Shortwave
Shortwave frequencies are capable of reaching the other side of the planet because they can be refracted by the ionosphere (a phenomenon known as Skywave propagation). The selection of a frequency to use to reach a target area depends on several factors:
The distance from the transmitter to the target receiver.
Time of day. During the day, higher shortwave frequencies (> 12 MHz) can travel longer distances than lower ones; at night, this property is reversed.
The dependence to the time of the day is due to a particular transient atmosphere ionized layer forming only during day when atoms are broken up into ions by sun photons. This layer is responsible for partial or total absorption of particular frequences.
Seasons, during the winter months the AM broadcast band tends to be more favorable because of longer hours of darkness.
Solar conditions, including the number of sunspots, solar flares, and overall solar activity. Solar flares can prevent the ionosphere from reflecting or refracting radio waves.
Type of modulation. Independent from the frequency, the receiver must be capable to receive the same modulation type of the transmitter. USB, LSB, AM, CW are all modes of modulation.
Types of modulation frequently used in the shortwave frequency range are:
AM: amplitude modulation. Usually used for shortwave broadcasting, and some aeronautical communications.
NFM: Narrow-band frequency modulation. Because of the bandwidth required this is normally used for VHF communication, but some NFM transmissions occur in the higher HF frequencies.
SSB: Single sideband (USB/LSB): This is used for long-range communications by ships and aircraft, for voice transmissions by amateur radio operators, and for broadcasting. LSB is generally used below 10mhz and USB above 10mhz.
CW: Continuous/Carrier wave, which is used for Morse code communications.
DRM: Digital Radio Mondiale: digital modulation for use on bands below 30 MHz.
Various radioteletype, fax, digital, or other systems, which require software or special equipment to decode.
Some major users of the shortwave radio band include
Domestic broadcasting in countries with a widely dispersed population with few longwave, mediumwave, or FM stations serving them
International broadcasting to foreign audiences (which explains why shortwave is also known as "world band radio")
Speciality political, religious, and conspiracy theory radio networks, individual commercial and non-commercial paid broadcasts for the north American and other markets.
Utility stations transmitting messages not intended for a general public, such as aircraft flying between continents, encoded or ciphered diplomatic messages, weather reporting, or ships at sea
Numbers stations
Amateur radio operators
Time signal stations. WWV operates on these frequencies: 2500kHz, 5000kHz, 10000kHz, 15000kHz, and 20000kHz. CHU Canada operates on these frequencies: 3335kHz, 7335kHz, and 14670kHz.
The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave broadcast radio receivers in use in 2002.
The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunication Union, allocates bands for various services in conferences every few years. The next WRC is scheduled to take place in 2007.
At the World
Administrative Radio Conference (WARC) in 1997, the following bands
were allocated to international broadcasters (listed in the table):
| Meter Band | Frequency Range | Remarks |
|---|---|---|
| 120 m | 2,300 - 2,495 kHz | tropic band |
| 90 m | 3,200 - 3,400 kHz | tropic band |
| 75 m | 3,900 - 4,000 kHz | shared with the amateur radio 75/80 meter band |
| 60 m | 4,750 - 5,060 kHz | tropic band |
| 49 m | 5,900 - 6,200 kHz | |
| 40 m | 7,100 - 7,300 kHz | shared with the amateur radio 40 meter band |
| 41 m | 7,300 - 7,350 kHz | |
| 31 m | 9,400 - 9,900 kHz | |
| 25 m | 11,600 - 12,100 kHz | |
| 22 m | 13,570 - 13,870 kHz | |
| 19 m | 15,100 - 15,800 kHz | |
| 16 m | 17,480 - 17,900 kHz | |
| 15 m | 18,900 - 19,020 kHz | |
| 13 m | 21,450 - 21,850 kHz | |
| 11 m | 25,600 - 26,100 kHz |
Analog AM shortwave broadcasting channels are allocated with a 5 kHz
separation.
International broadcasters, however, may operate outside the normal WARC-allocated bands or use off-channel frequencies to attract attention in crowded bands. The new digital audio broadcasting format for shortwave DRM operates in 5khz, 10khz or 20 khz channels -- so there are some ongoing discussions with respect to specific band allocation for DRM.
The power used by shortwave transmitters ranges from less than one watt for some experimental transmissions to 500 kilowatts and higher for intercontinental broadcasters. Shortwave transmitting centers often use specialized antenna designs to concentrate radio energy on a bearing aimed at the target area.
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