| Summary: | The pointing accuracy of the tracking antenna used in a radar guided homing missile is reduced by a streamlined radome. This can have serious effects on the missile's performance. The pointing error of a microwave antenna inside a radome can be predicted with fair accuracy provided that the radome and antenna diameters are ten or more wavelengths. This is done, basically, by determining the modified aperture distribution, taking account of the varying phase shift through different parts of the radome. The polar diagram of the modified aperture distribution is then calculated. If the radome diameter is only about five wavelengths this procedure (the 'insertion diffraction theory') is not usually adequate. The pointing error can be calculated from known aperture fields, under reasonable conditions, for small antennas. The inadequacy of the theory is therefore mainly due to the inability to calculate the aperture fields with sufficient accuracy. The reasons for the failure of the ray tracing procedure are discussed and it is shown that scattering by the tip of the radome, surface waves guided by the radome and multiple scattering (interaction) between the antenna and the radome would be expected to modify the aperture fields and introduce pointing errors. The propagation of surface waves on radomes is investigated and a calculation of the pointing error due to the surface wave is carried out. Experiments showed that surface waves can be troublesome on a very lossy radome but for the small ceramic radomes used in this research they are not significant. A method is described for calculating the pointing error caused by interaction between an antenna and an infinite plane dielectric sheet of uniform thickness. This method is not generally applicable to radomes and a method for radomes is developed. It is found that interaction is a serious cause of error, especially if the radome has a low dielectric constant. The tip scattering effect is also found to give large pointing errors in small radomes. The effect of radome diameter (measured in wavelengths) on each of the sources of error is examined and it is shown that, whereas the pointing error due to phase variation effects is inversely proportional to size, the errors due to the other effects vary much more rapidly. This explains why the latter are only of second order importance in large radomes but become predominant in small ones. It is shown that a high dielectric constant is essential for small radomes which are to be used over a narrow frequency band.
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