ducting

In physics and communications, "ducting" refers to a phenomenon where radio or sound waves are confined to a restricted path, often due to atmospheric conditions. This occurs when a layer in the atmosphere, under abnormal circumstances, traps waves, preventing them from dispersing normally.1

This atmospheric ducting can significantly impact radio wave propagation, as described in the context of line-of-sight microwave links. Temporary atmospheric disturbances can alter the refractive index, causing radio beams to bend in unusual ways. In severe cases, this bending can lead to a phenomenon called ducting, where the beam is trapped and guided along a specific path, potentially causing signal fading or enhancing transmission ranges beyond the geometric horizon. This is analogous to how changes in air duct area can filter sound by reflecting undesirable frequencies.23