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Computer Modelingof Wideband Tapered‐SlotMicrowave Antenna FeedsMicrowave Antenna Feeds
S t b 19 2007September 19, 2007
Gabe A. Cohn
OverviewOverview
• Background on the Lindgren Feedg g• Goals of Computer Modeling• Modeling Software• Modeling the Lindgren Feed
– Return LossF fi ld P tt– Far‐field Patterns
• Postprocessing Calculations– Phase Center VariationPhase Center Variation– Taper and Spillover Efficiencies
• Conclusions and Future Work
2
3
4
Goals of Computer ModelingGoals of Computer Modeling
• Increase Efficiency and Reduce Noisec ease c e cy a d educe o se
• Don’t want to just starting making modificationsDon t want to just starting making modifications to the actual feed
• Would like to simulate the effect of proposed p pchanges
• Allow fast and simple testing of design improvements
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Modeling Using QuickWaveModeling Using QuickWave
Features:• Finite Different Time Domain (FDTD) solver• Conformal polygon mesh
Advantages:Advantages:• Very fast solver and postprocessing• Complete control of mesh
Disadvantages:Disadvantages:• Many bugs! (crashes way too much)• Very poor UI and documentation• Drawing directly linked to the mesh• Drawing must be made using code which• Drawing must be made using code which
builds the mesh as well• Modifications are very difficult to make• Can’t model large structures• Can’t obtain good return loss data• Can t obtain good return loss data
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Modeling Using CST Microwave StudioModeling Using CST Microwave Studio
Features:• Contains three solvers:Contains three solvers:
– Transient Solver (Time Domain)– Frequency Domain Solver– Eigenmode Solver
• Hexagonal and tetrahedral mesh• Supports Adaptive Subgridding• Automatic meshing utilities (with many options)• Built‐in postprocessing calculations (including phase
center)• Built‐in optimizer can sweep parameters to find
optimal
Advantages:• Very simple and powerful UI with good
documentation• Very stable• The structure drawing is completely separate fromThe structure drawing is completely separate from
the mesh and therefore each can be modified independently
Disadvantages:• Slow extraction of far‐field patterns• Subgridding is very resource intensive
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Time Comparison of Modeling fSoftware
Time to… QuickWave CST MWS
Become Familiar with Software
Weeks Days
Create a new structure Days 1 hour
Modify the mesh <10 min Seconds (hours when using subgridding)
Run full simulation of 1‐2 hrs. 1‐3 hrs.Lindgren Feed
Export far‐field patterns with 5 degrees azimuthal
10 hrs. 50 hrs.
resolution
Total (excluding becoming familiar with software)
3 days 3 days
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Modeling the Lindgren Feed
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Model of Lindgren FeedModel of Lindgren Feed
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Good Agreement Verifies Model
‐2
0
|S11| Comparison for Feed OnlydB
‐8
‐6
‐4
‐12
‐10
8
‐18
‐16
‐14
‐20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Frequency (GHz)
Measured Data CST Model
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Lindgren Feed without Radiation ShieldMeasured Data CST MWS Model Simulation
EPlane
HHPlane
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Data and Model ComparisonLindgren Feed without Radiation Shield
4 GH 11 GH4 GHz 11 GHz
EPlane
HHPlane
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Feed Inside Radiation ShieldFeed Inside Radiation Shield
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Lindgren Feed in Radiation ShieldMeasured Data CST MWS Model Simulation
EPlane
HHPlane
15
4 GH 11 GH
Data and Model ComparisonLindgren Feed in Radiation Shield
4 GHz 11 GHz
EPlane
HHPlane
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Feed in Radiation Shield Lined with AN‐73 Mi Ab bMicrowave Absorber
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Modeling Microwave AbsorberModeling Microwave Absorber
• Need the Following Electrical Properties:Need the Following Electrical Properties:– εr – Relative Permittivity
μ Relative Permeability– μr – Relative Permeability
– σ – Electrical Conductivity [S/m]
M ti C d ti it [Ω/ ]– σm – Magnetic Conductivity [Ω/m]
• This data is not provided by manufacturer
