About DX

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About DX By Jacob Norlund, KC0LTV

Have you ever turned your TV (not connected to satellite or cable) on, wanting to watch your local Channel 2 newscast only to discover it has been overridden by a station from 1,200 miles away? Or have you ever accidentally pressed the autoscan button on your TV set, and found 45 channels scanned in even though you don't have cable? Or changed the channel from 3 to 4 only to discover an unfamiliar station coming in like it's 2 miles away, even though the advertisements are from Montana (and you live in northeastern Minnesota!). If you have, you've probably experienced DX before. DX is sort of a break from the normal distance limits imposed on TV signals by nature.
Under normal conditions, off-air TV stations (that is, without the assistance of cable or satellite technologies) travel about 10 to somewhere around 150 miles, depending on factors such as transmitter power and especially line-of-sight (the transmitter must "see" the receiver). However, the rules of line-of-sight are broken when DX conditions fire up. There are primarily two types of DX (although other types, such as meteor and lightning scatter also exist): Tropo and E-Skip.

Tropo is related to the weather, and is known to occur during temperature inversions, when warm air is higher in the atmosphere than cold air (the normal situation is the opposite). Tropo signals typical travel between 50 and 2,000+ miles, depending on what is located between you and the transmitter. If the path the signal must travel is over water, distances are often higher than land, where 900+ mile tropo is rare. Over water distances can be well over 1,000 miles (in the case of the Carribean Sea) to well over 2,000 miles (in the case of the Pacific, when signals from Hawaii can be received in California). William Hepburn's tropo forecasts.

The signal quality of tropo receptions can vary greatly from so poor that the signal is only discernible from absolute snow to better than your local stations. The pictures above (WNDU-South Bend, IN, 466 miles from here) and to your right (WHBF-Rock Island, IL, 371 miles from here) are good examples of what tropo might look like. Tropo tends not to fade as much as skip (which I will describe next). Tropo is best on UHF (channels 14-69), second on VHF-High (channels 7-13), and third on VHF-Low (channels 2-6). It is not unusual for a long-time DXer to have a 950 mile over-land UHF tropo logging, but to have the same distance on channel 2, for example, would be very rare. Typically the best time of the year for tropo in Minnesota is the summer, with late August/Early September being the optimum time for DX. My tropo experiences here.

What actually causes skip still remains an enigma, but we do know that it may be related to lightning strikes and a little bit about how it works. Signals bounce off ionized clouds in the E-layer of the ionsphere, located very high above ground allowing the signals to travel great distances. Typical distances are between around 700 and 1,500 miles. If the signal can bounce twice (rare), distances can be double the normal range. Skip is unpredictable, but it is common in early-mid Summer and mid-late Spring. In many cases, skip will fade in and out rapidly and multiple stations will come in at once, "fighting" for a channel. Skip can also be very intense, with local-like signal strength.

Most of the time, skip only affects channels 2 through 6. Skip will begin to come in on channel 2, and rise to channel 6 over time (the highest channel or frequency affected is known as the MUF, or maximum usable frequency). After it reaches channel 6, it will hit the FM radio band, allowing FM stations from faraway to come in just like stations 20 miles distant from you. Rarely does skip affect channels 7 and above. Skip can also shift direction, and sometimes you will even see two totally different parts of the country coming in-at once. For example, I once seen CKAL-TV-1 from Lethbridge, AB coming in-and shortly after, WSB from Atlanta, GA destroyed the signal! (If my memory serves me right, it was WSB). Skip is very enjoyable but requires constant checking for conditions in those skip months-April, May, June (especially), and July.

DXING? HUH?

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