LimeSDR TX output harmonics

I need only the LO frequency with maximum output power for a project. So if i stream ones with the SopaySDR writeStream() to the LMS I get big output harmonics when the LO frequency is set to below 100MHz. Does this imply that the default TX1_1 matching network is not matching the 40Ohm differential? Or is there another known issue? The harmonics don´t change relatively to the fundamental if the TX gain is changed.

Thank you in advance.
Best regards,

I made some measurements some time ago:
Here they are:S11 Measurement Results
In Short: There isn’t any matching below 2 GHz.