Can anyone tell me how to find permittivity of a dielectric sample using a rectangular waveguide
See if this works:

A New Method to Determine the Complex Permittivity and Complex Permeability of Dielectric Materials at X-Band Frequencies, Hassan Elmajid, Jaouad Terhzaz,Hassan Ammor, Mohamed Chaïbi, Angel Mediavilla, INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.10, NO.1, JANUARY 2015