Simultaneous measurements of L- and S-band tree shadowing for space-Earth communications

This paper presents results from simultaneous L- and S-band slant-path fade measurements through a pecan, a cottonwood, and a pine tree employing a tower-mounted transmitter and dual-frequency receiver. A single circularly polarized antenna was used at each end of the link. The objective was to provide information for personal communications satellite uplink power control design on the correlation of tree shadowing between frequencies near 1620 and 2500 MHz. Fade time series obtained simultaneously did not exhibit any significant correlation, but their means were weakly correlated (r² from 0.31 to 0.58). Fades were measured along a 10-m lateral distance with 5-cm spacing. Instantaneous fade differences between L- and S-band exhibited a normal distribution with means usually near 0 dB and standard deviations from 5.2 to 7.5 dB. The cottonwood tree was an exception with 5.4 dB higher average fading at S- than at L-band. More than 90% of the spatial variations occurred with periods larger than 1~2 wavelengths. Simultaneous swept measurements over 160-MHz spans showed that the standard deviation of the power levels as function of frequency increased from about 1 dB at locations with mean fades less than 4 dB to near 6 dB at locations with mean fades of 20 dB. For a 5-dB fade, the central 90% of fade slopes were within a band of 0.7 dB/MHz at L- and 1.9 dB/MHz at S-band