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DXing Above 30 Mhz

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Talk to most people about how far a VHF signal (above 30 Mhz) will travel, and most times, the answer will be 'line of sight'. However, as there sometimes is in nature, there's exceptions to the rule. Many phenomenon can allow VHF and UHF transmissions to travel hundreds of miles or more. Events such as stalled weather fronts, meteor showers and solar weather events can temporarily change conditions in the atmosphere which allows transmissions to travel much further than normal.

One common type of atmospheric event that is of benefit to scanning scanner hobbyists and amateur radio operators is called "Tropospheric Ducting". Tropospheric Ducting occurs as a result of low-level temperature inversions. low-level temperature inversion is a phenomenon in which the temperature in the lower portion of the atmosphere, near the earth's surface, is considerably cooler than a layer farther up in the atmosphere. This causes signals to be refracted thus resulting in a tunneling or ducting of the signals. Low-level temperature inversions can occur any time of the year but are often most dramatic (producing the most significant tropospheric ducting events) during the summer months. They are most often associated with surface high pressure which results in light near-surface winds allowing for radiative cooling affect. These events also commonly coincide with fog.

VHF, UHF and even 800 MHz and higher can be impacted quite significantly by these ducting events. One good example of very high frequencies being impacted is the anomalous propagation that plague the Doppler weather radar (WSR-88D) during the most extreme tropospheric ducting episodes.

Hams (and folks involved in TV/FM DXing) have been studying this for years. Many of these phenomenon have been cataloged, but not are all well understood. If you are interested in a description of how a VHF signal (or above) can travel, take a look at this article on the WikiPedia site. Keep in mind that FM and TV broadcasts, as well as ham frequencies, are found above 30 MHz. Techniques used in TV, FM and 144/220/432 Mhz DXing are applicable to DXing in the scanner bands.

The links below are not intended to be exhaustive, but rather be a starting point for getting more information. Additional links with descriptions and maps are always welcome.

The RR forum for discussing VHF/UHF skip can be found at the Skip and Tropospheric Ducting Forum

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