How to explain such coil coupled signal increases when tested in water?

Hi everyone:

I have two coils designed in order to transmit squared pulses. They are standing face to face with distance of 10mm. The attenuation of received signal is supposed and tested to be -20dB in air, when I place a bag of salt water in between, the attenuation became to be about -12dB, that is, the coupled signal "increases" when in the water.

In my mind, the salt water will absorb such signal resulting in more attenuation; but it is not in practical test. I don't know how to explain this. The frequency band of such received signal lies in 100-300MHz.

This is the test setup:

This is the result:
airwater.pdf

Could anyone help me to explain this? Is there anything that I've ignored in the test and made mistakes? Thanks in advance!

Increased capacitive coupling due to the high permittivity of water?

Are you using balanced loops (so are you using baluns). It can be because of increased capacitive coupling where the water acts as a conductor reducing the coupling capacitance between the coils.

Do I see non-disolved salt in the bag (in other words is the solution saturated)?

Maybe there is a resonance?
I would try to measure the coupling vs. frequency, with and without water, in order to understand what happens.
Regards

Z

The geometry is unclear. You are talking about coils, but the photo shows a ground plane that shorts the magnetic field. So we would rather describe the setup as planar antenna and assume the dominant coupling mechanism as capacitive. In this case, a dielectric media increases the coupling. The result won't be probably much different for pure water, which also has an Er of 78.