The Study on Stealth Design Technology
Abstract—In this report we study some basic ideas on stealth technology. Loosely speaking, the stealth technology can be divided into two categories: the study on target shape design and the study on radar absorbing materials. In this report, we explain the basic principles of these two categories, and give some simulations by CST Microwave Studio and Feko to show the effects of stealth technology.

I. I NTRODUCTION The purpose of stealth design is to protect the target from being detected by the enemy’s radar and to increase the target survivability. Ideally, for an object to be ”stealth”, it needs to have a low optical visibility in addition to being low observable in the infrared spectrum and at all radar frequency bands, also the emission of acoustic noise should be low. However, this goal is difficult to achieve, and the recent research is mainly focus on the low observability in a certain radar bandwidth. In some degree, the term ”stealth design” is equivalent to the technology of reducing target’s the Radar Cross Section (RCS). RCS is the parameter to characterize the strength of backward scattering of the target, the formal definition is σ = lim 4πR2
R−>∞

Fig. 1: B-2 stealth bomber

|Es |2 , |Ei |2

(1)

Fig. 2: F117 stealth fighter plane

where, R is the distance from target to radar, |Es | is the scattered electric field intensity and |Ei | is the incident electric field intensity. Radar equation can be used to estimate the power range of radar. It is given as R= Pt Gt Ar σ (4π)2 Pr,min
1/4

,

(2)

where Pt is the transmission power, Gt is the gain of the transmitting antenna, Ar is the effective aperture of the receiving antenna, Pr,min is the minimal receiving power, R is the power range. From the equation we can see, the RCS is the only parameter can be controlled by the target, and the reduction of RCS by an order will reduce the detection range by 44%. Some typical examples of the stealth design include F-22 stealth fighter plane, F-117 stealth fighter planes, B-2 stealth bomber, sea shadow stealth vehicle, etc. Fig. 1 Fig. 3 are pictures of F22, F117 and sea shadow, respectively. Generally speaking, the research on stealth design can be divided into two categories: the target shape design and the research on radar absorbing materials. Each of these two categories includes a board range. The low-reflection shaping requirements have strong negative influence on the aircraft’s aerodynamic properties. There are two distinctly different approaches to establishing the overall shape of a stealth object: 1) by adopting a compact, smooth blended external geometry. This technique is exemplified by the Northrop B-2. 2) By

employing a faceted configuration, using flat surfaces arranged to minimize normal reflections back toward the illuminating radar and eliminate glint. The Lockheed-Martin F-117A adopted this design. The design of radar absorber materials also has two major distinct ways: 1) by admitting the signal and then attenuating its intensity. This type has a wide range of radar frequencies but is usually heavy and expensive. 2) By generating internal reflections which interfere with the waves reflected from the outer surface. It is called resonant radar absorber material because it is only effective at a number of discrete frequencies. The remainder of the paper is organized as follows. The scatter mechanism is briefly discussed in Section II. Section III presents some discussion on the effect of faceted design. Section IV discusses the resonant radar absorber material; and

Fig. 3: Sea Shadow

in Section V, we offer some concluding remarks. II. S CATTER M ECHANISM We focus the microwave bandwidth radar, at which frequency, the wave length is usually much smaller than the target; therefore, the RCS of the target can be discussed under the optical region. When the EM wave that hits on the target are scattered by a variety of mechanisms: 1) Specular: Specular wave scattering is essentially a reflection of the incoming wave. The main contribution arises when the Poynting vector of the incoming wave vector is perpendicular to the local surface. 2) Diffraction: Diffraction occurs when there is a discontinuity in the target geometry or a discontinuity in the electromagnetic material properties of the object. 3) Diffracted surface waves: A surface wave may result when the incoming wave is more or less aligned along the length of a long thin coated body. The scattering arises when the surface wave encounters surface discontinuities, the end of the body or changes in the electromagnetic properties of the surface of the body. 4) Radiation from creeping waves: When the surface which supports a surface wave makes a gentle bend in the longitudinal plane of the surface wave, the surface wave will convert into an attenuated creeping wave that continues to follow the surface and space waves that radiate away from the surface. The first two mechanisms are much larger than the other two. In plane, the empennage, wings and the head easily evolve the specular reflection, as shown in Fig. 4 and 5.

