L RD-R169 M AN INTRODUCTION TO RRDAR RBSORDENT MATERIALS ( )U) ROYAL SIGNALS AND RADAR ESTRBLISHNENT NALVERN (ENGLAND) I P G LEDERER FEB 86 RSRE-85916 DRIC-R-99572 UNCLASSIFIED F/G 01/9 Ill II 3.6~ 1A L III" I-I" IB~ ' 11111.25 11'- u111111. .4 -III MICROCOPY RESOLUTION TEST CHART N.TO.AL A O35F1RA NOAROS- 91 -- p. ,,,. "°? -.',.**:.... -:, - - ." . . ~.- -. .-,' - .-. . . . . **.' .. *, '.,--'. .-.. -. . .." " ". : " ,. , ' ., .,. ,? .. -~~~- - r~ -7 - - - - UNLIMITED803z loll Report No. 85016 * co ROYAL SIGNALS AND RADAR ESTABLISHMENT, zMALVERN * - 0 Lfl * 00 O(n AN INTRODUCTION TO RADAR ABSORBENT MATERIALS (RAM) Author: P G Lederer RSRE I i 111 Worcestershire. RSe'm g7': 3 /5 ~ ~ ~ F 1986' Fbiua a, UNLIMITED 8 6 7 15 103 ROYAL SIGNAtS ANID RADARESTABLISHMENT Report No 85016 TITLE: AN INTRODUCTION TO RADAR ABSORBENT MATERIALS (RAM) AUTHOR: P G Lederer DATE: February 1986 :-I. SUMMARY The purpose of this Introduction is to present, in a straightforward way, the electromagnetic principles of Radar Absorbent Materials (RAM) for the benefit of the non-electromagnetic-specialist who finds himself involved in this field. The fundamental theory of electromagnetic wave propagation in media and at the interfaces between different media is reviewed and the various approaches to absorber design are described in the light of this. The types of materials required and the techniques - for measuring both their electromagnetic properties and the performance of the finished abosrber are outlined. Finally a means of designing absorbers from a knowledge of the properties of its constituent materials is described. J . - - Copyright , . Controller HMSO London 1989 S83/16 CONTENTS 1. INTRODUCTION 2. ELECTROMAGNETIC FUNDAMENTALS 2.1 Material Properties 2.2 Reflection and Refraction at Boundaries and Impedance Matching 2.3 Finite Layers 3. ABSORBER TYPES 3.1 General Requirements 3.2 Pyramidal Absorbers 3.3 Tapered Loading 3.4 Matching Layer 3.5 Tuned Layer 3.6 Salisbury Screen 3.7 Techniques for Improving Bandwidth 4. MATERIALS FOR RAM 4.1 Composite RAM 4.2 Binder Materials 4.3 Lossy Fillers 4.4 Ferrites 5. MEASUREMENTS 5.1 Reflectivity Measurement 5.2 Material Constant Measurement 5.3 Transmission Line Methods 5.4 Time Domain Methods 6. ABSORBER DESIGN 6.1 Transmission Line Model 6.2 Matrix Elements 6.3 Oblique Incidence 6.4 Design of Absorbers 7. REFERENCES APPENDICES: A.1 Effective Impedance of Multilaver Structures A.2 Propagation and Wavelength in Lossy Media A.3 Dielectric Slab Sandwiched Between Two Media A.4 The Decibel (dB) for Expressing Reflectivitv C. °° - 7_ -W. 1. INTRODUCTION .4 There are many circumstances in which it is necessary to control the reflection of microwave radiation from surfaces. For example, in order that a microwave anechoic chamber may fulfil its purpose of simulating free space in a confined volume, its walls must be rendered non-reflecting by lining them with an electromagnetically absorbent material. The severely limited performance of many surveillance radar systems, particularly those at sea, due to very strong return signals from -e.arby objects such as masts, * buildings or bLidges may be improved by coating these objects with an absorbent layer. A similar approach is being used to solve the problem of television 'ghost images' in Japanese cities due to multiple reflections * of the broadcast signal by nearby buildings. Nor has the potential contribution of microwave absorbing materials to the reduction in the detectability of targets by radars escaped the notice of the military planners, as recent press reports on the American 'Stealth' programme bear witness. The electromagnetic demands placed on radar absorbent materials, or RAM, *as it usually called, vary according to the specific application, and there is no panacea capable of universal use. The lining of an anechoic chamber, for example, must provide very high levels of energy absorption over a wide frequency range whereas radar obstructions and television ghost images * require treatment only at a single frequency. In the military field, wideband coverage is generally required but environmental conditions and restrictions on size and weight are much more severe. The design of absorbers, therefore, must be tailored to each application, and this requires an understanding of the factors which can influence performance. Most absorbers rely on the bulk electromagnetic properties of materials, but, at microwave frequencies, the wavelength of the radiation is of the same order a- the dimensions of absorbing structures (a few mm to a few cm) * and, crc.i,,equently, geometrical factors also play an important role. A practical electromagnetic absorber must satisfy two main requirements- it must reduce the reflection of incident radiation by the specified degree over the frequency band of interest and it must achieve this - and continue( tO * achieve it throughout its life - in its operational environment. Environmental conditions may range from the benignity of an anechoic *chamber to the ferocity of the external surfaces of a supersonic jet aircraft, *while mechanical constraints may demand anything from a simple self-supporting layer to a full