Communications, Radar & Electronic Warfare (201.. - Index of

........VHF to SHF Radio Prediction 171There can be as many as three different dominant propagation regimes for such longlinks. These are:• line of sight propagation mechanisms (free space loss for highly directionalantennas, two-ray for most links);• diffraction;• tropospheric scatter.There will also be variability due to atmospheric variations such as:• variations to the k-factor;• ducting and other anomalous conditions;• variations in troposcatter response.To account for these, propagation models designed for long range prediction at VHFand UHF will express the expected loss for specific percentages of time, with 5 %, 50%and 95% being the most often quoted. Models like ITU-R P.528 (aeronauticalpropagation) are based on long-term measurements that by their nature include allof the atmospheric variations likely to be accounted for these percentages of time.10.1.4 Variability at VHF and UHFFigure 10.12 shows a graph derived from ITU -R P.1406, which shows the standarddeviation for variability for VHF and UHF links over land and sea paths. The valuesStandard deviation of time variability against distanceco1 1 5 1-- ________"D 10 ..._-i I5US 0 ./... I ,---_-_-_·. ___ .... -::_:; :;;; L __ _.",.. ... .. " '",','"., ". '-y" "'.- .-, ... "-,." .. .-,'.,-' .. ,..,.... .. - ..0 100 1 5 0 200Distance (km).5.,-·'·-------··--·--· -...,l:o.., ···-·-····1: . ==Vf-lF' land & sea)'i: - . . UHF landI: ,;; __ UHF sea i···'·'''· ·' .. ·'1---·--- -_ ....iFigure 10.12Standard deviation for links as VHF and UHF over land and sea paths, derived fromITU-R P.1406. The variation increases with distance, which is an intuitive result.