Path Loss and Antenna Gain Considerations for Unattended Ground Sensor (UGS) Systems

摘要

Unatttended Ground Sensor (UGS) systems are adversely affected by the physics of RF propagation at low elevations. Units are often located at or below ground level in an effort to reduce the visual signature. Two key elements of the link budget work are compromised by the ground level antenna height: path loss and antenna gain. A two-ray reflected path model predicts that path loss increases by R4 with distance separating units and is inversely related to the height of the antennas. Strict adherence to this model indicates that infinite path loss is incurred by antennas located at the ground (height = 0). Ground wave propagation allows for a minimum effective antenna height to be assigned thereby eliminating this theoretical anomaly. Regardless, ground reflections for low elevation RF propagation result in up to 40 dB or more of extra path losses when compared to the free space model. Antenna modeling programs used to predict antenna patterns also paint a grim picture for UGS units. Monopole antennas have a null at the horizon and can be predicted to have antenna gains less than -20 dBi at low takeoff angles. Combining path loss and antenna gain to obtain an overall picture of the decrease in signal level is not straightforward. Simply combining the two sources of signal loss results in predicted performance that is much less than what is measured with real hardware. This paper examines the interaction of path loss and antenna gain and presents a reasonable approach for accounting for both in the link budget. Experimental results are presented to support the models.