I am designing a PCB in which there are 3 SIM800 GSM modules sharing a (commercial) external antenna. The external antenna is connected to the PCB via an antenna connector, and from that connector, the antenna signal is “shared” via PCB traces running to the GSM_ANT port of the GSM modules (please see photo)
The reason I prefer this design is that it is both cheap and convenient.
But without in-depth knowledge on antenna design and EMI, my question is Could this be a bad design? Or should there be scenarios that this design does not work reliably?
Just to provide more info, the PCB will be housed inside a steel enclosure, and its main PSU is an AC-DC adapter (good brand) which is already sealed for EMI. The adapter is also inside the steel enclosure. The steel enclosure will be deployed in a residential area where there are no large machineries around.
Following some of your answers, I update the question as follows:
Below is the old design, which uses 3 separate antenna connectors for the 3 GSM modules. However, these 3 antenna cables are combined (soldered) into one (with seal) outside the PCB and go to the external antenna.
The 3 GSM modules are supposed to be sending signal one at a time.
This old design has been tested and worked (though I suspect that signal strength are just around 8/10 of what it should be – good enough for the application though)
I think a more reasonable way would be to use an external three way splitter and the old design to connect your three GSM modules to one antenna.
You don't seem to have the knowledge to do RF PCB designs (not knocking you, I don't either) so it would be safer for you to use an external device that "Just Works."
The old design may have been more advanced than you think - proper lengths of coax joined correctly can form an RF splitter. See "Wilkinson power divider" on the Wikipedia power divider page. For the uninformed eye, this would look like somebody just lashed a bunch of coax cables in parallel and hoped for the best - when in reality the lengths and connections were carefully selected to make a proper splitter that isolates the three sources from each other.
You can also make a Wilkinson divider on your circuit board, though that may take too much space.
Microstrip Wilkinson divider: