1 NEC Features INPUT INPUT Geometry Geometry Wires & Patches Wires & Patches Environment Environment Free Space Free Space Perfect Ground Perfect Ground Real Earth Real Earth Sources Sources Voltage & Current Voltage & Current Plane Wave Plane Wave D I I C C S U U T R R R R R I I E E B N N U T T T I I O N OUTPUT OUTPUT I & Q I & Q Distributions Distributions Z IN Y IN P IN Power Budget Power Budget P IN P RAD P LOSS Efficiency Efficiency Fields Fields Near & Far Near & Far Gain Gain Power, Power, Directive, Directive, Average Average
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1 NEC Features INPUT Geometry Wires & Patches Environment Free Space Perfect Ground Real Earth Sources Voltage & Current Plane Wave D ICSUTRRR IEBNUTT.
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11
NEC Features NEC Features
INPUTINPUT
GeometryGeometryWires & PatchesWires & Patches
EnvironmentEnvironmentFree SpaceFree Space Perfect GroundPerfect GroundReal EarthReal Earth
Repetitive Use of Matrix and Exploit Partial SymmetryRepetitive Use of Matrix and Exploit Partial Symmetry Create Numerical Greens FunctionCreate Numerical Greens Function WGWG Use Numerical Green’s FunctionUse Numerical Green’s Function GFGF
66
NEC Output FeaturesNEC Output Features
CommentsComments Structure Specifications (Wires and Patches)Structure Specifications (Wires and Patches) Segmentation DataSegmentation Data FrequencyFrequency Structure Impedance LoadingStructure Impedance Loading Network DataNetwork Data
Excitation at Network Connection PointsExcitation at Network Connection Points Antenna EnvironmentAntenna Environment Matrix TimingMatrix Timing Currents and LocationCurrents and Location Power BudgetPower Budget
77
NEC Output FeaturesNEC Output Features
Charge DensitiesCharge Densities Near Fields Near Fields Input Impedance DataInput Impedance Data Radiation PatternsRadiation Patterns Average Power GainAverage Power Gain Scattering Cross SectionScattering Cross Section Radiated Fields Near GroundRadiated Fields Near Ground Normalized GainNormalized Gain Coupling DataCoupling Data Plane Wave ExcitationPlane Wave Excitation Receive PatternReceive Pattern
Corrected Gain = Corrected Gain = Computed Gain x (k/GAVE)
G
G d
G E x H P
G kP P k for free space
for perfect ground
AVE
P
P IN
AVE RAD IN
41
21
2
Re /
/
*
2020
Wire ModelingWire Modeling
Wires near lossy earthWires near lossy earth Reflection coefficient approximation is reasonable for:Reflection coefficient approximation is reasonable for:
vertical wires at least 0.1 vertical wires at least 0.1 to 0.2 to 0.2 above the ground above the ground horizontal wires at least 0.4 horizontal wires at least 0.4 above earth above earth
Sommerfeld/Norton works for:Sommerfeld/Norton works for: wires as close as 10wires as close as 10-6 -6 height should be several time radius height should be several time radius
{h{h22 + a + a22} } 1/2 1/2 10 10-6 -6 , h , h3a3a
AreaArea should be less thanshould be less than 0.04 0.04 22 (.2(.2x .2 x .2 ))
Since no defined shape, avoid long, thin patchesSince no defined shape, avoid long, thin patches SinceSince currentscurrents defined at center only, not good for edge defined at center only, not good for edge
currentscurrents Where radius of curvature is small, use smaller patchesWhere radius of curvature is small, use smaller patches Surface must beSurface must be closedclosed and not tooand not too thin (no plates, no thin (no plates, no
fins or wings)fins or wings) WiresWires must connect at patch centersmust connect at patch centers Increase definition at connection pointsIncrease definition at connection points
2323
Surface ModelingSurface Modeling
Wire Grid ModelingWire Grid Modeling
Use wire grids whereUse wire grids where edge connections are needededge connections are needed Wire grid = surfaceWire grid = surface if mesh is “small enough”if mesh is “small enough”
Problem:Problem: - can’t afford real fine meshes - can’t afford real fine meshes - sparse meshes have too much - sparse meshes have too much L, not enough C L, not enough C
PossiblePossible Solutions: Solutions: - negative L distributed loading - negative L distributed loading - fat, rod-like wires- fat, rod-like wires
2424
Surface ModelingSurface Modeling
Wire gridding is acceptable for Wire gridding is acceptable for thin structures, plates, thin structures, plates, wings, etcwings, etc.. and for far field responses / not for surface and for far field responses / not for surface charge or currentscharge or currents
Grid size not too criticalGrid size not too critical (~ 0.1 (~ 0.1 at midband) at midband)
/a not critical /a not critical (10< (10< /a < 30/a < 30 good forgood for wires attaching to wires attaching to surface)surface)
UseUse equal radii and segmentationequal radii and segmentation at junctionsat junctions
2525
Patch vs. Wire Grid/ResourcesPatch vs. Wire
Grid/Resources
Patch:Patch: 2LW2LW Grid:Grid: 2LW + L + W2LW + L + W
But Patch can be .2But Patch can be .2 on a side … on a side …
Connection between 2 segments containing Connection between 2 segments containing admittances (impedances)admittances (impedances)
Segments do not have to be nearbySegments do not have to be nearby(not so in real life)(not so in real life)
2-port Y-parameters2-port Y-parameters
R
V2
V1
++ ++
Y11
Y12
Y22
I2I1 Y V Y V I
Y V Y V I11 1 12 2 1
12 1 22 2 2
Y YR
YR
11 22
12
1
1
Series ResistorSeries Resistor
Example:Example:
3232
TL -- Transmission Lines TL -- Transmission Lines
NEC’s transmission lines areNEC’s transmission lines are equations, equations, not wiresnot wires If If transmission line (TL)transmission line (TL), , load (LD)load (LD), , andand voltage voltage
source (Ex)source (Ex) are on the same segmentare on the same segment …
ZL = load on LD
ZT = load on TL
Transmission LineTransmission Line
VSegmentSegment
ZT ZL
3333
Transmission Line Application
Transmission Line Application
Transmission line equations are forTransmission line equations are for balancedbalanced conditions only!conditions only!
OK!OK! NOT BALANCED!NOT BALANCED!
3434
Crossed Feeds Crossed Feeds
Turnstile radiators Turnstile radiators present a challenge at the feed present a challenge at the feed pointpoint
Positive current flows from Positive current flows from END 1END 1 to to END 2END 2
3636
Current DirectionsCurrent Directions
Ex: Ex: VEE dipole, fed at cornerVEE dipole, fed at corner
GW X Y Z X Y Z a
GW X Y Z X Y Z aC C C B B B
C C C A A A
1 4
2 4
, , , , , , , ,
, , , , , , , ,
GW X Y Z X Y Z a
GW X Y Z X Y Z aC C C B B B
A A A C C C
1 4
2 4
, , , , , , , ,
, , , , , , , ,
11BB
22AA
CC
++++
11
22AA
11BB
CC
++
++
3737
E- and H-Fields for a Desired Power
E- and H-Fields for a Desired Power
NECNEC usesuses peak valuespeak values for for voltage, current, and fieldsvoltage, current, and fields
We usually apply 1 volt to an unknownWe usually apply 1 volt to an unknown Zin
E Erms NEC 1 voltsource
Desired Power
2NEC Power for
1 volt source
E Erms peak / 2
38382323
NEC User Notes - Feeding of ArraysNEC User Notes - Feeding of Arrays
Problem:Problem: Array excitations are in terms of feed point Array excitations are in terms of feed point currents (amplitude currents (amplitude
& phase)& phase) NECNEC does not allow does not allow current drives,current drives, only voltage only voltage (amplitude (amplitude
& phase)& phase) at feed points at feed points You can’t drive a feed with a specified You can’t drive a feed with a specified current current unless you unless you
know the driving point impedance. But the driving point know the driving point impedance. But the driving point impedance depends on the impedance depends on the current drivecurrent drive and, of course, the and, of course, the physical arrangement of the array elements.physical arrangement of the array elements.
