1. By Neil Kruger Supervisor: Prof. KD Palmer University of Stellenbosch
Chaff RCS Modelling
By Neil Kruger
Supervisor: Prof. KD Palmer
University of Stellenbosch

2. Introduction Chaff background Chaff cloud characteristics
Using Matlab & FEKO
Modelling development & results
Dipole RCS Characteristics
Dipole RCS Simulation
Conclusion
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3. Chaff Background Chaff was used for the first time during World War II
Though numerous research has been done on Chaff since WWII, it still is effective as a passive countermeasure against enemy radar
Chaff consist of very thin dipoles cut to resonant length
With different dipole lengths a larger radar bandwidth can be covered
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4. Chaff Background
Dispensed in the atmosphere to form a cloud of scatterers
Dispensed by dropping or firing from ships, aircraft and vehicles.
The technique used determines the intension, but overall the purpose of chaff is to mask the radar target
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5. Chaff Cloud Characteristics
A number of factors influence the RCS a chaff cloud
Aerodynamics solution (coordinates):
Initial conditions
Method of dispersion
Fall speed
Electromagnetic solution (scattering):
Dipole characteristics
Coupling between dipoles
Masking within the chaff cloud
With reasonable assumptions modelling can be simplified
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6. Using Matlab & FEKO
Matlab code previously developed was modified & expanded for the RCS modelling
Chaff is generated in Matlab from which FEKO is called up for simulation, Matlab then extracts the data for processing and interpretation
Symmetry is used to speed up calculation since chaff modelling is approached statistically.
Due to processing capabilities of hardware and software the number of chaff elements is still quite limited
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7. Modelling development & results
Spherical Chaff Cloud
Top view
Right view
3D Chaff Sphere
Front view
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8. Modelling development & results
Creating a sphere of randomly orientated and uniformly distributed dipoles
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9. Modelling development & results
RCS of a sphere
Fig 2.9, Skolnik, Introduction to radar systems
2πa/λ
= 39
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10. Modelling development & results
Determining average normalized RCS for a spherical chaff cloud as density increases to pieces
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11. Modelling development & results
Spherical chaff cloud; Correct model vs. Error model
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12. Modelling development & results
Direction of
Incident Field
One of the effects seen in
chaff clouds are shielding
within the chaff cloud self
Polarization is linear and
perpendicular to the
direction of incidence
1000 pieces
Smallest current -44.1dBA
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13. Modelling development & results
Direction of
Incident Field
5000 pieces pieces
-56.2 dBA dBA
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14. Modelling development & results
E-field line Points through the spherical region (Near-field)
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15. Modelling development & results
E-field line through the spherical region (Near-field)
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16. Dipole RCS Characteristics
The average RCS of a random orientated dipole was investigated through theory and simulations
From theory the average value differs between 0.15λ² and 0.28λ² depending on approach used.
Further literature study grouped these values as below
0.15λ² λ² for a dipole uniformly distributed over a sphere
0.27λ² λ² for a dipole uniformly distributed over a disc
0.22λ² is the value associated with the Scattering Cross Section
For SCS the polarization is not taken into account
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17. Dipole RCS Simulation
Geometry of dipole in space defined by dipole length and theta & phi angle
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18. Dipole RCS Simulation Uniform distributed over Disc (in angle) Sphere
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19. Dipole RCS Simulation
In determining the average RCS of a dipole the first approach led to the uniform angle distributed RCS value of 0.28λ²
From a uniform spherical distributed simulation the average RCS value was found to be 0.187λ²
Value was verified for incidence on
the x-axis and z-axis
The reason for the larger than expected value is possibly due to a finite thickness of the dipole element
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20. Dipole RCS Simulation Confirming the Cos^2 relationship with Ø = 90°
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21. Dipole RCS Simulation Ø = 0°, no cos^2 relationship
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22. Dipole RCS Simulation
The numerical simulation data has been compared with mathematical derivations from literature
The average RCS values has been found to agree quite well with the literature
The individual RCS values however did not agree
This is the current status of investigation…
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23. Conclusion
The angular average RCS of a dipole has been computed and compared to literature
The individual angle dipole RCS and coupling need to be investigated to determine assumptions that can simplify modelling
Realistic chaff positioning and orientation must be incorporated in modelling to make it applicable
Practical effects like “bird nesting” & shielding within the chaff cloud need investigation to make modelling accurate
© CSIR