Design and Analysis of FSS Radomes

Design and Analysis of FSS Radomes

Gopinath Gampala and C. J. Reddy*

Copyright © 2014 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Outline

• Introduction

• Radome Analysis Methods

• Radome Design Process

• Characterizing FSS Radome Layered Configuration

• Impedance Sheet Approximation

• Conclusion

FDTD

FEM

MOM

Hybridisation to

solve large and

complex problems

MLFMM

EL

EC

TR

ICA

L S

IZE

COMPLEXITY OF MATERIALS

PO/GO

UTD


Radome

• Radome (Radar Dome) is a Structural, Weatherproof Enclosure that

Protects an Antenna/Radar System

• Radome Should Minimally Attenuate the Electromagnetic

Signal Transmitted or Received by Antenna/Radar System

• Radomes Can be Constructed in Several Shapes

Depending on the APPLICATION


Applications

Aircraft

Missile

Maritime

Vehicular

Telecom

SatcomAir Traffic Control

Weather


Radome Wall Types

• Radomes can be Classified into Monolithic and Sandwich Designs,

based on Wall Construction

Monolithic

A-Sandwich: ε1 > ε2

B-Sandwich: ε1 < ε2

C-Sandwich: ε1 > ε2

A or B

ε1 ε1 ε2

C

ε1 ε1 ε1 ε2 ε2

Dennis J. Kozakoff, Analysis of Radome-Enclosed Antennas,

Boston: Artech House, Inc., 1997

Sandwich Design


Radome-Induced Effects

• Radome Changes the Electrical Performance of Antenna Because of

Following Factors:

— Dissipative Losses Within a Dielectric Material

— Electrical Phase Shifts Introduced by Presence of Radome

— Internal Reflections

• The Changes (above) Produced by Radome Manifest Themselves

into Radome Performance Parameters

— Radome Insertion Loss (Radome Loss)

— Increased Antenna Sidelobe Levels

— Boresight Error (BSE)




FDTD

CEM Methods in FEKO

Methods for

Geometrically

Complex

Problems

FEM

MOM

Hybridisation to

solve large and

complex

problems

Methods for

Electrically

Large

ProblemsMLFMM

EL

EC

TR

ICA

L S

IZE

COMPLEXITY OF MATERIALS

PO/GO

UTD


Radome Analysis Techniques

• FEKO Offers Several Numerical Methods to Analyze the Radomes

for Radome-Induced Effects

— MoM and FEM for Electrically Small Radomes

— MLFMM and RL-GO for Electrically Large Radomes

— TDS Approximation for Thin Walled Radomes

• TDS can be used in combination with MoM, MLFMM

and RL-GO

MoM : Method of Moments

FEM : Finite Element Method

TDS : Thin Dielectric Sheet

MLFMM : Multi-Level Fast Multipole Method

RL-GO : Ray-Launching Geometrical Optics


Specialty Radomes

• There is More to the Types of Radomes than Just Dielectric-Layered

Wall Radomes

— Frequency Selective Surface (FSS) Radome is One Such Specialty

Radome

• FSS Radome Includes a Curved FSS within a

Multilayered Dielectric Wall Radome

— FSSs are Periodic Surfaces in Two Dimensions

— FSSs Have Inherent Filtering Characteristics

• FSS Radomes are Useful to Reduce RCS Over a

Wide Frequency Range Due to Bandpass/Bandstop

Characteristics


Radome Design Process

• The First Step in Designing a Radome Would be the

Order and Choice of Radome Wall Layer Materials

— The Radome Wall Materials Can be characterized

Through the Transmission Loss Analysis

• Next Step in the Design Process Would be Building

the Complete Radome and Analyze for Radome

Induced Effects

— Wall Layer Configuration with Optimal Transmission

Performance is Used to Build the Complete Radome

— Radome Induced Effects are Computed with the

Radome Protecting the Antenna it is Built for


Radome Wall Material Characterization

• FEKO Offers Two Fast and Accurate Methods to Calculate the

Transmission Loss Through the Radome Layers

Planar Green’s Functions Periodic Boundary Conditions

– Planar Green’s Functions

• No Unknowns

• Highly Efficient i.t.o. Both Time and Memory

– Periodic Boundary Conditions

• Infinite Layers Realized with Single Small Unit Cell

• Huge Reduction in No. of Unknowns


Special (Planar) Green’s Function Example

• Consider an A-Sandwich Radome (Style-c)

