Hanyang University 1/15 Antennas & RF Devices Lab. MODERN ANTENNA HANDBOOK by CONSTANTINE A.BALANIS ch Jeong Gu Ho

Hanyang University 2/12 Contents 5.3 Fundamental design principles Aperture illumination and spillover Axial defocusing Lateral defocusing Root-mean-square surface error Phase error Blockage Sidelobes Polarization

Hanyang University 3/ Fundamental design principles Aperture illumination and spillover –The efficiency of a reflector antenna is primarily determined by (1) the ability of the feed system to illuminate only the reflector (“spillover efficiency”) (2) the ability of the feed system to uniformly illuminate the parabola (“illumination efficiency”). –Ex) The illumination efficiency is 100% when the energy density on the entire main reflector aperture is a constant. –A convenient representation for asymmetric feed pattern Peak gain

Hanyang University 4/12 The gain of the feed is G=2(2n+1) Sharp beam

Hanyang University 5/ Axial defocusing For a surface revolution, the pattern function written by If we use the primary feed representation shown defocusing

Hanyang University 6/12 If the source is an infinitesimal electric dipole, gain loss as a function of defocusing can be shown to be proportional to (sin x/x) P. G. Ingerson and W. V. T. Rusch, Studies of an axially defocused paraboloid, IEEE Int., Antennas Propag. Symp. Dig., Vol. 7, pp. 62–68, December 1969.

Hanyang University 7/12 As the taper is increased, the minima become less pronounced. (-) Taper is increased

Hanyang University 8/12 Consequently, highly tapered feeds can be used in beam-broadening applications while less-tapered feeds are generally unsuitable. (-)

Hanyang University 9/ Lateral defocusing When a feed is laterally displaced from the focal point, there is a loss in peak gain, and the beam is scanned from the boresite an amount equal to BDF x θ. Vector and scalar analysis were used to examine reflector performance under the conditions of large lateral feed displacements. Results from the two formulations were compared to experimental data obtained from a precision F/D=0.4 reflector with minimum blockage. boresight Coma lobe

Hanyang University 10/12 The beam-peak angle position is accurately predicted by both the vector and scalar theories. The peak gain is accurately predicted by the vector theory, but the scalar theory is several decibels in error for large scan angle. The scalar theory can be greatly in error for predicting the coma-lobe peak. W. A. Imbriale, P. G. Ingerson, and W. C. Wong, Large lateral feed displacements in a parabolic reflector, IEEE Trans. Antennas Propag., Vol. 22, pp. 742–745, November 1974.

Hanyang University 11/ Root-mean-square surface error Equation for computing the gain loss in antenna caused by random surface errors. R. Levy, Structural Engineering of Microwave Antennas, IEEE Press, Piscataway, NJ, 1996, pp. 289–296.

Hanyang University 12/12 The six parameters are indicated in figure 5.17

Hanyang University 13/ Phase error Phase errors are similar in effect to surface deformation errors. And they may be due to the causes as feed phase errors. If the phase errors are symmetic, there is generally a gain loss and an effect on the sidelobe structure. If the errors are asymmetric over the structure, there may also be a beam squint. surface feed

Hanyang University 14/ Blockage Opaque devices are between the feed system and the reflector and between the reflector and the far-field observation point. Region 1 : blockage by the feed system itself Region 2 : projection of a feed support structure Region 3 : the part of the feed support structure The major effect on the radiation patterns is to reduce the on-axis gain and modify the sidelobe structure.

Hanyang University 15/ Sidelobes The feed typically produces a monotonically shaped amplitude distribution as shown by the solid curve, whereas a low sidelobe distribution generally requires an inflected shape as shown by the dashed curve. feed The distribution shape and the edge taper determine sidelobe levels achievable. However, in addition to the sidelobes caused by the aperture distribution, there are also sidelobes caused by the primary illumination that misses the reflector and diffraction effects at the reflector edge.

Hanyang University 16/ polarization Any monochromatic wave is elliptically polarized. The orthogonal components of the incident radiation(wave traveling along the positive z-axis) at any point in space can be written J. D. Kraus, Electromagnetics, McGraw-Hill, New York, 1984.

Hanyang University 17/12 Linearly polarized in the y-direction Linearly polarized x-direction Circular polarized if If the polarization of the incident wave does not match the polarization of the receiving antenna, a polarization loss occurs due to the mismatch.

Hanyang University 18/15 Antennas & RF Devices Lab. Thank you for your attention Antennas & RF Devices Lab.

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Hanyang University 20/12 R. Levy, Structural Engineering of Microwave Antennas, IEEE Press, Piscataway, NJ, 1996, pp. 289–296. Region 2 Region 3 Region 1

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