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Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the range between groundwave and conventional skywave distances—usually 30–400 miles (50–650 km). It is used for military and paramilitary communications, broadcasting,〔Broadcasting in Band 7 (HF) in the Tropical Zone, International Radio Consultative Committee, International Telecommunications Union, Geneva, 1969〕 especially in the tropics, and by radio amateurs. The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter.〔The Emergency Communications Antenna, By Stephen C. Finch, AIØW http://www.w8ne.com/Files/NVIS%20nvis_AI0W.pdf〕 If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction fails to occur and if it is too low, absorption in the ionospheric D layer may reduce the signal strength. There is no fundamental difference between NVIS and conventional skywave propagation; the practical distinction arises solely from different desirable radiation patterns of the antennas (near vertical for NVIS, near horizontal for conventional long-range skywave propagation). == Frequencies and propagation == The most reliable frequencies for NVIS communications are between 1.8 MHz and 8 MHz. Above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz. Usable frequencies are dictated by local ionospheric conditions, which have a strong systematic dependence on geographical location. Common bands used in amateur radio at mid-latitudes are 3.5 MHz at night and 7 MHz during daylight, with experimental use of 5 MHz (60-meter) frequencies. Broadcasting uses the tropical broadcast bands between 2.3 and 5.06 MHz, and the international broadcast bands between 3.9 and 6.2 MHz, Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight. Optimum NVIS frequencies tend to be higher towards the tropics and lower towards the arctic regions. They are also higher during high sunspot activity years. The usable frequencies change from day to night, because sunlight causes the lowest layer of the ionosphere, called the D layer, to increase, causing attenuation of low frequencies during the day 〔AN ANALYTICAL STUDY of HF COMMUNICATIONS between PROVINCIAL PREOC’s and the NORTH SHORE EMERGENCY MANAGEMENT OFFICE at VE7NSR http://www.nsarc.ca/tech_archive/HF%20PREOC%20Study/Report/Report_9%20Mar10.pdf〕 while the maximum usable frequency (MUF) which is the critical frequency of the F layer rises with greater sunlight. NVIS is most useful in mountainous areas where line-of-sight propagation at VHF or UHF frequencies is ineffective or when the communication distance is beyond the 50-mile (80 km) range of groundwave, and less than the 300–1500-mile (500–2500 km) range of lower angle sky-wave. Another interesting aspect of NVIS communication is, that direction finding of the sender is more difficult than for ground-wave communication (i.e. VHF or UHF). For broadcasters, NVIS allows coverage of an entire medium-sized country at much lower cost than with VHF (FM), and daytime coverage similar to MW (AM) nighttime coverage at lower cost and often with less interference. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Near vertical incidence skywave」の詳細全文を読む スポンサード リンク
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