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: 
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.
This is a horiblle bodge (whether implemented in coax or on the PCB).

Firstly the direct interconnections will create horiblle impedance mismatches which will reduce signal integrity. Some high power RF transmitters can even be damaged by signal reflections.

Secondly whenever any module transmits a substational fraction of the transmit power will feed back into the other modules. This may blind the modules to incoming signals or in extreme cases may even damage them.

A proper impedance matched splitter is a better soloution but reflections from the splitter and antenna may still cause problems as impedance matches are rarely exact.