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I would like to know if UHF signal quality can be affected by hot weather? And which of UHF and VHF is better on that case of weather ?

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Propagation of UHF and VHF signals can definitely be affected by weather.

In particular Tropospheric propagation (radio waves traveling through the Troposphere, the lowest portion of the earth's atmosphere) and Tropospheric Ducting are seasonal:

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not common when the refractive index of the atmosphere is fairly high.

Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.

As to which is "better", UHF or VHF that depends on your goals. Lower frequencies generally travel farther while higher frequencies are more affected by rain and snow. Often you do not want your signals to travel too far as in the case of a Wi-Fi router or cordless phone in a home for example.

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