Сontrol of backscattering by corrugated structures and small-period gratings

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control of backscattering by corrugated structures and small-period gratings

Y. N. Kazantsev, V. N. Apletalin, A. N. Kozyrkov, V. S. Solosin, A. S. Zubov

The Diffraction Problem Laboratory,

the Institute of Radioengineering and Electronics of the Russian Academy of Sciences

Received June 07, 2000

A possibility of backscattering controlling by means of corrugated structures and a small period gratings is considered. It is shown that applying of these structures allowed to reduce the backscattering from rear edges and from metal-impedance border.

1. Introduction

measuring backscattering electromagnetic border

A possibility of controlling the backscattering from typical scattering centres by means of structures with anisotropic conductivity has been demonstrated in our preceding work [1,2]. Here we consider corrugated planar structures with inclined grooves and small-period gratings with variable spacing. A possibility of controlling the backscattering from a rear edge (using the corrugated structures) and from a boundary between metal and impedance surfaces (using the small-period grating) is demonstrated.

For TM incident waves (vector H is perpendicular to the incidence plane), the backscattering from the rear edge of corrugated impedance slab is known to be substantially lower than that from the rear edge of smooth metal slab. As for TE waves (vector E is perpendicular to the incidence plane), the backscattering from the rear edges of both corrugated and smooth metal slabs are approximately the same.

The corrugated structure with inclined grooves (Fig. 1a) provides the desired surface impedance with less thickness than that of usual corrugated structure (Fig. 1b).

Fig.1

The small-period grating with variable spacing enables to match the surface impedance of a metal and of another surface, thereby the backscattering from the boundary between these surfaces is decreases.

2. Methods and facilities for measuring of the backscattering

Measurements of the backscattering were made in the 2.816.0 GHz frequency band by the use of two types of measurement facilities. The Reflectometer described in [1] was used in the 8.016.0 GHz frequency band. Measurements in the 2.88.0 GHz frequency band were performed with a Horn Measuring Set shown in (Fig. 2), where 1 - the Frequency Synthesizer (FS), 2 - coaxial directional coupler, 3 - pyramidal horn with coax-waveguide adapter, 4 - rotary arrangement with an object under test, 5 - the Microwave Network Analyzer (MNA), 6 - computer for control, detection and processing the measurement data.

ISAR method was employed to separate the individual scattering centres on the objects surface under test.

Fig.2

3. Backscattering from rear edge of corrugated slab

The corrugated structures of two kinds were studied:

- Structures with grooves filled with non-absorbing dielectric;

- Structures with grooves filled with radio-absorbing (lossy) material.

The basic dimensions of two experimental corrugated structures are shown in Fig. 3a,b.

Fig.3

Fig.4 Fig.5

Groove's depth of the sample with absorbing filling was chosen sufficient so far the wave is practically attenuated after a double-run of the groove. Effective permittivity of the absorbing filling was The depth of the grooves with non-absorbing filling was equal to a quarter of the central wavelength in the dielectric. Mylar with permittivity was used as low-loss dielectric. To absorb surface waves, a thin 70 mkm resistive film was placed on the corrugated structure. The permittivity of the film material was . The backscattering from the rear edge of experimental samples was measured for 3 observation angles (20o, 30o and 40o). The backscattering level of TE waves was low and practically the same as in the case of a sample with smooth metal surface. However, the backscattering of TM waves proves to be substantially lower as compared with the case of a sample with smooth metal surface.

Experimental results for samples shown in Fig. 3a and 3b are presented in Fig. 4 and Fig. 5, respectively. In these Figs. dash line marks an echowidth from metal half plane for observation angles =200. As it follows from Fig. 4 for a sample with lossy filling, a gain in the backscattering level more than 10 dB is obtained in a wide frequency band 3.016.0 GHz. As for the sample with non-lossy filling, the gain is significant only in a relatively narrow frequency band.

4. Backscattering from the matching grating transition between surfaces with different electromagnetic properties

It is known that a sharp boundary between surfaces with different electromagnetic properties is responsible for scattering of waves incident to this surface.

For example, it may be a boundary between metal surface and a surface coated by lossy material. TE waves incident to this boundary are scattered rather insignificantly, whereas the backscattering for TM waves may be sufficiently intense. This scattering may be reduced by a grating with variable period, which matches the surface impedances of the metal and lossy coating (Fig. 6).

Fig.6

The grating is located on surface of the lossy coating. The grating edge with maximal period is joined directly to the metal surface. Away from the edge the grating period decreases, and eventually the grating becomes transparent for an incident wave and does no longer screen the surface of lossy coating. As a result, the surface impedance varies smoothly along the grating from zero to the impedance of the lossy coating. Such smooth matching of impedances reduces level of the backscattering. The matching efficiency was studied on a number of experimental samples. A typical sample is shown schematically in Fig. 7.

Fig.7

A layer of lossy material with complex permittivity placed on a ground plane was used. The layer thickness was 1 mm. The grating was made of layered composite material consisting of aluminium foil of 12 mkm in thickness between a layer of mylar of 100 mkm in thickness and a layer of polyethylene 50 mkm in thickness. The grating length was about 84 mm. The grating period smoothly varied from 8 mm (near the metal side of the structure) to 0.5 mm (near the opposite side) by square law. The grating consisted of 56 slits, the width of each slit being about 0.04 mm.

Fig.8

The backscattering from the sample with grating structure was measured in the 8.015.0 GHz frequency range for different observation angles (20o, 30o and 40o ). The experimental results are shown in Fig. 8a, b, c. The measured echowidth/ values for the sharp metal-lossy coating boundary was about of -(1213) dB in the operating frequency range.

References

1. Yu. N. Kazantsev, V. N. Apletalin, A. N. Kozirkov, V. S. Solosin, A. S. Zubov. Backscattering Investigation of Structures with Anisotropic Conductivity. Proc. of 7th Russian Seminar "Wave phenomenon in homogeneous medium, Krasnovidovo, Moscow reg., 22-27 May, 2000 v.2, pp.76-77.

2. Yu. N. Kazantsev, V. N. Apletalin, V. S. Solosin, A. S. Zubov. Study of Electromagnetic Wave Backscattering from Structures with Anisotropic Conductivity. Journal of Radioelectronics, N4, 2000.

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