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  • 1
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    A simple circular-polarized antenna: Circular waveguide horn coated with lossy magnetic material (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A circular waveguide horn coated with a lossy material in its interior wall can be used as an alternative to a corrugated waveguide for radiating a circularly polarized (CP) field. To achieve good CP radiation, the diameter of the structure must be larger than the free-space wavelength, and the coating material must be sufficiently lossy and magnetic. This device is cheaper and lighter in weight than the corrugated one.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-177092 , NAS 1.26:177092 , UILU-ENG-86-2554 , ELSR-86-8
    Format: application/pdf
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  • 2
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    Wave attenuation and mode dispersion in a waveguide coated with lossy dielectric material (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The modal attenuation constants in a cylindrical waveguide coated with a lossy dielectric material are studied as functions of frequency, dielectric constant, and thickness of the dielectric layer. A dielectric material best suited for a large attenuation is suggested. Using Kirchhoff's approximation, the field attenuation in a coated waveguide which is illuminated by a normally incident plane wave is also studied. For a circular guide which has a diameter of two wavelengths and is coated with a thin lossy dielectric layer (omega sub r = 9.1 - j2.3, thickness = 3% of the radius), a 3 dB attenuation is achieved within 16 diameters.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-173820 , NAS 1.26:173820 , EL-TR-84-13 , UILU-ENG-84-2552
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  • 3
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    Normal modes in an overmoded circular waveguide coated with lossy material (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The normal modes in an overmoded waveguide coated with a lossy material are analyzed, particularly for their attenuation properties as a function of coating material, layer thickness, and frequency. When the coating material is not too lossy, the low-order modes are highly attenuated even with a thin layer of coating. This coated guide serves as a mode suppressor of the low-order modes, which can be particularly useful for reducing the radar cross section (RCS) of a cavity structure such as a jet inlet. When the coating material is very lossy, low-order modes fall into two distinct groups: highly and lowly attenuated modes. However, as a/lambda (a = radius of the cylinder; lambda = the free-space wavelength) increases, the separation between these two groups becomes less distinctive. The attenuation constants of most of the low-order modes become small, and decrease as a function of lambda sup 2/a sup 3.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-176186 , NAS 1.26:176186 , TR-85-8
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  • 4
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    Numerical methods for analyzing electromagnetic scattering (1985)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Attenuation properties of the normal modes in an overmoded waveguide coated with a lossy material were analyzed. It is found that the low-order modes, can be significantly attenuated even with a thin layer of coating if the coating material is not too lossy. A thinner layer of coating is required for large attenuation of the low-order modes if the coating material is magnetic rather than dielectric. The Radar Cross Section (RCS) from an uncoated circular guide terminated by a perfect electric conductor was calculated and compared with available experimental data. It is confirmed that the interior irradiation contributes to the RCS. The equivalent-current method based on the geometrical theory of diffraction (GTD) was chosen for the calculation of the contribution from the rim diffraction. The RCS reduction from a coated circular guide terminated by a PEC are planned schemes for the experiments are included. The waveguide coated with a lossy magnetic material is suggested as a substitute for the corrugated waveguide.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-176141 , NAS 1.26:176141
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  • 5
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    Numerical methods for analyzing electromagnetic scattering (1985)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Numerical methods to analyze electromagnetic scattering are presented. The dispersions and attenuations of the normal modes in a circular waveguide coated with lossy material were completely analyzed. The radar cross section (RCS) from a circular waveguide coated with lossy material was calculated. The following is observed: (1) the interior irradiation contributes to the RCS much more than does the rim diffraction; (2) at low frequency, the RCS from the circular waveguide terminated by a perfect electric conductor (PEC) can be reduced more than 13 dB down with a coating thickness less than 1% of the radius using the best lossy material available in a 6 radius-long cylinder; (3) at high frequency, a modal separation between the highly attenuated and the lowly attenuated modes is evident if the coating material is too lossy, however, a large RCS reduction can be achieved for a small incident angle with a thin layer of coating. It is found that the waveguide coated with a lossy magnetic material can be used as a substitute for a corrugated waveguide to produce a circularly polarized radiation yield.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-175507 , NAS 1.26:175507
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  • 6
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    Radar cross section of an open-ended circular waveguide Calculation of second-order diffraction terms (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The radar cross section of an open-ended, semi-infinite circular waveguide is calculated by the geometrical theory of diffraction and the equivalent-current (EC) method. Both single- and double-diffraction terms are included. It is found that the double-diffraction term is stronger than the single-diffraction term for the horizontal polarization at wide-angle incidence. Its inclusion is necessary in order to check with experimental data and the asymptotic expansion of the exact Wiener-Hopf solution. The EC method is used for the single diffraction. On the other hand the double-diffraction term is obtained by using the EC method for axial incidence and geometrical theory of diffraction for wide-angle incidence. These two solutions are matched with a proper matching function. This technique is computationally more efficient than the matching technique of integration over the equivalent currents.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Radio Science (ISSN 0048-6604); 22; 2-12
    Format: text
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