Tue July 24, 2012
TheEC: Tropospheric Ducting
Welcome to the Engineer's Corner and since it's high summer I thought we'd talk a bit about TROPOSPHERIC DUCTING, an interesting and unpredictable phenomena that happens when it gets hot out, say, the mid 80's and hotter. When an FM or TV signal goes out into the upper atmosphere, if it encounters air that is warmer, instead of cooler, the higher "refractive index" can cause the signal to be bent. If conditions are right, it can be repeatedly "bent" into a "tunnel" or "duct" and carried for hundreds, if not thousands, of miles.
I personally have experienced WHBM from Park Falls, Wisconsin, completely overriding WZBC in Chestnut Hill/Boston, while listening only a few miles from WZBC's transmitter. The Farm Report faded in over a minute or so, stayed strong for 45 minutes, then faded away. Some radio enthusiasts would consider that a great "DX" (Distant Reception).
Here in Rhode Island, the heat and humidity interactions caused by Narragansett Bay can cause more stable ducting conditions, and that can impact our 102.7 signal noticeably for folks along the Bay. During warm days 102.7 can often be heard more strongly further north. At night, when it cools off, the effect tends to fade.
Since ducting is an atmospheric effect, it tends to be fairly unstable and unpredictable. Conditions like temperature, solar activity, proximity to the ocean, and wind can all create/disrupt a "duct" on a minute by minute basis. But there are some factors that allow for general prediction of the likelihood of ducting and where it can occur. William Hepburn's www.dxinfocentre.com/tropo.html has daily "forecast maps" for the eastern United States, although these don't really show enough detail regarding the Bay itself. But it's interesting to watch the forecast shift with other weather conditions!