SE868-V3 Product User Guide
1VV0301205 r4 – 2016-01-20
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 53 of 76
Mod. 0805 2015-02 Rev.4
11.3.
Active versus Passive Antenna
If the GNSS antenna is placed near the receiver and the RF trace losses are not excessive
(nominally 1 dB), then a passive antenna may be used. This would often be the lowest cost
option and most of the time the simplest to use. However, if the antenna needs to be located
away from the receiver, then an active antenna may be required to obtain the best system
performance. An active antenna includes a built- in low noise amplifier (LNA) to overcome RF
trace and cable losses. Also, many active antennas have a pre-select filter, a post-select filter,
or both.
Important specifications for an active antenna LNA are gain and noise figure.
11.4.
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 the next section 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.
