Long wave, it can be usedSkywaveorGround waveIn the form of communication.Long wave band has the strongest emission capabilitybandIt is most suitable for propagation in the form of ground waves around the earth's surface, but the maximum distance of ground waves propagation does not exceed (3~4) thousand kilometers, so the propagation mode of ordinary long waves is still dominated by sky waves.[1]
Its transmission mode is mainly around the earth's surfacethe ionosphereWaves can propagate in the form of thousands to tens of thousands of kilometers. In addition, they can also be transmitted bySurface waveSpread, thebandOfelectric field intensityAt night, the wavelength is larger than that at daytime, and the shorter the wavelength is, the greater the increase is;The influence of electric field intensity on season is small;The propagation condition is less affected by the ionospheric turbulence, and has good stability. It will not produce sharp changes in acceptance intensity and sudden interruption of communication.Applicable toRadio direction finding，Radio navigationAnd so on.As long wave requires huge antenna equipment, China's radio stations do not use the long wave (LW) band, which is mainly used forsubmarineCommunication and ocean going ship communication.Therefore, domestic radios generally do not have long wave (LW) bands.
Frequency below 30kHzUltralong waveIt can travel around the world.When long wave propagates, it has stable propagationnuclear explosion, small impact of atmospheric turbulence, etc.It spreads in seawater and soil, and the absorption loss is also small.[2]
Communication characteristics
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The frequency (LF) of long wave is 30-300kHz (wavelength 1000-10000m).It mainly depends on ground wave propagation, is less affected by climate change, and can communicate reliably all day long.Especially in the middle and short distance (50-100km).Stable propagation can be used for medium and long distance communication and as a means of communication between shore and ship.
The characteristics of long wave propagation are as follows:
① The change of electric field intensity is not obvious
The electric field intensity changes insignificantly and slowly in the long wave range. The longer the wavelength, the smaller the change.The measured data prove that under the influence of the irregular changes of the ionosphere, the average change of the electric field is not more than lo~30%, and the length of time for such changes is calculated in tens of minutes or even hours.
② Diurnal variation of electric field intensity
Long wave propagation is affected by layer D in the daytime and reflected by layer E in the evening, so the electric field intensity changes.Generally, with the advent of darkness, the electric field intensity increases. The shorter the wavelength, the more significant.When the wavelength is 17500m, the electric field intensity changes from daytime to nighttime. The increase value is about 50%.
③ Seasonal influences
In the long wave range, the electric field intensity changes very weakly in a year, which shows that the electric field intensity increases by 20% - so in the daytime in summer than in the daytime in winter, while the electric field intensity at night in summer is weaker than that at night in winter
④ Influence of 11 year cycle of solar activity
The 11 year cycle of solar activity has little effect on the process of long wave propagation. Only when solar activity increases, the diurnal electric field intensity increases slightly. It can be seen that when solar activity changes from minimum to maximum, the electric field intensity increases twice.
⑤ Influence of ionospheric turbulence
The repeated ionization disturbance mainly affects several layers above the ionosphere, which have nothing to do with the long wave propagation, so it has little impact on the long wave propagation. It will never causelong-wave communicationInterruption. This is a major advantage of long wave communication.
application
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Usually, long wave can be used for: medium and long distance communication;Ground wave broadcast;Ground wave emergency communication;Long wave mine communication;Underground communication；Standard frequency and time audio broadcasting andRadio navigationThe following are introduced respectively:
（1） Long wave early warning broadcast network
Long wave early warning broadcast network belongs to DIDS, the discrimination information distribution system of American national early warning network.The role of the Discriminant Information Distribution System is to send warnings about natural disasters or other very urgent situations to the federal government, military and civil authorities of the United States.It will provide a communication network that can more effectively transmit emergency information. DIDS's long wave broadcast network has ten regional broadcast stations. Its scope of action includes 48 states in the United States and may be extended to Hawaii and Alaska. The selected operating frequency ranges from 163-197kHz. This is because ground wave propagation in this frequency band can provide very stable signals in a large areaThe earth is free from day and nightIonospheric disturbanceImpact.
The locations of the ten regional broadcast stations selected by DIDS are: Richland, Mondope, Riverton, Winslow, Mazumane, Kamak, Maynard, Edgewood, Asheville and Dunford. The early warning information entering the network is located in ColoradoThe national early warning centers in Texas and Washington, D.C. introduced each other. This low level early warning signal was sent to the regional relay stations through the automatic telephone network (Kansi Hu) and dedicated lines, and then was relayed in low frequency.The wired transmission line also connects the turning arena to the monitoring equipment of the remote control maintenance center and the alarm display of the regional police station and fire station.[3]
（2） Long wave emergency communication network (GWEN ')
GWEN is a ground strategic communication system in the United States.It is reinforced by a large number of electromagnetic pulse resistant low-frequency unattended distributed throughout the United States mainlandRadio relay stationComposition. The long wave emergency communication network is a part of the minimum basic emergency communication network (MEECN). It will develop into an invulnerable and durable system that connects the top national command authority NCA with the strategic command center and retaliatory forces.The damage measures adopted by the system are: long wave propagation, decentralized relay nodes and packet switching technology. Therefore, when the United States is under nuclear attack and after the attack, and when part of the network is destroyed, it can still ensure the strategic forces of the country's highest command authority to implement the overall nuclear war plan are connected.[3]
