SARA-R4 series - System integration manual
UBX-16029218 - R20
System description
Page 33 of 128
C1-Public
1.7.2
GNSS antenna RF interface (ANT_GNSS)
☞
The GNSS antenna RF interface is supported by SARA-R422M8S modules only.
☞
For additional information and guidelines regarding the GNSS system, see the u-blox SARA-R4 /
SARA-R5 positioning implementation application note
SARA-R422M8S modules provide an RF interface for connecting the external GNSS antenna. The
ANT_GNSS
pin represents the RF input reception of GNSS RF signals.
The
ANT_GNSS
pin has a nominal characteristic impedance of 50
and must be connected to the Rx
GNSS antenna through a 50
transmission line to allow proper RF reception. As shown in
the GNSS RF interface is designed with an internal DC block, and is suitable for both active and/or
passive GNSS antennas due to the built-in SAW filter followed by an additional LNA in front of the
integrated high performing u-blox M8 concurrent position engine.
1.7.2.1
GNSS antenna RF interface requirements
summarizes the requirements for the GNSS antenna RF interface. See section
suggestions to correctly design antennas circuits compliant with these requirements.
Item
Requirements
Remarks
Impedance
50
nominal characteristic impedance
The impedance of the antenna RF connection must match the
50
impedance of the
ANT_GNSS
port.
Frequency
range
BeiDou 1561 MHz
GPS / SBAS / QZSS / Galileo 1575 MHz
GLONASS 1602 MHz
The required frequency range of the antenna connected to
ANT_GNSS
port depends on the selected GNSS constellations.
Return
loss
S
11
< -10 dB (VSWR < 2:1) recommended
S
11
< -6 dB (VSWR < 3:1) acceptable
The return loss or the S
11
, as the VSWR, refers to the amount of
reflected power, measuring how well the antenna RF connection
matches the 50
characteristic impedance of the
ANT_GNSS
port.
The impedance of the antenna termination must match as much
as possible the 50
nominal impedance of the
ANT_GNSS
port
over the operating frequency range, reducing as much as possible
the amount of reflected power.
Gain
(passive
antenna)
> 4 dBic
The antenna gain defines how efficient the antenna is at receiving
the signal. It is important providing good antenna visibility to the
sky, using antennas with good radiation pattern in the sky
direction, according to related antenna placement.
Gain
(active
antenna)
17 dB minimum, 30 dB maximum
The antenna gain defines how efficient the antenna is at receiving
the signal. It is directly related to the overall C/No.
Noise
figure
(active
antenna)
< 2 dB
Since GNSS signals are very weak, any amount of noise degrades
all the sensitivity figures of the receiver: active antennas with LNA
with a low noise figure are recommended.
Axial ratio
< 3 dB recommended
GNSS signals are circularly-polarized. The purity of the antenna
circular polarization is stated in terms of axial ratio (AR), defined
as the ratio of the vertical electric field to the horizontal electric
field on polarization ellipse at zenith.
Table 8: Summary of GNSS antenna RF interface requirements
