But for the people that already have these, i have changed the antenna to a external one to make the reach/distance a little bit better.
I know it is against the rules in some country's but i still wanted to share.
If you want to open up the sensor, beneat the sticker on the bottom or backside of the sensor is a small screw.
Dont forget to unscrew and if all goes ok you will see the following pcb.
as you can see on the picture on the pcb trace i used something sharp like a screw driver or something (watch out you don't hit other components).
The antenna length for this frequency would be 16.4 Cm long, i used this website to calculate it. (http://www.csgnetwork.com/antennaevcalc.html)
upper right corner
There is improvement in the signal, i forgot to check the strength in the devices list but when i was in the garage, about 15 meters away from the RFXcom that has to look trough 3/4 walls i managed to get a reading when looking on te log file by teamviewer.
At some places in the garage there is to much interference i think but when i went to the windows/doors were they would be mounted i got everytime a reading if not opening and closing to fast (rapid every second)
Then i walked up front , thats less difficult actually from where the RFX is located, just 1 wall/windows to go trough but the distance is about 25/30 meters.
i did the same for my mailbox sensor.. already had it open to extend the reed switch with some cable to fit in the mailbox the signal at the frontdoor was "poor" there so made the antenna 16.3cm as you already have 0.1cm from the last filter component on the PCB 