PCB trace GSM antenna be affected by EMI
There is a PCB design with 3 SIM800 GSM modules sharing a
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.
As I do not know more about antenna design and EMI, I have a
question that is Could this be a bad design? Or should there
be scenarios that this design does not work reliably?
If you want to use three SIM800 modules at the same time with one antenna, it will not work.Firstly, due to impedance split. Secondly, the modules output will affect each other which will disrupt the communication.
This is an interesting way to combine the signals. Certainly it is not good, but really I think you're asking if its any worse than your old bodged solution of paralelling the coax.
A proper impedance matched splitter is a better solution but reflections from the splitter and antenna may still cause problems as impedance matches are rarely exact.
A good way would be to use an external three way splitter and the old design to connect your three GSM modules to one antenna. It would be safer for you to use an external device that "Just Works." as you do not have the knowledge of RF PCb designs.