HE910 Hardware User Guide
1VV0300925 Rev. 32
Page
64
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2019-11-25
7.3.3.
GPS 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 25mm by 25m square patch antenna on a reference ground plane (usually
70mm by 70mm) will give an antenna gain at zenith of 5 dBic. A smaller 18mm by 18mm
square patch on a reference ground plane (usually 50mm by 50mm) 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) they should supply 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 these specifications, such as ground plane required 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 mean the
difference between being able to use a particular satellite signal or not.
7.3.4.
Active versus Passive Antenna
If the GPS antenna is placed near the HE910 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 HE910 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 either a pre-select filter, a post-select
filter, or both.
Summary of Contents for HE910
Page 1: ... 01 2017 Mod 0805 2017 01 Rev 6 HE910 HW User Guide 1VV0300925 Rev 32 2019 11 25 ...
Page 40: ...HE910 Hardware User Guide 1VV0300925 Rev 32 Page 40 of 111 2019 11 25 ...
Page 71: ...HE910 Hardware User Guide 1VV0300925 Rev 32 Page 71 of 111 2019 11 25 ...
Page 111: ... 01 2017 Mod 0805 2017 01 Rev 6 ...
