DXing Above 30 Mhz
From The RadioReference Wiki
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 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
Applications and Databases
- Broadcast AM and FM Radio Reports and Lists
- FCC FM Query Broadcast Station Search
- FCC TV Query Broadcast Station Search
- LPFM Database
- Antennas Direct
- FM Radio DX Radio Reviews
- FM Tuner Information Center
- Godar DXR-1000 AM/SW/FM Antenna
- Live Tunable Receivers
- FM Reflect 2 Dipole Antenna C Crane Co.
- Software Defined Radios
- Radio Station Finder
- Tropospheric Propagation Forecast
- TV Reception Map - rabbitears.info
- VHF Propagation Map
- William Hepburn's Worldwide Tropospheric Ducting Forecasts
Other Propagation Links
TV/FM DX Links
- DX Central
- FCC Tweaks LPFM Technical Rules
- FM Radio DX
- Long Distance FM Reception from Paul Hogan - Ireland
- TV FM Skip log
- Worldwide TV/FM DX Association
- WTFDA Propagation Page old but still lots of useful information. Written by well known author and DXLD owner Glenn Hauser