Composite materials in electromagnetism - Radar absorbant materials

Radar absorbing materials (MAR) were developed in 1940, following the installation of the first radar networks. In English, they are often referred to as RAM (Radar Absorbing Materials).

The ideal MAR would resemble a paint that is effective for all polarizations over a wide frequency band and incidence range. Unfortunately, such a material does not exist, and the likelihood of one appearing in the near future is quite low.

In practice, the most effective type of absorber in a given situation is highly dependent on a number of parameters (radar frequency, shape of transmitted wave, bandwidth, target shape, etc.).

Absorbent requirements and properties are determined by the following considerations:

• operating frequency: the absorber can be designed for single-frequency absorption or for multiple discrete frequencies, or for broadband applications;

• wave incidence: the absorption of a material, especially if it is anisotropic, can be highly dependent on incidence;

• composite or homogeneous medium: the absorbent is made up of a homogeneous material or a series of discrete or gradient media;

• absorption range: a function of transmission losses through the absorbers ;

• power handling: governed by the heat dissipation ranges of the absorbing materials and the breakdown voltages in any metallic patterns;

• geometric considerations: thickness and surface area needed to achieve the required absorption levels;

• Temporal stability: ageing of materials, taking into account physical and thermal degeneration due to continuous exposure to electromagnetic radiation;

• ease of manufacture: ease of manufacturing, moulding or forming an absorbent on a given template;

• Weight considerations: as low a weight as possible for airborne and aeronautical applications (3 to 4 kg/m ² is a maximum value) or high weight ;

• product production, sales and installation costs as low as possible; for example, Boeing's large anechoic chamber in Seattle, which can measure an entire Boeing 747, has a cost excluding electronics of over €20 million (in 2009).

The aim of this article is to enable readers to acquire or refine their knowledge of absorbent materials.

We'll be looking at :

• the various types of broadband flat screens ;

• resonant structures ;

• ...