A simple approximate mathematical model using Fresnel-Kirchhoff diffraction theory is proposed to investigate the frequency dependence of propagation attenuation by natural obstacles with lateral profiles having general shapes. Measurements carried out in several links in the frequency band from 270 to 330 MHz have shown variations of ± 10 dB. Results obtained from this model agree reasonably with experimental data. The design reliability of UHF communication systems is therefore improved by its use.
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