posted on Feb, 26 2017 @ 01:37 PM
Most two-way radio enthusiasts are aware of distant communications during atmospheric conditions commonly known as “skip”. Skip occurs when “sky
wave” signals in the HF range (frequencies between 3 and 30 MHz) are refracted off the ionosphere excited by sunspot activity or cosmic radiation.
20 MHz is considered the best frequency for distant skip communications, however, skip can occur up into the lower VHF range under certain
Because the higher VHF and UHF signals are line of sight and will normally shoot straight out through the ionosphere into space, range is normally
increased through antenna height and higher transmitter output levels. However, certain weather conditions will allow for VHF and UHF signals to
greatly extend their range. Such conditions are known as tropospheric ducting that occur in the lowest layer of the atmosphere called the troposphere.
The troposphere, or lower atmosphere, is where almost all weather occurs. The troposphere begins at the Earth's surface and extends from 4 to 12
miles high. Tropospheric ducting happens during weather when the signal encounters a temperature inversion.
A temperature inversion flips the normal change of atmospheric temperature in the troposphere from warm to cold with increasing altitude. During an
inversion, the normally warmer surface air temperature is colder than the warm air mass above it. During an inversion the higher refractive index
between the inverted layers will cause the signal to be refracted or bent. Tropospheric ducting will affect all frequencies, and signals enhanced this
way can travel up to 800 miles.
Generally speaking, mild temperature inversions occur in the mornings and evenings when the angle of sunlight is hitting the upper troposphere,
heating it up to a higher temperature than the lower surface layer. This effect will typically last for a couple of hours before and after sunrise and
sunset. During temperature inversions when a warm front enters an area over the top of a colder layer of air, the intensity and area covered increases
tropospheric ducting allowing for greater range and clarity of VHF and UHF two-way communications. The duration of a tropospheric ducting event like
this depends on how long the inversion lasts.
Inversions are caused by a number of factors and will often cause fog to develop, or in urban areas smog will increase. One way to be aware of
tropospheric ducting is through the observation of broadcast TV signals. Apparently all long-distance reception of broadcast digital television occurs
by tropospheric ducting, so when you’re monitoring broadcast TV on an antenna and notice an increase in signal strength and the reception of distant
channels, then you should have an increase in range when communicating on VHF/UHF two-way radios.
Due to the limitations of the unlicensed multi-use radio service (MUR) and family radio service (FRS) two-ways, communications on these frequencies
during a temperature inversion would be the best opportunity to increase your range from a mile or two to considerably farther. A well matched, full
wave outdoor antenna mounted as high as possible in a flat open area always helps and in the case of handheld two way radios, a higher elevation in
flat open topography is best for communications range as well. Using a directional antenna will increase forward gain as well as range and if pointed
skyward it will help project sky waves toward the refractive atmospheric layer during a tropospheric ducting event.