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Draft:TDR and TDT soil moisture sensor

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TDR and TDT soil moisture sensors measure the dielectric constant of a certain volume element around the sensor by measuring the speed of propagation along a short transmission line. From that the soil moisture can be derived.

In TDR (Time-domain reflectometry) method a short open ended transmission line formed by three rods are placed in the soil. A pulse is sent on the transmission line and received back. The bulk end conductivity (BEC) distorts the signal and processing is required to derive the correct transmission delay. The open-ended rods make it easy to probe soil.

TDT (Time-domain transmission) method uses a transmission line that loops back where on one end sits the sender and on the other the receiver. Due to better signal quality their design can be simpler and more tolerant to the porosity of the soil. They are more suitable when being buried permanently.

See also