User's Manual

HE910 Hardware User Guide

1vv0300925 Rev.28 – 2015-06-24

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.