Radio Propagation Theory — Page 37, Lesson 39

Right, let's pick this up with the two abnormal propagation conditions that affect VHF and above. We've covered normal refraction, so now we're looking at what happens when the atmosphere misbehaves. First, super-refraction. This is a phenomenon significant at frequencies above 30 MHz — that's VHF and above. In super-refraction, radio waves experience greater refraction than normal. That means they're bent downwards towards the earth's surface more than usual. The practical effect is a notable increase in line-of-sight range — up to 40% above the usual. Now, what causes it? Four conditions. First, a decrease in relative humidity with height. Second, temperature falling more slowly with height than standard — so a less steep lapse rate. Third, fine weather and high pressure systems. And fourth, warm air flowing over a cooler surface. Here's the key mechanism. In extreme cases, you get a low level temperature inversion combined with a marked decrease in humidity with increasing height. Simply put: warm dry air above cool moist air. When that happens, a low level duct may form. This duct traps radio waves at frequencies above 30 MHz, giving extremely long ranges. That's duct propagation — and it can lead to exceptionally long ranges. A classic real-world symptom: when UK television channels suffer interference from continental stations, the cause is exactly this — a duct has formed and is carrying those signals far beyond their normal range. Where does this happen most? Where warm desert areas border oceanic areas — the Mediterranean and Caribbean seas are prime examples. It can also occur in temperate latitudes when high pressure predominates, particularly in winter months. Why winter? Because the dry descending air in the high pressure system is heated by the adiabatic process — compression heating as it sinks — and ends up warmer than the underlying cool, moist air. That sets up the inversion and the duct. Now the opposite case: sub-refraction. This is much rarer than super-refraction, but still significant. Sub-refraction causes a reduction in the normal refraction, which gives a decrease in line-of-sight range by up to 20%. Its causes are the mirror image. First, an increase in relative humidity with increasing height. Second, temperature decreasing with increasing height at a greater rate than standard — so a steeper lapse rate than normal. Third, poor weather with low pressure systems. And fourth, cold air flowing over a warm surface. So hold the two side by side. Super-refraction: humidity decreasing with height, temperature falling slowly, fine weather, high pressure, warm air over cool surface — range increases up to 40%. Sub-refraction: humidity increasing with height, temperature falling fast, poor weather, low pressure, cold air over warm surface — range decreases up to 20%. One bends the wave down more, the other bends it down less, and that directly stretches or shrinks your usable line-of-sight range at VHF and above.