I think I found the reason, it seems that my used antenna only has the L1 band or the L2 is broken. My used antenna is not labeled.
To give you an example:
In the RTCM 1074 of my BS RTK “Skytraq”, there are only messages:
MT1074 ToW 128366.0000 Signal L1_C/A
In RTCM 1074 commercial BS RTK “TRIMBLE BD970”, the messages:
MT1074 ToW 128585.0000 Signal L1_C/A
MT1074 ToW 128585.0000 Signal L2_Z-Track
MT1074 ToW 128585.0000 Signal L2C(M+L)
MT1074 ToW 128585.0000 Signal L5_I+Q
I need to check my guess