Received signal error when loop connections from Tx to Rx

I would like to use LimeSDR-USB to design a VNA to calculate S parameters.

To do this, I wrote a C++ Script (based on SoapySDR API) with four threads to control the transmission and receiving functions. It can work correctly by connecting the Tx to a spectrograph analyser and Rx to a signal generator,the transmitted and received signal levels are similar to what have been set. But when I use a straight connection from the Tx to the Rx,the signal power I received (RSSI=sqrt(I^2+Q^2)) is much lower than the expected value. So I wonder when I do the above connection,do I need to modify any register in LMS7002 ?


System:Ubuntu 16.04