Hello again !
I found a webshop which sales -90 dBm receiver modules ! That is about 6 dBm more than the usual receiver modules from airwave.
Here is the brandnew schematic :

Send me an Email if you are interessted into the layout and the C programm.
The programm switchs between the receiver moduls and automatically chooses the highest RSSI signal. I use it for a 6 dBi Omni antenna and a additional 8 dBi Patch antenna. Works well.