UNUSUAL FM RECEPTION, EXPLAINED…
No doubt you have read several postings to Airwaves Journal about some very unusually long-distance FM reception reports. As a Radio Engineer and LONG-TIME DX’er, I thought I would give you a little explanation on what is going on. During the warmer (especially the Summer) months, two types of long-distance FM reception can occur. The first is what we call “trops” or in other words… tropospheric ducting. This happens mainly during temperature inversions. During the day, the ground absorbs the heating rays of the sun. At night, radiational cooling occurs where that absorbed heat rises from the earth and goes straight up into the atmosphere to about 1-3 miles up to the higher areas of the troposphere. If the conditions are ripe, mainly during times of high humidity, the troposphere can act like a mirror, a sort of waveguide. Trops are more prevalent during the late night, early morning hours. Since the ducts are 3 to 4 miles above the earth, the line of sight can be roughly 100-500 miles around. If a stalled weather front occurs, that ducting can bring stations over 1000 miles away. I witnessed such an occurrence in 1993 when I got local reception of WHTK/99.7 in Port Royal, SC , over 800 miles away from my vantage point here in the Boston area. UHF- TV also can get into the act during trops. A viewer in Tulsa, Oklahoma got a perfect picture of Boston’s WSBK-TV, Channel 38 in 1979 and got it verified!
Now, the fun part. The other type of long-distance reception is called “E-skip” or Sporadic-E. This happens during the months of late-May through late-July. But it can happen at any time of the year but on a very limited scale. This involves the “E” layer of the Ionosphere, roughly 50-100 miles above the earth. E-Skip mainly occurs during times of high thunderstorm activity. Without getting technical, if you are getting a station from South Florida (and you live in Massachusetts, for example).chances are there is a band of severe weather somewhere between those two locations. The highly powerful electric activity of these thunderstorms has been linked to the occurrence of E-skip. In 1991, during a VERY powerful batch of T-storm activity in MY area, I was getting some very usual skip conditions. I was getting WBVE-FM “Beaver 96.5” in Hamilton (Cincinnati), OH clear as a bell and WKDD-FM (96.5) from Akron, OH started coming in a few minutes later. After the storm abated, I was getting WMT-FM (96.5) in Cedar Rapids, IA and WDZQ-FM (95.1) from Decatur, IL. And just before the skip died, I was getting KKDL-FM from Fargo, North Dakota (95.1). There is NOT a way to predict E-Skip (as far as I know). But you can get a hint of a chance that E-skip is going to occur. If you have a police scanner, capable of LOW band (30-50 MHz), set one of your memories to one of several paging channels NOT being used in your area or a police channel NOT being utilized in your area. My one is 42.12 MHz (Missouri Highway Patrol, and others). If you are beginning to get some activity on that channel, chances are the bands are going to “open-up”. After that, check the any low band TV channel that is not very active (Channels 2-6) in your area. If you are beginning to see some video or if you experiencing some co-channel interference on that channel, you’ve got SKIP! SKIP conditions move up in frequency depending upon the strength of the opening. The highest frequency where skip occurs is called the MUF (maximum useable frequency). Sometimes it can level off at Channel 2, or it can level off upwards to 107.9 and maybe beyond!
73’s Peter Q. George, N1GGP Randolph, Massachusetts USA