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Many vendors provide positioning modules that support active antennas as well as ones that don't. I am using 4 positioning modules out of which 3 of them support active antennas and the other , CC4000 by TI , supports only passive antennas. To be honest I am testing modules for performance, in terms of TTFF for SGEE, CGEE and A-GPS.

Take a look if its necessary.

http://www.u-blox.com/en/gps-modules/pvt-modules/max-m8-series-concurrent-gnss-modules.html

https://www.linxtechnologies.com/en/products/gps-modules/fm-gps-receiver-module

http://www.ti.com/tool/cc4000gpsem

However all three claim to have almost same acquisition gains,about 143 dBi. But let me make my question general so that it will help more people.

For all 4 positioning modules that claim to have almost same acquisition gains, how would the antenna type (active/passive) effect its performance? I mean with a passive antenna if the module can have a acquisition gain of 143dBi why bother fixing it an active antenna?

Is an active antenna necessary for applications that have really short antenna cables(about 2-3cm)?

If I have gone wrong somewhere, kindly direct me.

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Active antennas contain a low noise amplifier and possibly a filter and line driver. It is very important to put the LNA as close to the antenna as possible to get some gain before the cable loss. The trick with GPS is that the signal is extremely small. The GPS signal is actually below the noise floor of the LNA, so adding more gain really does not help at all above mitigating the loss in the cable. What does help is the coding gain in the receiver when it despreads the signal. This does not depend on the receiver as the coding gain is dependent on the design of the code.

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  • \$\begingroup\$ I have around 4 GPS devices with me. How do you suggest I figure out which devices are coded better? Should I run tests under same conditions and find the TTFF? \$\endgroup\$ – Denis Sep 24 '14 at 1:16
  • \$\begingroup\$ By code I mean the GPS spreading code, not the software on the modules. The spreading code is generated by the satellites so it doesn't matter what module is receiving it. So long as the implementation is correct, the coding gain will be identical across all of the modules. \$\endgroup\$ – alex.forencich Sep 24 '14 at 6:47
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The only reason for a active antenna is to improve signal quality from long cable lengths. This is true for any antenna if its only being used for receiving. If you don't have long cable lengths then the non active will be the same. If you don't have a long cable you will experience no improvement using active over passive. So given your length passive is the way to go.

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  • \$\begingroup\$ tman, isn't it better to have 2 SAW filters, one in the active antenna and one in the positioning module? what sort of an effect would that make in the total performance? Wouldn't it improve the signal quality? \$\endgroup\$ – Denis Sep 28 '14 at 10:12
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    \$\begingroup\$ I would presume very minimal. Note that active antennas does not imply a saw filter. This is a extra feature. If one of the saw filters was considerably better then the other then a performance improvement could be achieved. I'd put my money on the positioning module as opposed to the antenna having the better saw filter. \$\endgroup\$ – tman Sep 28 '14 at 12:51
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In addition to mitigating cable loss, most active antennas have narrowband SAW filters to reject interference.

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  • \$\begingroup\$ Does every positioning device have a RF front end which has a saw filter? Provided the device has one, then active antenna is not useful is it? \$\endgroup\$ – Denis Sep 24 '14 at 1:21
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    \$\begingroup\$ Not every GPS front end or module includes a SAW, some SAWs are better than others, and sometimes two SAWs are better than one. YMMV! \$\endgroup\$ – pericynthion Sep 24 '14 at 1:25
  • \$\begingroup\$ since manufacturers nor does any antenna manufacturers mention specific properties about the SAW filter, before buying, how do we know which one to is good to go? Its impossible to read a spec sheet and decide which device to purchase. Am I right? The only way is to test test and test. \$\endgroup\$ – Denis Sep 24 '14 at 1:37

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