• Layer Configuration

Quartz Polycyanate Skins

εr = 3.23

tan δ = 0.016

h = 0.0762 cm

Phenolic Honeycomb Core

εr = 1.10

tan δ = 0.001

h = 1.0160 cm



Radome Layers Designed using Planar

Green’s Functions


FEKO vs Literature

Transmission Loss at Different

Incidence Angles

FEKO Agrees Very Well With

Published Results




SGF vs PBC

• Consider a Monolithic Thin-Walled Radome

• Layer Configuration

Dielectric Constant = 4

Loss Tangent = 0.015



Radome Layer Designed using

PBC

Radome Layer Designed using

SGF


SGF vs PBC

Both the Methods Agree with Each Other

SGF – Special (Planar) Green’s Functions

PBC – Periodic Boundary Conditions

Wall Thickness = 0.1 λ Wall Thickness = 0.2 λ


X-Band FSS Radome Characterization

• A-Sandwich Radome FSS Radome

—Jerusalem Cross FSS Layer in the Middle of the Core of A-Sandwich

Configuration

• A-Sandwich Layer Configuration

— 0. 319 mm Thick Quartz Polycyanate Skins

• εr = 4.8, tan δ = 0.0002

— 1.595 mm Thick Phenolic Honeycomb Core

• εr = 1.3, tan δ = 0.001

• FSS Layered Configuration Can be Characterized Through Transmission

Loss Analysis

— PBC is the Best Approach to Characterize FSS Layered Configuration


Pass Band FSS Radome

• The FSS Radome Was Designed for X-Band (9.4 GHz) Radar Applications

— The A-Sandwich FSS Configuration Has a Passband Centered at 9.4 GHz


Full Radome Analysis

• Next in Design Process is Building a Full 3D Model of the Radome to

Analyze for Radome Induced Effects

• A Radome with All Explicit Layers is a Computational Nightmare

— The Analysis is Near-impossible if the Layers are Electrically Too Thin

• TDS Approach Won’t Account for the FSS Layer

• FEKO Offer an Impedance Sheet Approximation Solution


Impedance Sheet Approximation

• FEKO Offers the Ability to Define a Frequency-Dependent Impedance

Sheet

— LUA Scripting is Integrated into FEKO

— Use the Script to Convert Transmission/Reflection Coefficients into

Impedance Parameters

— Use the Impedance Parameters to Define an Impedance Sheet in Place of

Explicit Radome Layers

• Huge Reduction in Number of Unknowns → Tremendous Savings in

Computational Resources

LUA Script

Τ, Γ → Z


Explicit Layers vs. Impedance Sheet

• The Impedance Sheet Approximation is as Accurate as Modelling Explicit Layers

• Computational Resources:

— Explicit Layers: 1.7 GB of Memory and 14.5 hours Run-time

— Impedance Sheet: 2.4 MB of Memory and 0.1 hours Run-time


Radome Induced Effects at 9.4 GHz

• A Nosecone Shaped Radome

Protecting the Vivaldi Antenna

— The Full 3D Radome is Built Using the

Impedance Sheet Approximation

• As Intended, the Radome is

Completely Transparent to the RF

Signal at the Center of Pass Band

• The Sidelobe Levels are Unaffected

• Negligible Boresight Error


Radome Induced Effects at 12.5 GHz

• At 12.5 GHz (Outside the Passband),

the Radome Rejects Most of the

Radiation

• This Characteristic is Extremely Useful

in Reducing the Out-of-Band RCS


Conclusion

• FEKO Offers Various Solvers for the Design and Analysis of

Radomes

• Periodic Boundary Conditions are Extremely Useful in Characterizing

a Layered FSS Radome Configuration

• The Impedance Sheet Approximation is an Efficient and Accurate

Solution to an Otherwise Complex Thin-Walled FSS Radome

Analysis

Design and Analysis of FSS Radomes

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