For convenience, the shortwave part of the radio-frequency spectrum is subdivided into bands. Each band has its own unique propagation characteristics. A detailed discussion of propagation is provided elsewhere. What follows here is a summary discussion, intended to get the new shortwave listener off to a quick start.
The lowest-frequency broadcast band is the 75m band. It runs from 3900 kHz on up to the bottom edge of the old 4 mHz Marine band at 4080 kHz. Parts of this band (3900 to 4000) kHz are shared in North America with ham radio operators. You will hear few broadcasters in this segment; the much closer hams drown them out. This band is useless during the day unless you live within a few hundred miles of a shortwave relay station. At night, you will hear few stations in North America, although this band is quite popular in Europe.
Next is the 61m band, from 4500 to 5100 kHz. This band is also known as the Tropical band because so many broadcasters in the world's tropics for day-to-day broadcasting. These broadcasts are obviously in the local language; however, they provide a source of music second to none. This band is closed during the day, except for a handful of American broadcasters; start listening around sunset.
The next band is the 49m band, from 5730 to 6250 kHz. Many shortwave listeners consider this the lowest frequency "real" band. You will hear considerably more stations, from further off, than on the other bands we have already discussed. This is a popular band to cut dipoles for. It is quiet during the daylight hours; start listening around sunset.
Continuing up in frequency, we come to the 41m band. This runs from 7100 to 7600 kHz. From 7100 to 7300 kHz, the band is shared with ham operators in North America, and American listeners will hear few English broadcasts in this segment. Reception is limited to a few hundred miles during the day, but at night signals from this band travel all around the world. As a nostalgia note, in the old days Radio Moscow had a signal every 50 kHz or so on this band, transmitting in English to North America. While listening to the Russian view during the Vietnam war, I was annoyed by interference from unintelligible voices having conversations. I learned to use my radio's BFO to tune them in, et viola! Another ham was born.
The next band is the 31m band, from 9250 to 9800 kHz. This is next band above 49m that is considered a "must hear." This band is open at night.
In fact, all the bands we have discussed so far are known as "darkness path" bands, because the signals propagate best after dark. Use your knowledge of the earth's rotation to clue you into which parts of the world you can hear when. For example, in the evening, it is already dark in Europe, but still daylight in Japan. So you would listen in the evening for 41m European stations and in the morning for Japanese ones.
Now, though, we come to the 25m band, which is an anomaly. Depending on the time of year and signal path, this band is open to some part of the world all day. It runs from 11500 to 12160 kHz.
Next is the 19m band. This is the lowest-frequency "daylight path" band, although it is often open after dark. It runs from 15000 to 15800 kHz. It is one of the most popular bands, and the lowest frequency one that sees good performance from built-in whip antennas.
The 16m band is next, from 17480 to 17900 kHz.
Now we come to the bands that are open at intervals dependent on the eleven-year sunspot cycle. In peak years these bands are open long and often; during low spots in the cycle they are rarely open. Use of these bands never stops, but does become infrequent during the lows in the solar cycle.
The 13m band is the highest-frequency one that is open often. It runs from 21450 (the top edge of the amateur 15m band) through 21850 kHz.
Finally we have the 11m band, from 25670 to 26100 kHz. It is rarely open during lows in the sunspot cycle, and has almost no activity then. As a result, it is under-utilized even during sunspot peaks.
When using the "daylight path" bands, you can again use the earth's rotation to predict when the bands will be open to which parts of the world. Simply take the opposite approach to the "darkness path" bands. For example, in North America, listen for Europe in the morning, and Asia in the evening.
There are other bands besides the ones discussed above, but these are the most popular ones, so they make a good place to start listening. In addition, the ITU (the international body that regulates shortwave radio) is constantly adjusting the exact frequency coverage of bands, removing under-used bands and adding new ones. This is one reason why a general coverage receiver is a better buy than one that only hears specific broadcast bands.