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: 
I think a more reasonable way would be to use an [external three way splitter](http://www.instockwireless.com/power_divider_3way.htm) 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."

Three-way splitter:

[![enter image description here](https://i.stack.imgur.com/q8uEZ.jpg)](https://i.stack.imgur.com/q8uEZ.jpg)

---

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.](https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers) 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.

Wilkinson divider:

[![enter image description here](https://i.stack.imgur.com/4BSbd.png)](https://i.stack.imgur.com/4BSbd.png)

You can also make a [Wilkinson divider on your circuit board,](https://en.wikipedia.org/wiki/Wilkinson_power_divider) though that may take too much space.

Microstrip Wilkinson divider:

[![enter image description here](https://i.stack.imgur.com/1ff0I.png)](https://i.stack.imgur.com/1ff0I.png)
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.