Soil moisture probe

Gabel Corporation (Victoria, B.C., Canada), design and manufacture soil moisture probes which operate by measuring the time taken for pulses to travel along a fixed length of the probe. An increase in the level of moisture around the probe slows down the pulses. The TLM method has been used to directly model the operation of various probe configurations in the time-domain, for soils of different properties.

The following plots show the magnitude of the electric field around the probe for four different soils. A 30cm length of probe with an open-circuit termination is considered. The probe is placed 10cm from the bottom of the plot (the feed point) and 15cm from the top. Symmetry has been exploited so that only one half of the problem is modelled and the centre of the probe is placed along the left edge of the plot. The scale is logarithmic with a range of 3 decades (60dB) and the grid spacing is 5cm. The plots show a snapshot of the field 2.5ns after the start of the simulation. Click on one of the plots to receive an MPEG animation (the largest of these is about 430Kbytes). The movies contain 120 frames with an elapsed time of 80.2ps between each frame.

circular probe 10% moisture content no loss	circular probe 10% moisture content 0.05 S/m conductivity	circular probe 20% moisture content no loss	circular probe 20% moisture content 0.05 S/m conductivity

The total voltage in the 50 ohm, co-axial feed cable is shown below:

J L Herring, 25 September 1996.