DXing Above 30 Mhz
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 cause a VHF and UHF signal to travel hundreds of miles or more. Events such as a stalled weather front to a meteor shower or even the solar wind can change things in the atmosphere and make a signal travel much further than normal.
The most common type of tropospheric ducting to impact scanner hobbyists and amateur radio operators results from a low-level temperature inversion. A low-level temperature inversion is a phenomena in which the temperature in the lower portion of the atmosphere, near the earth's surface, is considerably cooler than a layer further 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 broadcast, as well as ham frequencies, are found above 30 mhz. Techniques used in TV, FM and 2/432 Mhz DXing are applicable to DXing in the scanner bands.
The links below are not intended to be comprehensive, 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 here
Other Propagation Links
TV/FM DX Links
- Worldwide TV FM DX Association (WTFDA)