Weather, not the sun

VHF and UHF normally go little farther than the horizon. But the lower atmosphere can bend them over the horizon too, and here the driver is weather, not space weather. When it happens, a handheld can hit repeaters it has never heard, and simplex contacts stretch to 300 km or more.

How it works

Usually air gets colder with height. In a temperature inversion, a layer of warm air sits on top of cooler air, with a sharp change in moisture between them. That boundary bends VHF and UHF signals back toward the ground, and sometimes traps them in a duct that carries them a very long way.

When to watch for it

• Stable high pressure sitting over the region for several days.
• Calm, clear nights and early mornings, especially in late summer and fall.
• Warm air moving over cooler ground, like a warm front, or air over a cold lake or reservoir.
• Long, flat paths, like across the prairies, tend to do well.

Chinooks are interesting here: the warm air that flows down off the mountains over cold air at ground level can create exactly the kind of sharp inversion that bends signals.

How to take advantage

• Check a tropo forecast map, such as the Hepburn tropospheric ducting forecast.
• Listen on 144.200 MHz SSB and the FM simplex calling frequencies, and watch FT8 on 144.174 MHz.
• Horizontally polarised antennas, used for SSB and CW, get the most out of tropo.
• Note which distant repeaters you hear. It’s a quick sign an opening is on.

Last updated October 2026.