• Guessed parameters using a test simulation:εr = 5 μr = 5 σ = 1.2 S/m σm = 1.2 Ω/m
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Lindgren Feed in Absorber Lined Radiation Shield
Measured Data CST MWS Model Simulation
EPlane
HHPlane
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4 GH 11 GH
Data and Model ComparisonLindgren Feed in Absorber Lined Radiation Shield
4 GHz 11 GHz
EPlane
HHPlane
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Phase Center Variation
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Phase Center Variation ComparisonPhase Center Variation ComparisonPhase Center of Lindgren 3164‐05 Feed
Based on CST MWS Model
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17
18
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[cm]
11
12
13
14
15
ve th
e feed
point)
7
8
9
10
11
Phase Ce
nter (a
bov
4
5
6
7
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
P
Frequency [GHz]
Feed Only Feed in Radiation Shield Feed in Absorber Lined Radiation Shield
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Taper and Spillover Efficiencies
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Efficiency Calculations
Taper efficiency – uniformity of the amplitude distribution of
Efficiency Calculations
the feed pattern over the surface of the reflector
Spillover efficiency – fraction of the total radiated power that i ll t d b th fl tis collected by the reflector
The product of the two efficiencies is then easily calculated:
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Spillover and Taper Efficiencies3164‐05 Feed Only
80
90
100
60
70
80
%]
40
50
Efficiency [%
10
20
30
0
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
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2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
Spillover Eff.
Taper Eff.
Product of Spillover and Taper Efficiencies3164‐05 Feed Only
80
90
100
60
70
80
]
40
50
Efficiency [%
]
20
30
0
10
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
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Subtended Half‐Angle (theta) [deg]
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
Spillover and Taper Efficiencies3164‐05 Feed in Radiation Shield
80
90
100
60
70
80
%]
40
50
Efficiency [%
10
20
30
0
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
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2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
Spillover Eff.
Taper Eff.
Product of Spillover and Taper Efficiencies3164‐05 Feed in Radiation Shield
80
90
100
60
70
80
]
40
50
Efficiency [%
]
20
30
0
10
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
28
Subtended Half‐Angle (theta) [deg]
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
Spillover and Taper Efficiencies3164‐05 Feed in Absorber Lined Radiation Shield
80
90
100
60
70
80
%]
40
50
Efficiency [%
10
20
30
0
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
29
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
Spillover Eff.
Taper Eff.
Product of Spillover and Taper Efficiencies3164‐05 Feed in Absorber Lined Radiation Shield
80
90
100
60
70
80
]
40
50
Efficiency [%
]
20
30
0
10
0 10 20 30 40 50 60 70 80 90
Subtended Half‐Angle (theta) [deg]
30
Subtended Half‐Angle (theta) [deg]
2.2 GHz 3.0 GHz 4.0 GHz 8.4 GHz 11 GHz 14 GHz 18 GHz
ConclusionsConclusions
• Can successfully model wideband feeds in either yQuickWave or CST Microwave Studio
• Highly recommend using CST MWS• The CST model of the Lindgren 3164‐05 Feed appears to be an accurate representation
• Can obtain return loss far field patterns phase• Can obtain return loss, far‐field patterns, phase center, taper efficiency, and spillover efficiency from a simulated model
• Simulated models can be used to predict the effect or proposed changes
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Future WorkFuture Work
• Need a better model of absorber material– Need to take measurements on the absorber materials that we
have in order to get params.• Improve Return Loss
– Change backshort, gap dimensions or fin shape• Alternatives with absorber
– Absorber strips on fin edgesAbsorber strips on fin edges– Absorber strips around cylinder– Absorber in backshort– Using different types of absorberUsing different types of absorber
• Cuts in fins– Slot to reduce back edge current
Different shape to change low frequency characteristics– Different shape to change low frequency characteristics
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