How can I optimize multiple FM transmitter modules and their antennas
 

Dear forum,
I am using a number of small, low power FM transmitters from Adafruit to send various audio signals to various receivers in a room/hall.

Things are basically working OK, but I would like to improve the performance (noise level/distance).
At the moment the modules sit very close to each other and I've noticed that if their antennas are too close to each other, performance degrades quickly.
Another thing I noticed is that when I increase the TX power beyond a certain point, the quality gets worse and eventually the whole system stops working.
The "antennas" are currently just ordinary wire cut to quarter wavelength.

So I wonder if it would be advisable to place the modules further apart from each other?
Or can/should the modules be shielded from each other?
Which type of antenna would be preferrable over the simple wires?
Or would it maybe even be possible to mix their outputs and use a single antenna??! (I've heard about the Wilkinson combiner/divider but I have no clue if that might work with my setup and how the schematic would look like. Nor if it would actually improve things)

Sorry for these newbie questions - I'm really more of an audio/computer guy.
Thanks much in advance,
Sukandar



Link to module:
https://learn.adafruit.com/adafruit-...s-rdbs-support