Tropospheric propagation extends VHF/UHF signals

When I’m down at the Hands-On Museum, talking to the visitors there, I frequently get asked if the weather affects radio propagation. I normally respond that the weather has no effect on propagation at all. That’s true, of course, for HF radio propagation, but I now know that’s most definitely not true for VHF, UHF, and even microwave propagation.

Above 30 MHz, the troposphere, the lowest layer of the Earth’s atmosphere refracts and scatters radio waves. This effect occurs every day and can allow amateurs to make contacts of up to 500 miles, although typically the distance is about half that.

Atmosphere

Normally, air temperatures are warmest near the Earth’s surface and decrease as you go up. Sometimes, though, weather conditions are such that a temperature inversion forms and the atmospheric temperature increase with altitude. When this occurs, a phenomenon called ducting can take place. The duct can act like a waveguide and propagate radio waves for long distances with relatively low losses. If a radio wave of the right frequency enters such a duct, it can propagate up to 900 miles. Sometimes these ducts can exist for days.

For more information on tropospheric scattering, you can read up on it in the ARRL Handbook, or go to the Web page, “Tropospheric DX Modes.” On that website, you’ll also find tropospheric ducting forecasts produced from weather forecasts.

Being an HF guy, I blew off learning about this phenomena. Even when teaching my Tech classes, I wouldn’t attempt to discuss this much. Instead, I’d just recite the answers to the questions and plead ignorance. Now that I understand this more—thanks to a great presentation by Russ, KB8U, at our club meeting on Wednesday—I actually find it kind of interesting.

As if on cue, yesterday while I was fiddling around with the Baofeng UV-5RA that I just purchased (more on that later), I heard a couple of guys from Buffalo, NY access the W8UM repeater EchoLink node. They’d access the link, then identify every minute or so. I thought that was kind of odd, so I called one of them directly. As it turns out, what they were doing is keying repeaters in areas where they thought a ducting path to Buffalo might exist. They would key the repeater via EchoLink and then listen for the repeater with their radios. They weren’t successful with W8UM, but they had been successful with other repeaters that they’d accessed. Very cool stuff.

Comments

  1. Huh, that might explain why I was able to tune in a Cleveland FM station last night? I had a drive back and forth to Toledo, and the signal was strong in Toledo and then gradually faded in and out as I got closer to Ann Arbor.

  2. Tropo is a lot of fun, especially if you have one of those newer DC-daylight radios. Put up a small beam in a high location. We’re getting toward the end of tropo season, though. I heard a 2m repeater about 250 mi away on the way into work this morning and there was a lot of co-channel interference on the NPR affiliate I listen to as well. One of the really fun things about tropo is that the duct usually gets better as you go up in frequency, making “impossible” QSOs happen at UHF and microwave.

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