In the room where the sender is, there should be nothing interfering with the transmission. In the receiver room, I have a 433 MHz transmitter for a weatherstation near by, but it is only transmitting once every half minute, so this shouldn't be causing any big problems.
I have already tried 34 cm at the transmitter end, but it didn't make much difference. I might try again with 34 cm at both ends.
Are there any other tricks I can do with the antenna to improve the gain?
The antenna at the receiver side needs to be omni directionally, since I am also using it for receiving data from my weather station.