Using transmission line effects to measure soil moisture - will this idea work?

[miceuz]

I'm working on this idea - oscillator drives a transmission line which is burried in a soil and XOR gate measures the phase shift between the driving signal and reflection that comes back.

I've build a prototype which uses xor gates as inverters to make an oscillator - it kinda works, but I want to go higher frequency and I want a more stable frequency.

So, I want to use a ready made oscillator (something like http://www.ctscorp.com/components/Datasheets/008-0256-0_F.pdf), but I'm not sure if it will be able to drive a transmission line. Could anybody comment on this idea?

[jerry507]

Done in most quality sensors:

http://www.decagon.com/products/sensors/soil-moisture-sensors/ec-5-soil-moisture-small-area-of-influence/

[Daving]

If you're not worried about spacial variation of moisture, you could just measure the capacitance.

It's difficult to give advice without more specifics, but if we assume that you are using 1mm wire and the wires are spaced 1cm apart, then you can expect to see your capacitance range between 30pF to 250pF / meter, and the wavelength of your signal is going to vary from (at 50Mhz) 1-3 meters.  These are just rough numbers.  You can reduce the capacitance by spreading the wires apart, and that will also increase the characteristic impedance, so make sure that you're series resistor matches to minimize unwanted reflections.  Could work, just make sure to balance everything out.

You might look into TDR, if you want to know how the soil changes with depth or distance, for example.