Re: How can I calculate the relative permittivity of a salt solution?

It seems the effective permittivity of inhomogeneous mixture and of
homogeneous solution are quite different problems.

Two references for the former problem are:

Effective permittivity of dielectric mixtures 
Sihvola, A.H.; Kong, J.A. 
Geoscience and Remote Sensing, IEEE Transactions on 
Volume: 26 4 , July 1988 , Page(s): 420 -429 

Macroscopic permittivity of dielectric mixtures with application
to microwave attenuation of rain and hail 
Sihvola, A. 
Microwaves, Antennas and Propagation [see also IEE 
Proceedings-Microwaves, Antennas and Propagation], IEE Proceedings H 
Volume: 136 1 , Feb. 1989 , Page(s): 24 -28 

Two references for the latter problem are:

Noertemann, K. Hilland, J. Kaatze, U. Dielectric properties of aqueous NaCl 
solutions at microwave frequencies Journal of Physical Chemistry. v 101 n
37 Sep 11 1997. p 6864-6869

High-frequency, complex dielectric permittivity of saline solution
at elevated temperatures 
Han, K.-F.; Butler, C.M.; Shen, L.-C.; He, H.Y.; Harris, M.A. 
Geoscience and Remote Sensing, IEEE Transactions on 
Volume: 29 1 , Jan. 1991 , Page(s): 48 -56 

The last paper provided a useful bibliography for the problem.