Shortwave Radio
Frequencies between 3 MHz and 30 MHz are set aside for use in long distance radio communication. This frequency band is generally referred to as the short-wave band. The wavelength that lies between 1 m and 100 m is short compared to that of the medium wave. It was pointed out in Section 2.9 that the increased frequency of the carrier used in short-wave communication gives its electromagnetic transmission wave increasingly directional properties. Transmission antennae can be designed to concentrate the power of the transmitter in a given direction. The ionosphere and the Earth's surface are used as reflectors to direct the signals from the transmitter to the receiver when very long distances have to be covered.
The superheterodyne arrangement is used in short-wave radio receivers. However, because the radio frequency is much higher the intermediate frequency used is also high. Intermediate frequencies in use with short-wave radio are usually between 1.5 and 28 MHz. In some special cases, a double superheterodyne system is used. The radio frequency is translated to the first intermediate frequency, a high frequency, and then translated the second time to a lower frequency, between 455 kHz and 5 MHz. The advantage of this system is improved selectivity and image rejection.
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