SL876Q5-A
Product User Guide
1VV0301333 Rev. 2
Page 49 of 68
2017-06-27
GNSS Antenna Gain
Antenna gain is defined as the amplified signal power from the antenna compared to a
theoretical isotropic antenna (equally sensitive in all directions).
Optimum performance is realized only if the firmware build and hardware configuration match
the type of antenna used (active or passive). The firmware must set the internal LNA gain to
correspond to the installed antenna.
For example, a 25 mm by 25 mm square patch antenna on a reference ground plane (usually
70 mm by 70 mm) may give an antenna gain at zenith of 5 dBic. A smaller 18 mm by 18 mm
square patch on a reference ground plane (usually 50 mm by 50 mm) may give an antenna
gain at zenith of 2 dBic.
An antenna vendor should specify a nominal antenna gain (usually at zenith, or directly
overhead) and antenna pattern curves specifying gain as a function of elevation, and gain at
a fixed elevation as a function of azimuth. Pay careful attention to the requirement to meet the
required design, such as ground plane size and any external matching components. Failure to
follow these requirements could result in very poor antenna performance.
It is important to note that GNSS antenna gain is not the same as external LNA gain. Most
antenna vendors will specify these numbers separately, but some combine them into a single
number. Both numbers are significant when designing the front end of a GNSS receiver.
For example, antenna X has an antenna gain of 5 dBic at azimuth and an LNA gain of
20 dB for a combined total of 25 dB. Antenna Y has an antenna gain of -5 dBic at azimuth and
an LNA gain of 30 dB for a combined total of 25 dB. However, in the system, antenna X will
outperform antenna Y by about 10 dB (Refer to
Section 4.16 Internal LNA
for more details
on external LNA gain).
An antenna with higher gain will generally outperform an antenna with lower gain. However,
once the signals are above about -130 dBm for a particular satellite, no improvement in
performance would be realized. But for those satellites with a signal level below about -135
dBm, a higher gain antenna would amplify the signal and improve the performance of the
GNSS receiver. In the case of really weak signals, a good antenna could mean the difference
between being able to use a particular satellite signal or not.
System Noise Floor
The receiver will display a reported C/No of 40 dB-Hz for an input signal level of -130 dBm.
The C/No number means the carrier (or signal) is 40 dB greater than the noise floor measured
in a one Hz bandwidth. This is a standard method of measuring GNSS receiver performance.
The simplified formula is:
C/No = GNSS Signal level – Thermal Noise – System NF
Equation 10-1 Carrier to Noise Ratio
Thermal noise is -174 dBm/Hz at 290K.
We can estimate a system noise figure of 4 dB for the module, consisting of the pre-select
SAW filter loss, the LNA noise figure, and implementation losses within the digital signal
processing unit. The DSP noise is typically 1.0 to 1.5 dB.
However, if a good quality external LNA is used, the noise figure of that LNA (typically better
than 1dB) could reduce the overall system noise figure from 4 dB to approximately 2 dB.
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