Low-angle propagation measurements at HF over farmland

High fidelity electromagnetic modeling of the wave propagation field is essential to accurately assess the impact of wind-turbine modulated ground clutter on high frequency (HF) Over-The-Horizon (OTH) radar performance. To support the modeling effort, field measurements were conducted in 2014 near the AN/TPS-71 HF Relocatable Over-the-Horizon Radar (ROTHR) in Virginia to investigate wave propagation near the ground. Extensive propagation data were collected by transmitting from a helicopter-borne HF transmitter to a number of monopole antennas on the ground near the ROTHR receive site at three HF frequencies (7.5 MHz, 14.5 MHz, and 23.5 MHz) and various elevation angles from five to zero degrees (grazing incidence) as measured from the receive antenna. The measurement ranges varied from 5 to 40 km and the helicopter altitudes varied from ground level up to 6000 feet. The measurement results are well correlated with model-based predictions for a smooth-homogeneous-earth surface provided that the “effective” dielectric constant and conductivity of the terrain are reduced from their expected values as estimated from in situ soil samples and the National Land Cover Database.