Open.Theremin Antenna Connectors
Removable Antenna Connectors
Finally I found a nice way to connect the antennas to the circuit board.
I simply took a banana connector as shown on the picture above. By removing the plastic part you get a connector that can be easily screwed (or soldered) to the pcb. The antenna inner diameter must be about 4mm. It is really nice and easy to put an remove the antennas. 
For the pitch antenna I cut a (M3) screw thread in the whole at the end of the connector and fixed it to the circuit board by a small screw.
For the volume antenna I simply shortened the banana connector a little at the end and screwed it to the circuit board with an traversal screw. 
This is how it looks with the antennas connected.
