require tuning to obtain the required
performance for compliance with all the applicable required certification schemes. It is recommended to
consult the antenna manufacturer for the design-in guidelines for antenna matching relative to the
custom application
