HF GROUND CONSTANT MEASUREMENTS AT THE LAWRENCE LIVERMORE NATIONAL LABORATORY (LLNL) FIELD SITE

Abstract

The SRI International open-wire-line (OWL) kit was used 3AE5 July 1987 to measure the HF ground constants at the Lawrence Livermore National Laboratory (LLNL) field site in Livermore, CA. Data were acquired at 11 locations about 250 ft west of the LLNL facility fence in the vicinity where a longwire and broadband dipole were erected in August 1987 for making impedance measurements for the purpose of validating the Numerical Electromagnetics Code (NEC). An additional location was measured to the north of the antenna site where field strength data were to be taken. Several samples were taken at most locations. Best estimates of the conductivity, relative permittivity (relative dielectric constant), dissipation factor and skin depth were computed as the median values versus frequency for 2 through 30 MHz. Data were acquired at l-MHz intervals from 2 MHz through 8 MHz, and the interval was increased to 2 MHz from 8 MHz to 30 MHz. The maximum and minimum values were also determined as bounds on the conductivity and relative permittivity values for use in parameter sensitivity analyses. The conductivity values for the relatively dry, densely packed light brown clay fell between those typical of pastoral land and rich agricultural land at about 4xlO-2S/m. The relative permittivity values exhibited more variation with frequency. At the low end of the HF band, the relative permittivity values exceeded those of a non-flooded rice paddy (e.g., about 150 at 2 MHz); whereas, at the high end of the band, the relative permittivity approximated values typical of rich agricultural land (about 17 at 30 MHz). The skin depth varied from about 2 m at 2 MHz to 0.7 m at 30 MHz. The dissipation factor was about 1.5, so the soil acted almost as a semiconductor rather than as a lossy conductor or a lossy dielectric. Both the relative dielectric constant and conductivity are important in modeling antennas and propagation over the ground at the LLNL site. Data from nearby wells indicated that the water table was at least 20 m below the surface. Therefore, a one layer slab model adequately described the ground at this site for HF down to the skin depth. [Vol. 3, No. 2, pp. 131-165 (1988)]