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.
First of: all of the combiners from the other answers can only work on a single band. You can't use the same combiner for GSM900 as for GSM1900. That is physically impossible.

The fact that GSM devices can use the same antenna for transmit and receive at all is due to the fact that up- age downlink in GSM are on different frequencies, so that you can use a frequency-selective device, called a *diplexer* to avoid TX signal going into the receiver (and destroying it).

Now, the way a diplexer works is that you basically have at least one filter in the RX branch that blocks out the uplink frequencies and only lets through the downlink. Together with the same principle and a splitter, the TX branch is connected to the antenna. Since this is very cost-, space- and energy-efficient microwave technology, the two filters and the splitter are basically combined into one device.

The problem with that is that it's very hard to achieve perfect impedance matching on both RX and TX at the same time over all frequency bands that GSM exists on (800,900,1800,1900,...).

Since receiving successfully is more important than transmitting at perfect efficiency, typically a mismatch of the TX branch's impedance compared to the nominal antenna impedance is accepted. In the single-transmitter-per-antenna case, that's OK.

This means that quite a bit of the TX energy gets reflected back into the TX amplifier in the single antenna case.

Now, imagine you have two TXing senders A and B connected to a splitter. Not only will a bit of transmitter A's energy be reflected back into A at the diplexer, the remainder that actually goes into the splitter will inevitably at least partially be seen by B's diplexer, where it's going to be partially reflected back, and by the way all directive couplers work, destructively interfere with A's signal at the antenna port.

So, you'd need to build three diplexers, one for each GSM module, build an impedance matching network for the TXes, a three-way splitter (or, while you're at it and have singled out the RX signal, anyway, add an LNA) for RX, and then you'd have a truely better system.