Measurements of single tree attenuation for vehicular satellite communications at X-band

The objective of this study is to obtain empirical experience with the expected signal degradation due to roadside trees for vehicular satellite communications at X-band. Broadband measurements of single tree attenuation were carried out at six locations at the campus of FFI, Kjeller, Norway. The excess vegetation attenuation was measured, utilising a network analyser, between 7.25 and 8 GHz during dry summer conditions. The species investigated were maple, spruce, lime, pine and birch. The elevation angle resembled typical elevation towards geostationary satellites. Reference measurements were performed besides the trees to calibrate the equipment, enabling extraction of the time, and frequency dependent attenuation variations. The signal component propagation through the trees was found to dominate the received power. The effect on attenuation from the main trunk obstructing the path was compared to measurement through branches only, and found to be significantly higher. The measured attenuation was compared with one existing model, showing a relatively large spread around predicted values. The attenuation (in dB) closely resembled a lognormal distribution. The observed attenuation values are expected to cause degraded communication quality during vegetation shadowing for vehicular satellite communication located close to trees.