When I was a kid, I often listened to the radio late at night, trying to pull in baseball games from other Midwestern cities. This wasn't very difficult, because the stations that broadcast the games of the St. Louis Cardinals and Cincinnati Reds were 50,000-watt monsters; meanwhile, other stations that operated in the AM band at low power — less than 10,000 watts — had to shut down their operations at nighttime perrules, which protected the so-called "clear channel" stations from interference from these secondary stations. So, the signals transmitted by the big boys had a clear path to my radio.
One night, as I slowly turned the tuning knob, something very interesting happened — I stumbled upon a station that was broadcasting from San Francisco. I was stunned. I live in Chicago, so pulling in a signal from 500 miles away is one thing — but San Francisco not only is 2,000 miles away, but it's also on the other side of the Rocky Mountains. How could this be? For four decades I've been asking myself that question.
A few weeks ago, I learned the answer. I attended the Radio Club of America's technical symposium in New York City, during which RCA Fellow and ARRL Life Member Gordon West offered a presentation on tropospheric ducting, which acts as a kind of slingshot for radio signals. As part of his presentation, West demonstrated how he was able to communicate over a ham radio set in Southern California with another ham in Hawaii — 2,500 miles away.
The troposphere is the lowest part of the Earth's atmosphere. It is about 20 kilometers deep at the Equator and becomes shallower as one advances toward the poles. In the middle latitudes, for example, the troposphere is about 17 km deep, while it is just 7 km deep at the poles. West calls the troposphere the "weather layer."
"This is the one that will affect those of you in the land-mobile industry and those of you in the microwave industry, because the weather has everything to do with how far, or how close, signals will go," West said.
West then explained the phenomenon. It begins when a high-pressure system stalls. "On the West Coast, we call that a Pacific High," he said. "As a sailor, we always hated the Pacific High, because when you get stuck in one, there's no wind, it's warm, and there's a haze."
The high-pressure air eventually becomes heavier than the air below it and begins to sink, a phenomenon known as "subsidence." West said the sinking air "squishes" the air immediately below it, causing that air to warm. As it does, it begins to stratify.
"That's because the cool air below it is keeping it up, while the cold, dry air above it is pushing it down," West said. It is within that sliver of squished air that tropospheric ducting occurs.
"And when it does, neat things happen out in the radio bands. A tropospheric duct acts much like a waveguide that channels [radio signals] over paths that are almost unheard of." West said.
This was the first time that I attended the RCA technical symposium, and solving the mystery of my youth by itself was the worth the time and effort. I am planning to make this an annual trek, because I learned a few other interesting tidbits that I'll be sharing soon.
In the meantime, happy holidays.
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