Positive S11 S21 in free space meas. with VNA Agilient,

Need Help,
I have some planar samples and designed a free space measurement, I calibrated the rectangular horn antennas THRU-REFLECT than put the sample in the middle of system. The result gives S11 and S21, at some frequencies S11 and/or S21 have/has positive values. Is it correct ? does it have any physical meaning?
Thanks for your valuable helps...

same problem with coaxial waveguide measurement?

so you used some horn antenna setup with perhaps 10 db of insertion loss, and told the network analyzer to use it as a 0 dB loss thru section. Then you put some sample in the way and found a slightly positive S21. And, your question is what?

thanks for your reply;
what should I do to obtain the exact S11 and S21 than I try to calculate the physciala parameters of samples such as permitivity permeability.