Detecting ground conductivity anomalies from the pattern characteristics of VHF antennas

A way to measure the spatial ground conductivity at VHF frequencies without tedious preparation is suggested. The proposed test system includes a 6-8 element ground plane Yagi antenna, a tunable oscillator as a transmitter and a monopole with a diode detector as a receiver. The human interface can be just a hand-held multimeter having a 0.1 mV resolution. A directional bridge and two additional detectors to monitor the impedance match of the transmitter are optional. The operating principle is based on the arrangement where an artificial ground plane, made from three half-wave copper ropes is provided for the radiating element but the Yagi directors have nothing else except the real ground to be tested. Calibration can be done e.g. by temporarily mounting a chicken hatch below them, extending about one or two wavelengths to each side of the center line. With a transmitter power of 10 mW we have observed on a clay-like soil changes of gain from -2 dBi to 12 dBi and beam tilt from 10 to 30 degrees-these being related to the effectivity of the directors and thus to the radio frequency conductivity. A seasonal decrease of the soil water level of 100 mm has also been detectable