You could possibly “iterate” yourself to an approximate You could possibly “iterate” yourself to an approximate solution by twiddling voltage drivessolution by twiddling voltage drives
3939
NEC User Notes -Feeding of ArraysNEC User Notes -Feeding of Arrays
Solution:Solution: Current generators Current generators are realizable by high impedance, high voltage are realizable by high impedance, high voltage
series source:series source:
If you use in NEC, the large numbers used will swamp out theIf you use in NEC, the large numbers used will swamp out the drive segment voltage drive segment voltage and you won’t be able to use the resultsand you won’t be able to use the results
Can overcome this problem by replacing theCan overcome this problem by replacing the series resistor series resistor by an by an appropriate network but there is an easier method…..appropriate network but there is an easier method…..
AMP into circuits whose input impedance is <104
40404343
NEC User Notes -Feeding of ArraysNEC User Notes -Feeding of Arrays
Details:Details:
TheThe NT NT card(s) are used thusly:card(s) are used thusly:
Cards:Cards:
NT (Tag,NT (Tag, Seg Seg), 901, 1, 0, 0 ), 901, 1, 0, 0 0, 1 0, 0 0, 1 0, 0EX 0, 901, 1, 0, (j x FEED PT. CURRENT)EX 0, 901, 1, 0, (j x FEED PT. CURRENT)
Make sure these GW900’s do not interact with each other / squirt them off in all directionsMake sure these GW900’s do not interact with each other / squirt them off in all directions
(LATEST VIEWER ALLOWS YOU TO ELIMINATE A RANGE OF SEGMENTS FROM THE VIEW!)(LATEST VIEWER ALLOWS YOU TO ELIMINATE A RANGE OF SEGMENTS FROM THE VIEW!)
Far-WayFar-WaySegmentSegment }
Onesetperfeed
One feedOne feedsegmentsegmentof arrayof array
“Extra” added“Extra” addedsegment to supportsegment to supportthe generator. Usethe generator. UseGW 901, 1, …, orGW 901, 1, …, orsimilar large tagsimilar large tagnumber.number.
Set: Y11 = YA + YC = Y22 = 0
Y12 = YC = j I = I1 = jV
I1
VYA YB
YCI2
I
V1
V2
~
4141
NEC User Notes -Feeding of ArraysNEC User Notes -Feeding of Arrays
Process:Process: Set up enoughSet up enough GW900 GW900 cards for far-out feed segments, one for cards for far-out feed segments, one for
each array element. Make them very short w.r.t a wavelength so each array element. Make them very short w.r.t a wavelength so they will not radiate. Put them after any they will not radiate. Put them after any GS GS scaling to maximizescaling to maximizethe distance between dummy and actual geometry.the distance between dummy and actual geometry.
Add anAdd an NTNT card for connection between eachcard for connection between each GW900GW900 and its and its companioncompanion feed point segment.feed point segment.
Put the correctPut the correct current values current values on theon the EX0, 900 EX0, 900 cards to match the cards to match the array design.array design.
The input impedance at the feed points is in the The input impedance at the feed points is in the Network Excitation Network Excitation TableTable instead of under antenna input impedance. instead of under antenna input impedance.
Choose the dummies to be just one segment and set the radius so Choose the dummies to be just one segment and set the radius so /a /a 10 or more. 10 or more.
Process:Process: Set up enoughSet up enough GW900 GW900 cards for far-out feed segments, one for cards for far-out feed segments, one for
each array element. Make them very short w.r.t a wavelength so each array element. Make them very short w.r.t a wavelength so they will not radiate. Put them after any they will not radiate. Put them after any GS GS scaling to maximizescaling to maximizethe distance between dummy and actual geometry.the distance between dummy and actual geometry.
Add anAdd an NTNT card for connection between eachcard for connection between each GW900GW900 and its and its companioncompanion feed point segment.feed point segment.
Put the correctPut the correct current values current values on theon the EX0, 900 EX0, 900 cards to match the cards to match the array design.array design.
The input impedance at the feed points is in the The input impedance at the feed points is in the Network Excitation Network Excitation TableTable instead of under antenna input impedance. instead of under antenna input impedance.
Choose the dummies to be just one segment and set the radius so Choose the dummies to be just one segment and set the radius so /a /a 10 or more. 10 or more.
