User's Manual

SARA-R4 series - System Integration Manual
UBX-16029218 - R03 Design-in
Page 45 of 94
consult the antenna manufacturer for the design-in guidelines for antenna matching relative to the
custom application
In both of cases, selecting external or internal antennas, these recommendations should be observed:
 Select an antenna providing optimal return loss (or V.S.W.R.) figure over all the operating frequencies.
 Select an antenna providing optimal efficiency figure over all the operating frequencies.
 Select an antenna providing appropriate gain figure (i.e. combined antenna directivity and efficiency figure)
so that the electromagnetic field radiation intensity do not exceed the regulatory limits specified in some
countries (e.g. by FCC in the United States, as reported in the section 4.2.2).
2.4.1.2 Guidelines for antenna RF interface design
Guidelines for ANT pin RF connection design
Proper transition between ANT pad and the application board PCB must be provided, implementing the
following design-in guidelines for the layout of the application PCB close to the ANT pad:
 On a multilayer board, the whole layer stack below the RF connection should be free of digital lines
 Increase GND keep-out (i.e. clearance, a void area) around the ANT pad, on the top layer of the application
PCB, to at least 250 µm up to adjacent pads metal definition and up to 400 µm on the area below the
module, to reduce parasitic capacitance to ground, as described in the left picture in Figure 19
 Add GND keep-out (i.e. clearance, a void area) on the buried metal layer below the ANT pad if the top-layer
to buried layer dielectric thickness is below 200 µm, to reduce parasitic capacitance to ground, as described
in the right picture in Figure 19
Min.
250 µm
Min. 400 µm
GND
ANT
GND clearance
on very close buried layer
below ANT pad
GND clearance
on top layer
around ANT pad
Figure 19: GND keep-out area on top layer around ANT pad and on very close buried layer below ANT pad
Guidelines for RF transmission line design
Any RF transmission line, such as the ones from the ANT pad up to the related antenna connector or up to the
related internal antenna pad, must be designed so that the characteristic impedance is as close as possible to
50 .
RF transmission lines can be designed as a micro strip (consists of a conducting strip separated from a ground
plane by a dielectric material) or a strip line (consists of a flat strip of metal which is sandwiched between two
parallel ground planes within a dielectric material). The micro strip, implemented as a coplanar waveguide, is the
most common configuration for printed circuit board.