Water relative dielectric constant is there around at 50GHz and 25*C.
Conductivity is also an important antenna factor.
If knowing frequency and temperature and what body parts that will be involved, dielectric and conductive proprieties can be found at this site: **broken link removed**
Other sources may give minor variations of these values, and some sources have separate tables for man, child and woman.
Making a solution with similar properties as body tissue is commonly done by adding salt(NaCl) in a sugar and water solution.
Some common mixtures can be found here, p38: **broken link removed**
FCC's dielectric values are given for whole body-parts, not a certain tissue.
Mix boiling water and sugar and let cool down to room temperature.
Check dielectric value. I use a VNA for that.
Adjust by adding water or another water/sugar mixture. Do never add pure sugar, it will take forever to melt and become stable.
Finally add small amount of almost saturated salt-water for correct conductivity. Do not add pure salt directly.
If need to adjust, it requires a lot of slow stirring. To fast stirring and air-bubbles are added, which gives a unstable solution.
Add something that stops fermentation, which else changes electrical properties.
Observe that electrical properties changes with temperature also for final solution.