FREQUENCY-DOMAIN PI-PHASE-SHIFT REFLECTROMETRY FOR SOIL-MOISTURE MEASUREMENT .1. THEORY

The theoretical basis for a frequency domain pi-shift reflectrometer f or measuring soil moisture content has been presented. This method use s an interferometry approach. It is shown that this narrow-band techni que promises greater reliability and flexibility than the existing tec hnique of time domain reflectrometry (TDR). In particular, it measures both dielectric constant and conductivity, which enables salt interfe rence to be eliminated. A mathematical model has been developed to sep arate the effect of conductivity from that of the dielectric constant. Measurements are required at two distinct frequencies to minimize int erference and enable the dielectric constant and conductivity to be ca lculated numerically.