Urgent Matters

Decades later, a mystery solved

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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 per FCC rules, 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.

What do you think? Tell us in the comment box below.

Discuss this Blog Entry 10

VE4SIG (not verified)
on Dec 20, 2012

Ever consider getting your ham radio license? There's no requirement to know Morse Code anymore (a stumbling block for many). Come on and enjoy the Trop Ducting, Meteor Scatter and sunspots with the rest of us!

Anonymous (not verified)
on Dec 20, 2012

It's nothing special for hams to communicate over thousands of miles... using the ionosphere at HF. But when ducting conditions are present, they can close the link at VHF, UHF, or even microwave (according to the ARRL Operating Manual for Radio Amateurs, the California to Hawaii path has been used on 144 MHz through at least 5.6 GHz).
73 AB3NA

Ray L. (not verified)
on Dec 20, 2012

I remember early in my Air Force career while working ground control in the control tower at Laugthlin AFB, Texas, I suddenly heard a station issuing taxi instructions on 343.8 MHZ to military aircraft loud and clear. Now keep in mind that the nearest military field was at San Antonio, 150 miles east.

The controller was not identifying himself so I finally pickup the mic and ID myself and asked where he was, Laredo Ground Control, 180 miles south.

The "interference" ended after about thiry minutes.

on Dec 21, 2012

I first learned the physics of ducting as a seaman at the US Navy ET school in great Lakes, 1977. During my 20 years in the Navy, I witnessed ducting twice with a surface search radar. Each time it only lasted 10-15 minutes but that was too long for a warship at sea. And yes, both times we were traversing the "Triangle"!

Anonymous (not verified)
on Dec 21, 2012

Tropospheric ducting occurs at VHF and above. You weren't hearing that mode of propagation down on the AM broadcast band. It was undoubtedly F2-layer skip, on a night when the D-layer absorption was low enough to allow you to hear signals that would normally be too weak. Do a little searching for "MW DXing" and see the results people have listening to distant signals on these frequencies.

Ben Dawson (not verified)
on Dec 31, 2012

This commenter is correct. While ducting up and down the west coast and lots of other places is not at all uncommon, even down to "low" VHF (25 MHz or so), at MF it's skywave that results in long distance reception/communication. Just look at the FCC rules to see a commonly used propagation formula for it.
When I was very young and ch. 2 LA was the only west coast channel 2 station, it was often viewable in pretty amazing locations at great distances because of ducting - such as on the Oregon coast.
B. Dawson III, P.E.

on Dec 21, 2012

I can remember those days in the 1950s and early 60s when I'd tune my grandparents' 'waterfall'-fronted wooden-cased tube radios across the shortwave bands and log stations like Radio Havana and the Voice of America. That was my start in radio, and in radiocommunications. Altho I eventually became a ham, the best days of my radio hobby were logging stations in distant lands, finding the countries on my globe, and telling my classmates about it at 'show-'n-tell' the next week. I went on to work for Northern Electric (the installation arm of Bell Canada, and the Canadian counterpart of Western Electric), Fluke Electronics, then Transport Canada (the FAA equiv but in Canada) and now Industry Canada (Canada's equiv of the FCC). Radio has always been fun for me; it is great to have a job that always feels like an extension of my hobby.

73, de VA3ROD

W6KOZ (not verified)
on Dec 21, 2012

Had a lot of that on UHF military aviation channels. *** center could not work military aircraft at normal range. But could easily work them at double/triple normal. Took ages and a lot of data to convince them it was not an equipoment problem. As said above this is common an the Pacific coast -

on Dec 22, 2012

While I've always enjoyed the thrill and excitement of logging a rare shortwave broadcaster or "busting a pileup" to log a 2-way contact with a rare DX station on the HF ham bands, I think that weak signal operation via tropo, E-skip, FAI, scatter and aurora etc. is the "cats meow". I recall a particular extended tropo event about 11 years ago in the fall, when many hams from Texas and the nearby Gulf Coast enjoyed a 3 day long event that began on Friday. I personally logged many new grid squares and probably one or two new states, as far north as the upper Michigan peninsula and into Canada. Some of those signals on 144 MHz were as strong as 20 to 30dB over S9, and there was so much traffic that it sounded like the 20 meter ham band. It was fantastic!

N5IT, Waco, TX

Anonymous (not verified)
on Jan 11, 2013

Agreed, Ducting is very cool. I recall when the Mt. Greylock (NH) repeater was usable as a local repeater in south Jersey around 25 years ago, summertime. I think at the time it shared the same pair as a local machine but had no pl tone required.

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Donny Jackson is editor of Urgent Communications magazine. Before joining UC in 2002, he covered telecommunications for four years as a freelance writer and as news editor for Telephony magazine....
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