Question: 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.

Thanks in advance!
Dave
[![enter image description here](https://i.stack.imgur.com/3yuLW.jpg)](https://i.stack.imgur.com/3yuLW.jpg)

**UPDATES:**

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)

[![enter image description here](https://i.stack.imgur.com/9rny1.jpg)](https://i.stack.imgur.com/9rny1.jpg)

Answer: 
Apart from EMI issues, I have a hard time believing that this will work at all. Is there a reference design available for something similar? From what I would guess (I'm not an antenna specialist at all):

* Multiple drivers for one antenna
* No matching network (at least none which could be used if required)
* Your antenna line doesn't have a solid reference but there are traces on the other side.
* If this is a two layer board, I don't think that your trace is 50Ohms impedance
* The two connectors in the middle look like Ethernet. So it's probably metal, directly above your signal line.
* If you remove two of the SIM800, you will have crazy stub lines all around, nowhere terminated.
At the risk of giving a product recommendation you may want to look at an RF power splitter/combiner IC from someone like mini-circuits. [Mini-circuits splitters](http://194.75.38.69/products/Splitters.shtml)

These will allow you to combine the 3 outputs (and split the returning signal) on the PCB without the impedance mismatches and other nasties caused by your current design.