• Match points for both wires must lie outside the volumes
• Set a segment length limit to enforce this
5.0EK,with2or8and2or8
tansin2and
sintan2
2
2
1
1
212
211
aa
aaaa
:ALSO
:THUS
2/2
1/2
Wire 2 (Radius a2) = angle between wires
Match points from each wire atintersection
Wire 1 (Radius a1)
4545
Equivalent Radius for Non-Circular Cross-Sections
Equivalent Radius for Non-Circular Cross-Sections
Equivalent radiusEquivalent radius must lie between must lie between inscribedinscribed andand circumscribedcircumscribed circles which circles which bound the conductor boundary.bound the conductor boundary.
Best fit:Best fit: CirclesCircles formed with sameformed with same areaarea and and perimeterperimeter as the conductor boundary.as the conductor boundary.
Inner circle : Inner circle : Outer circle :Outer circle :
a Ai /
a P0 2 /
4646
Equivalent Radius for Non-Circular Cross-Sections
Equivalent Radius for Non-Circular Cross-Sections
Choose the mean:Choose the mean:
Aa
Pe
2
a
A P
e
22
47474747
Equivalent Radius for Non-Circular Cross-Sections
Equivalent Radius for Non-Circular Cross-Sections
• TRIANGLE:TRIANGLE: aaee = 0.425 = 0.425
• SQUARE:SQUARE: aaee = 0.65 = 0.65
• RECTANGLE OR STRIP:RECTANGLE OR STRIP: W/TW/T aaee
11 0.6 w0.6 w 2 2 0.44 w0.44 w 3 3 0.37 w0.37 w 5 5 0.32 w0.32 w 10 10 0.26 w0.26 w100100
Vary segmentation -- check Vary segmentation -- check convergenceconvergence CheckCheck reciprocityreciprocity Test for Test for average gainaverage gain Check grids vs.Check grids vs. patchespatches
SizeSize problem in wavelengthsproblem in wavelengths LocateLocate functional partsfunctional parts before modelingbefore modeling
Don’t forget the coupling toDon’t forget the coupling to baluns, etc.baluns, etc. bybynear fieldsnear fields
4949
Modeling Guidelines Modeling Guidelines
Always exploitAlways exploit symmetrysymmetry for large problemsfor large problems Model theModel the radiatorsradiators firstfirst
Check the literatureCheck the literature Duplicate literature before approaching the full Duplicate literature before approaching the full
problemproblem Strip out details/simplify structureStrip out details/simplify structure
Transmission lines and connectionsTransmission lines and connections Supporting structuresSupporting structures Environmental interactionsEnvironmental interactions
5050
Modeling Guidelines Modeling Guidelines
Wire grid modelingWire grid modeling Outline cornersOutline corners Grid size approximately 0.1 Grid size approximately 0.1 nominal nominal Try “equal area” ruleTry “equal area” rule Try two segments/sideTry two segments/side Minimize Minimize and and aa changes (< 2:1) at key changes (< 2:1) at key
junctions (near feedpoints)junctions (near feedpoints) Use denser gridding (2x) at connection points Use denser gridding (2x) at connection points
of of wireswires and and surfacessurfaces
5151
Modeling Guidelines Modeling Guidelines
Surface patch modelingSurface patch modeling Make sure surface is Make sure surface is closedclosed MaximumMaximum patch sizepatch size: (0.2 : (0.2 0.2 0.2 )) AvoidAvoid long narrow patches long narrow patches UseUse large patcheslarge patches on smooth surfaces:on smooth surfaces:
smaller patchessmaller patches on curved areason curved areas
Consider possible problem areasConsider possible problem areas Sharp bends in thick wiresSharp bends in thick wires Changes in wire radiusChanges in wire radius Wires connected to lossy groundWires connected to lossy ground Wires too thick?Wires too thick?
5353
Modeling Guidelines Modeling Guidelines
DetermineDetermine number of segmentsnumber of segments neededneeded < 0.1 in most cases< 0.1 in most cases too small? Low frequency limittoo small? Low frequency limit too small? Pencil lead vs. poker chipstoo small? Pencil lead vs. poker chips