I see that there are "pinless" moisture meters on the market, e.g., http://www.omega.com/pptst/HHMM257.html that look like this
My understanding is that they measure the amount of reflected RF. Perhaps, it's because liquid absorbs RF. However, different materials absorb RF differently - how could this type of measurement be reliable?
Also, why do they use a round RF transceiver rather than a dish for receiving?