未指导系统无线电波可用空间光学器件tlc9-2008.pdfVIP

9UnguidedSystems

Radiowavesandfreespaceoptics!

FreeSpaceOpticalComms

•Shortdistancesonly

•Atmosphereabsorption/scattering

•Highlydirective

RadioWave

•Canbehighlydirective–pointtopointlinksorforbroadcastcoverage–multiple

receivers

•Differentpropagationmechanismsandpropertiesdependinguponthefrequency

GroundWavePropagation

•Occursintheverylowfrequencyband(3-30kHz),lowfrequencyband(30-300kHz)and

mediumfrequencyband(300-3000kHz)

•Propagationoccursasverticallypolarised

wavesboundtothesurfaceoftheearth.

TheionosphericD-layeralsocontributes

behavingasiftheupperlayerofaterrestrial

waveguide!!TheEarth

•AttenuationforVLFislow,hencelong

distancetransmissionpossible

•Lowfrequencieslimitdatarate–usefulfor

longdistancenavigationsystems.

•Antennasarephysicallylargebutelectricallysmall(i.e.comparedtothewavelength).

•OftenradiateseveralhundredsofkW.

•D-layerofionospherevarieswithtimeofday–affectspropagation.

•Conductivityofthegroundeffectsrange–e.g.seawaterhasamuchhigherconductivity

thandryland.

SkyWavePropagation

•ConductiveionosphericlayersreflectandrefractRFsignals

1

•Overhorizoncoverage!

Ionosphere

•StartswithMF(300-3000kHz)often

causinginterferencweenskyand

groundwaves,especiallyatnight.