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[USER=1087]@vfone[/USER] is thinking of regular antenna operation (i.e. efficient radiation), but you want to use it as a sensor. That's perfectly fine.

The sqrt(eps_r) scaling is valid if you have an infinite hemisphere sphere filled with that material, not just a glas of water in the near field. At the moment, your antenna will "see" a layer of eps_r followed by a large air volume.

You will need to calibrate your setup with known water content. Conductivity of the wet soil will also have an effect, it changes attenuation and the width of your resonance.