Radio Propagation Theory — Page 37, Lesson 39

I want to walk you through two phenomena that bend radio waves in the atmosphere — super-refraction and sub-refraction. These matter because they change how far your VHF and higher-frequency signals actually travel, and that directly affects your line of sight range. Let's start with super-refraction. This is a phenomenon significant at frequencies above 30 MHz — that's VHF and above. Under normal conditions, radio waves bend slightly toward the earth. In super-refraction, they experience greater refraction — they're bent downwards toward the earth's surface more than normal. The result is a notable increase in line of sight range, up to 40% above the usual. Now, what conditions cause this? Four things. First, a decrease in relative humidity with height. Second, temperature falling more slowly with height than standard. Third, fine weather and high pressure systems. And fourth, warm air flowing over a cooler surface. In extreme cases, when you have a low level temperature inversion with a marked decrease in humidity with increasing height — simply put, warm dry air above cool moist air — a low level duct may form. This duct traps radio waves at frequencies above 30 MHz, giving extremely long ranges. That's called duct propagation. It can lead to exceptionally long ranges. When you hear about interference on UK television channels from continental stations, that's the forming of such a duct. Where is this most common? Warm desert areas bordering oceanic areas — for example, the Mediterranean and Caribbean seas. It can also occur in temperate latitudes when high pressure predominates, particularly in winter months. Why? Because the dry descending air in the high pressure system is heated by the adiabatic process, and it becomes warmer than the underlying cool and moist air. Now let's look at sub-refraction. This is much rarer than super-refraction, but still significant in radio propagation. Sub-refraction causes a reduction in the normal refraction, giving a decrease in line of sight range by up to 20%. The conditions that give rise to sub-refraction are the opposite of super-refraction. First, an increase in relative humidity with increasing height. Second, temperature decreasing with increasing height at a greater rate than standard. Third, poor weather with low pressure systems. And fourth, cold air flowing over a warm surface. So the key contrast here: super-refraction bends waves down more, extending range up to 40%; sub-refraction reduces the normal bending, cutting range by up to 20%. One extends your reach, the other shortens it. And the atmospheric conditions that cause each are essentially mirror images of each other.