## 1 Introduction
### Passive Methods (Passive Coatings)  
Passive methods to reduce the scattering field and radiating field require equipment of the protected body with some coating which uses wave absorption or interference, or a complicated coating based on metamaterials, but always without use of any outside source of energy.  
Absorption by coatings can be effective if: (a) the external surface of the coating does (almost) not reflect the incident wave; (b) the incident wave, passing through the coating and reflected from the internal surface of the coating, becomes very weak (due to absorption) when returning to the external surface of coating. The main reasons to choose the such passive coatings are their simplicity, their wideband frequency range of incident waves and the wideband range of angles of incoming incident waves of arbitrary structures. Besides this, such coatings also suppress the radiation of the protected body (Figure 1a). The main drawback of passive coatings are their large thickness \(h\) (equivalent to  
Figure 1: Traditional thick passive coating for absorption (_h\(>>\)\(\lambda\)_) suppresses the body’s radiation and its back scattering, but does not suppress forward scattering (a); (cloaking [3-7]) thick (_h\(>>\)\(\lambda\)_) passive coating, constructed from metamaterials, makes the body transparent, but does not suppress the body’s radiation (b); thin (_h\(<<\)\(\lambda\)_) active coating, which suppresses both scattering to the back and forward scattering and radiation of the protected body. The horizontal parallel lines on the left side of (a-c) represent the power flux lines.  
several incident wavelengths \(\lambda\)\(>>\)_h_). This precludes the use of passive coatings in low frequency applications. Besides this, good absorbing coatings can only effectively suppress backscattering, but cannot suppress forward scattering.  
Passive absorbing coatings can also be constructed from a large ensemble of passive oscillators with resonant absorption. In principle the thickness of such a coating can be much smaller than the length \(\lambda\) of the incident wave at a wide frequency band. However to absorb incident waves with oscillators, we need to compensate the dynamical interaction of oscillators, for instance, with special electroacoustical chains. In this case the parameters of each oscillator must depend on the arrival direction of each incident wave. This fact presents a great difficulty for effective absorption over a wide range of arrival directions, so a coating becomes very complicated oscillatory system, which is very sensitive to any small perturbation of its parameters. The body which does not produce backscattering and forward-scattering is called a \"transparent body\". We note again that absorbing coating reduces only back-scattering and creates a \"black body\", but does not reduce forwardscattering, and in principle cannot create a \"transparent body\".  
Recently work devoted to a passive coating with distributed mechanical connections was published [2]. This coating has less thickness than a classic passive coating, but it reduces (at most) only back-scattering and does not suppress the radiation of the protected body.