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I am using a Positioning module that has a SMA port to hot plug an SMA antenna. It also has pins available through its footprint to connect a GPS Antenna.

The module name is SIM908. I was not able to spot a place in its data sheet that describes if both antennas can be connected at once.

I want to solder a passive chip antenna about 5-10cm away from the GPS module to be used in case should the external SMA antenna is broken or disconnected. At least with a longer fix time, I would still have some fix, without having no fix at all.

Would having two antennas connected, one through PCB trace and other through direct SMA cable be a problem for the performance of the device?

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"Would having two antennas connected, one through PCB trace and other through direct SMA cable be a problem for the performance of the device?"

Absolutely. RF is a fickle creature and having stubs on a PCB is a good way of having neither antenna work.

However, all is not lost. The Hittite/ADI HMC849 is designed to switch between antennas and is not too expensive. If you need it to automatically switch, try detecting any DC current to the external (active) antenna. More than about 2mA and you can safely say an external antenna is plugged in.

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  • \$\begingroup\$ the second antenna is not going to be on the PCB. There will be only one GPS antenna on the PCB. The other one is external. Connected via the SMA cable. \$\endgroup\$ – Dina Dec 2 '14 at 11:03
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    \$\begingroup\$ Yes, but the connector will be on a stub, which is bad for RF. \$\endgroup\$ – Damien Dec 2 '14 at 11:04
  • \$\begingroup\$ I understand. I didn't know this method before. Many thanks for your time. \$\endgroup\$ – Dina Dec 2 '14 at 11:06
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Yes, you can connect both. Look at the block diagram. One port is for a passive GPS antenna, the other port is for an active antenna that outputs a serial signal. Then there is a third port that is for RF (GSM).

I would suggest you obtain the "SIM908-C AT Command Manual" listed in the back (but without a link) to figure out how, among other things, to select which GPS data to use.

enter image description here

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