load-bearing member. it is, therefore, clear that this is a cross-disciplinary field calling on expertise both in the structural and chemical integrity of materials and in their electromagnetic properties. - Unfortunately these are very diverse disciplines so that the majority of * chemists and materials technologists have little or no background in electro- magnetics and vice versa. The problem, therefore, tends to divide natural ly *into its technological and electrical aspects, but a successful design must *involve a collaboration between the two. The purpose of this Introduction is to help to improve the communication process by presenting the electromagnetic aspects of microwave absorbers in a straightforward manner for the benefit of the materials technologist. * The emphasis is on principles rather than a full and detailed treatment, but references are provided to point the way towards deeper study. Firstly, the electromagnetic fundamentals are briefly reviewed; a fuller explanation being given in any one of a number of standard text books of which (1a) is particularly recommended as it has been written very much from a materials * point of view. In chapter 3, the various types of absorber are explained in terms of these ideas, and the types of material that exhibit the desired properties are discussed in chapter 4. Chapter 5 reviews the techniques * available for measuring both the reflectivity of absorbers and the electro- magnetic parameters of materials. Finally, in chapter 6, the topics of * previous chapters are tied together in considering the electromagnetic design. * of absorbers. 2 e, PP% . 2.1 Material Properties In general, the electric and magnetic properties of a dielectric material are characterised by the complex permittivity, C and the complex permeability, p. *-e =t =pt - jp (2.1 or more frequently, by the relative permittivity, E and relative ,l permeability, p - E, El r- r~~ It (2.2) The universal constants, E and pot are respectively the permittivity and 0 permeability of free space and are related by cp 1/c2 = 0 0 where c is the velocity of electromagnetic propagation in free space and P is defined as 47r x 10 -7 H m-1 The real part of the permittivity (or permeability) is a measure of the extent to which the material will be polarised (or magnetised) by the application of an electric (or magnetic) field. The imaginary part is a measure of the energy losses incurred in re-arranging the alignment of the electric (or magnetic) dipoles in that applied field. If an ac electric field is applied across a dielectric slab (as in a capacitor), an ac "displacement" current is observed which is due to the oscillation of the electric dipoles with the field. The dielectric, therefore, possesses an ac conductivity, which is quite independent of any dc conductivity (due to the migration of free charge carriers) that may also exist. The relation- ship between the total conductivity (ac+ dc), a, and the dielectric loss is() 3 I ,,1 (2.3) where w is the angular frequency of the applied field. If a plane electromagnetic wave, of angular frequency, w, is propagating in the x-direction through such a dielectric material, the electric and magnetic field vectors, which are normal to each other and to the direction of propagation, are given as functions of time and space, by: E E exp(jwt-yx) (2.4) H H exp(jwt- yx) where y is the complex propagation factor, p. -Y a+ jB = jw(c . * (2.5) The real and imaginary parts of y define the way in which the amplitude and phase respectively of the wave vary with distance of propagation, a being called the attenuation factor and B the phase factor, given by B 2r (2.6) It is possible to define, in direct analogy to Ohm's Law, a wave impedance, as the ratio of the electric to magnetic fields, and since the E and H components of the wave are uniquely related to each other, this impedance, the intrinsic impedance, Z, is dependent upon the electromagnetic material constants, and is therefore a characteristic of the material: - .5 E Z =-=(2.7) The normalised intrinsic impedance, relative to the intrinsic impedance of free space, Z, where 4 01-I Z o~c (C 0)* is therefore z ~. .4S 2z Rflctonan Rfrcto (2.8) .. 2.2 Reflection and Refraction at Boundaries and Impedance Matching When a plane electromagnetic wave is propagating through medium 1 and is incident upon a second medium, medium 2, as shown in Figure 2.1, part of the energy is reflected at the interface and part is transmitted . . (refracted), the process being completely described by Snell's and Fresnel's Laws. If the incident electric and magnetic fields are E. and H. respec- tively, then the reflected components will be E and H and the transmitted r r components E and H . These are related by t t E. +E (a) 1 r t-.- (2.9) H. + H = H (b) 1 r t However, since electric and magnetic fields are related by the intrinsic impedances of the medium according to equation (2.7), it follows that E. , ZE 1 r = EZ t-C-c2(C) (2.9) 1 1 2 The negative sign associated with the reflected component arises as a result of the reversal of the direction of propagation. If the electric field reflection coefficient, r, is defined by E r E.' (2.10) 1 lit. .5