1TUNE THE "RUBBER" ANTENNA AT 433 MHz.
The original antenna is tuned around 470 MHz and therefore it ha a return loss of around -3dB at 433MHz. To tune it, you have 2 options. Without or With a Vector Network Analyzer.
2Carefully remove the rubber cover
Hold the SMA connector with a clamp and gently move the rubber cover until it gets away.
3Stretch the metallic (mine is made of copper) spring to obtain a total length of 17 cm, including the SMA connector
4Refine the tuning to obtain the minimum return loss at 433 MHz.
Stretch a little (to decrease the frequency o cut the wire no more than 3 mm each time, to obtain the desired return loss (yellow line).