Fig. 6: the simulation model of dihedral

Fig. 7: the mono-static RCS of the dihedral Diffraction is another major source for target’s RCS. Fig. 8 shows the surface current of NASA almond model illuminated by a plane wave. We can see the current density is especially high at the tip of the model, which will induce a large diffraction. This explains why many stealth planes adopt a compact, smooth blended external geometry to avoid sharp edges and tips.

Fig. 4: the specular reflection on the head of airplane

Fig. 5: the effective dihedral reflector on the empennage The following is a simulation of Dihedral reflector by FEKO. Fig.6 is the dihedral model and Fig. 7 is the monostatic RCS simulation result. In the simulation, the elevation is 0 degree, and the azimuth angle is from −15 to 105 degree. The two PEC plates are perpendicular to each other. For this dihedral, it is easy to see the reflection wave is always parallel to the incident wave; therefore, in the whole area from 0 to 90, the monostatic RCS main a large value.

Fig. 8: the surface current of NASA almond model

III. TARGET S HAPE D ESIGN In this section, we consider the small faceted stealth design. F117 is a typical example. Fig. 2 is a picture of F117, from which we can see it compose of many small facet, these small face located in certain angle to avoid the backward reflection. Fig. 9 is an illustration of this principle.

Fig. 9: the reflection on the head of F117 A group of simulation has been conducted to illustrate the effect of this design. The author builds a simple model for F117 (shown in Fig. 10) and a model for conventional airplane (shown in Fig. 11). Fig. 12 and 13 shows the simulation results by CST MWS. Notice the F117 model has much smaller backward reflection than the conventional one.

Fig. 12: The reflection property of F117

Fig. 13: The reflection property of the conventional airplane Fig. 10: a simple CST model of F117 If Rs = 377Ω, there will be no reflection from the Salisbury screen.

Fig. 11: a simple CST model of conventional airplane

IV. R ADAR A BSORBER M ATERIALS We discuss the resonant radar absorber materials. The most simple one is called Salisbury screen (shown in Fig. 14), which invented in 1952, named after the American engineer Winfield Salisbury. It is formed by placing a resistive sheet above the objects surface in order to create a resonant absorber. If the resistive film is λ/4 from the PEC, then a quarter-wavelength transmission line section transforms the short circuit at the conductor plate into an open circuit, i.e., 1 1 1 1 = + = . Zin Rs ∞ Rs (3)

Fig. 14: salisbury screen The following is the simulation of Salisbury screen by FEKO. Fig. 16 shows the comparison of Salisbury screen and PEC. The size of salisbury screen and PEC are both 4λ × 4λ square. The plane wave is normal incident, and the far field monitor is set to recode the reflection from −90 to 90 degree. From the simulation result, we can see the bi-static RCS of the Salisbury screen is much smaller than the PEC. The single layer Salisbury screen can only beat one single frequency. The bandwidth and the incident angle are relative

Fig. 15: FEKO model of salisbury screen

Fig. 16: The reflection property of salisbury screen and PEC narrow. If multi-layer is adopted, such as Jaumann absorbers, the radar absorbing material can enlarge the incident angle and bandwidth. Some optimal design on the location of layers in Jaumann absorbers are conducted by some other references. V. S UMMARY In this report, we discussed some basic ideas on stealth design technology. There are two major techniques to reduce target RCS: to focus on the target’s shape design and to focus on the radar absorber materials. Each of them contains a broad research range. In this report, we further make an explaination on the faceted stealth shape design technique and the resonant radar absorbing materials. The simulation by CST MWS and FEKO shows the effectiveness of these stealth design method.