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1 Tropospheric ducting

Main article: Tropospheric ducting

Virtually all DX reception of digital television occurs by tropospheric ducting (due to most, but not all, DTV stations broadcasting in the UHF band). Signals have a slow cycle of fading and will produce signals strong enough for noise-free stereo reception on FM or clear TV pictures, sometimes in full color.

2 E-skip and Sporadic E

E-skip, also called Sporadic E, is the phenomenon of irregularly scattered patches of relatively dense ionization that develop seasonally within the E region of the ionosphere and reflect and scatter TV and FM frequencies, generally up to about 150 MHz. When frequencies reflect off multiple patches, it is referred to as multi-hop skip. E-skip allows radio waves to travel many miles beyond their intended area of reception. E-skip is unrelated to tropospheric ducting.

E-skip usually affects the lower VHFVery high frequency (VHF is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). Common uses for VHF are FM radio broadcast at 88-108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestri channels (channels 2-6 and the FM band), and usually enhances stations from beyond 600 miles; however, under exceptional circumstances, a highly ionized cloud can propagate E-skip receptions over a distance as little as 450 miles, and can even go up to channel 10.

Another form of E-skip, called skywaveSkywave is the propagation of radio waves by reflection off the inner surface of the Earth's highly charged ionosphere. A skywave is a radio wave from a distant radio station (or rarely, a TV station) that is refracted in the ionosphere. It brings in dist, occurs every night in the mediumwaveMediumwave radio transmissions are those between the frequencies of 300 kHz and 3000 kHz. In most of the world, mediumwave serves as the most common band for broadcasting. The standard AM broadcast band is 535kHz to 1705kHz in North America, but remains o and lower shortwaveShortwave radio operates between the frequencies of 3,000 kHz and 30 MHz (30,000 kHz) and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at tha bands, allowing broadcast stationA broadcast station may be: a radio station a television station It does not include television networks or radio networks. See also: broadcasting.s and amateur radioAmateur radio commonly called ham radio , is a hobby enjoyed by many people throughout the world; as of 2004 about 3 million worldwide, 70,000 in Germany, 5,000 in Norway, 57,000 in Canada, and 700,000 in the USA. A holder of an Amateur Radio license has operators on those frequenciesFrequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the l to be heard from across the continent.

E-skip is a regular daytime occurrence over the equatorial regions and is common in the temperate latitudes in late spring, early summer and, to a lesser degree, in early winter.

At high, i.e. polar, latitudes, E-skip can accompany auroras and associated disturbed magnetic conditions.

E-skip can sometimes support reflections for distances of up to 2,400 km at frequencies up to 150 MHz or, on rare occasions, even higher.

Source: Federal Standard 1037C