Frequencies and propagation Near vertical incidence skywave
nvis radiation pattern
while groundwave (blue) cannot propagate, refracted skywaves (red) achieve hf coverage within common first hop (~500km).
the reliable frequencies nvis communications between 1.8 mhz , 8 mhz. above 8 mhz, probability of success begins decrease, dropping near 0 @ 30 mhz. usable frequencies dictated local ionospheric conditions, have strong systematic dependence on geographical location. common bands used in amateur radio @ mid-latitudes 3.5 mhz @ night , 7 mhz during daylight, experimental use of 5 mhz (60-meter) frequencies. during winter nights @ bottom of sunspot cycle, 1.8 mhz band may required. broadcasting uses tropical broadcast bands between 2.3 , 5.06 mhz, , international broadcast bands between 3.9 , 6.2 mhz. military nvis communications take place on 2-4 mhz @ night , on 5-7 mhz during daylight.
optimum nvis frequencies tend higher towards tropics , lower towards arctic regions. higher during high sunspot activity years. usable frequencies change day night, because sunlight causes lowest layer of ionosphere, called d layer, increase, causing attenuation of low frequencies during day while maximum usable frequency (muf) critical frequency of f layer rises greater sunlight.
nvis useful in mountainous areas line-of-sight propagation @ vhf or uhf frequencies ineffective or when communication distance beyond 50-mile (80 km) range of groundwave, , less 300–1500-mile (500–2500 km) range of lower angle sky-wave. interesting aspect of nvis communication is, direction finding of sender more difficult ground-wave communication (i.e. vhf or uhf). broadcasters, nvis allows coverage of entire medium-sized country @ lower cost vhf (fm), , daytime coverage similar mw (am) nighttime coverage @ lower cost , less interference.
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