What is the physical quantity you actually want to measure?

relative permittivity and quality factor of dielectric ceramics

Dear Jobin,

The techniques do not differ much from those at microwaves: coaxial probe (for liquids), transmission line/waveguides, free-space=quasi-optics, resonant cavity in waveguide or free-space form and parallel plate waveguide. In general, quasioptics is the preferred choice at millimetre-waves. And if you need to measure something with very very low absorption, you may need to use a quasi-optical cavity (Fabry-Perot cavity).

Dear Prof. Miguel,

   Thank you very much for your information.

Please look to my book "Microwave Dielectric Spectroscppy of Ferroelectrics and Related Materials" (Gordom and Breach Inc. Publ.)  It is described methods of ferroelectrics invastigation in the millimeter region and above.

Dear Prof. Grigas,

       Thank you very much for your timely replay, We are following your books.