Absorber Jaumann

So, it is difficult to achieve the width bandwidths generally desired for radar absorbers by using a thin single-layer absorber. Therefore, much work has been done in extending the bandwidth of absorbers by the use of multiple layers. The approach consists of slowly changing the effective impedance with distance into the material to minimise reflections. Two important types of multilayer absorbers have been developed, Jaumann absorbers and graded magnetico-dielectric absorbers. 

The bandwidth of a Salisbury screen can be improved by adding additional resistive sheets and spacers to form a Jaumann absorber. To provide... 

As with the Jaumann absorber, where sheet resistance values are tapered to reduce reflection, a graded dielectric can be used to help match the impedance between free space and a perfect conductor. 

Moreover, performances of multilayers can be enhanced either by increasing the overall thickness or by stacking several transparent layers (e4GHz = 4 — jO) as in a Jaumann type absorbant. Such a composite is depicted in Figure 8.43 with an attenuation of — 15 dB between 5 and 18 GHz. 

One paper in particular was investigating a combination of circuit analog and Jaumann absorbers. It was the result of a computer optimization process apparently without any consideration of physics. In less than 10 minutes 1 spotted the first fundamental and disastrous design flaw The interelement spacings were larger than a full wavelength For the author such an observation is an extremely emotional affair. It simply means that grating lobes are present in some bistatic... 

These devices create a resonant impedance to reduce reflections at multiple frequencies corresponding to multiple quarter-wavelengths of the dielectric material. Multilayers, or Jaumann absorbers, are generally more frequency broadband absorbers and may even be graded multidielectric or multimagnetic materials and thus increase the effectiveness at angles off of normal. Knott has shown that the thickness of the absorber can be reduced and the bandwidth considerably expanded if the sheets are allowed to have capacitive reactance in addition to pure resistance (Knott and Lunden, 1995). 

Knott, E.F. and Lunden, C.D. 1995. The two-sheet capacitive Jaumann absorber. IEEE Ant. and Prop. Trans 43(11). 

A. I Jaumann also supposed that chemical changes which occur with abso rption of heat occur along with other changes which are accompanied by a production of heat . Berthelot explained endothermic reactions as due to accompanying physical processes which absorb heat, as Berthollet did prevously. 

Chambers and coworkers [72-74] also explored the properties of semiconductive sheets of polypyrrole treated paper and cloth, which they characterize as a parallel RC circuit. They too report that the capacitive part of the complex impedance of the sheets depends on the morphology of the poly pyrrole coating. Measured reflectivity plots for Salisbury screen (narrowband) and Jaumann (broadband) absorbers fabricated from sheets of polypyrrole-treated material are also presented in their article. The measured results agree well with their model calculations, indicating the potential utility of conductive polymer-treated fabrics in radar-absorbing structures. 

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