User's Manual

GE910 Hardware User Guide
1vv0300962 Rev.12 2013-10-22
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 45 of 83
Mod. 0805 2011-07 Rev.2
7.4.4. GNSS Antenna Gain
Antenna gain is defined as the extra signal power from the antenna as compared to a theoretical
isotropic antenna (equally sensitive in all directions).
For example, a 25x25mm
2
patch antenna on a reference ground plane (usually 70x70mm
2
) will
give an antenna gain at zenith of 5 dBic. A smaller 18mm by 18mm square patch on a reference
ground plane (usually 50x50mm
2
) will give an antenna gain at zenith of 2 dBic.
While an antenna vendor will specify a nominal antenna gain (usually at zenith, or directly
overhead) antenna pattern curves should be supplied, 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 these specifications, such as required ground plane and any external matching components.
Failure to follow these requirements could result in very poor antenna performance.
It is important to note that GPS antenna gain is not the same thing as external LNA gain. Most
antenna vendors will specify these numbers separately, but some combine them into a single
number. It is important to know both numbers when designing and evaluating the front end of a
GPS 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 next chapter for more details on system noise floor).
An antenna with higher gain will generally outperform an antenna with lower gain. Once the
signals are above about -130 dBm for a particular satellite, no improvement in performance
would be gained. However, for those satellites that are below about -125 dBm, a higher gain
antenna would improve the gain and improve the performance of the GPS receiver. In the case
of really weak signals, a good antenna could make the difference between being able to use a
particular satellite signal or not.
7.4.5. Active versus Passive Antenna
If the GPS antenna is placed near the GE910 and the RF traces losses are not excessive
(nominally 1 dB), then a passive antenna can be used. This would normally 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 GE910 then an active antenna may be required to obtain the best system
performance. The active antenna has its own built-in low noise amplifier to overcome RF trace
or cable losses after the active antenna.
However, an active antenna has a low noise amplifier (LNA) with associated gain and noise
figure. In addition, many active antennas have a pre-select filter, a post-select filter, or both.