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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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172 Chapter 4 Coordinate systems

Example4.11 Show that

R = √ x 2 +y 2 +z 2

Solution From Figure 4.28

x 2 +y 2 =OQ 2

From △OPQ

OP 2 = OQ 2 +PQ 2

ButOP=RandPQ=z,so

R 2 =x 2 +y 2 +z 2

and so

R = √ x 2 +y 2 +z 2

Example4.12 Describethe surfaceR = 4.

Solution We have R = 4 and θ and φ can vary across their full range of values. Such points

generate a sphereofradius 4,centredon the origin.

Engineeringapplication4.6

Three-dimensionalradiationpatternofahalf-wavedipole

Oneofthesimplesttypesofpracticalantennaisthehalf-wavedipole.Thisconsists

of two conductor elements stretched out along a straight line having a combined

lengthofapproximatelyhalfthewavelengthatthefrequencyofthea.c.signalthatis

tobetransmitted.Thesignalisappliedtotheantennaatthecentreofthearrangement

byafeedcable.Theelectricfieldstrengthproducedbytheantennaatafixeddistance

is usually expressed using a spherical coordinate system. The coordinates for the

antennaandtheoriginoftheradiationitselfareassumedtobelocatedattheantenna

feedpointandtheelectricfieldstrengthisrepresentedbytheradius,R.Plotsproduced

likethisareingeneraltermedradiationpatternsandareausefulwayofvisualizing

the amountofradiated field inagiven direction, (θ,φ), foraparticularantenna.

Thehalf-wave dipole patternisdescribedby the equation

( π

)

cos

R =K

2 cos φ sin φ

∣ ∣

whereRrepresentstheelectricfieldstrengthandK isaconstantforagivendistance

fromtheantennacentrepoint.Theequationforthissimpleantennadoesnotinvolve

θ,whichindicatesthatRdoesnotdependonit,hencethereisradialsymmetrytothe

pattern. This function isplotted inFigure 4.29.

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