S-parameter measurement

Hi,

I am trying to electrically model printed transmission lines. First I measure the s-paramters of the line using VNA and then I try to extract the equivalent circit parameters from the measured s-parameters.

I am getting good results for frequency range greater than 300MHz. But for frequecy less than 300MHz the measured s-parameters are noisy and fluctuating. I can't explain why it is like that. Whether it is from the VNA or from some resonance in the DUT I don't understand.

Can anyone give me suggestions about what might be the problem?

thanks

Could it be over the air interference (nearby TV or FM station)?