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: 
A has been mentioned before, 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.

With the coax, you have a more symetrical circuit (each RF path looks similar, and has the same parasitic connections). You also have a longer transmission line providing a hint of isolation from the joining point (maybe only 0.1-0.2 dB, but it goes both ways, so is not nothing). I feel this design is inferior, but can't quantify it. You will for example be pushing more transmit power directly into the other two modules (but they need isolation for their own Tx, so this should be OK).

I would be tempted to try and lay this out as more of a star, and experiment with series resistors to improve the matching (you have certain losses here, but also better matching, so overall it will probably be better). Mis-matching the antenna pot on the modules might affect the tuning of filters, etc. About 25\$\Omega \$ should be right, I think.

Clearly this is the cheap and ugly option, a proper low loss spliter would be nicer.
How do you intend to use it? Do you want to use three SIM800 modules at the same time with one antenna? This will not work. First of all, due to impedance split. Secondly, the modules output will affect each other which will disrupt the communication. Just add two additional connections for the antennas. Remember that the antenna trace should be as short as possible. 

You are aware of the fact that with an antenna on a PCB you need to certify your device?

One tip: Add a via shielding around the antenna trace. Google -> via shielding.