View File - University of Engineering and Technology, Taxila

Two kinds of horns in common use are the pyramidal and conical. Ahorn’s interior surfaces can be smooth or corrugated, depending on polarizationrequirements. Pyramidal horns are easily designed and often used forearth coverage antennas because of their symmetrical radiation properties. Thefollowing equations are applicable to pyramidal horns, whose length is longcompared to a wavelength, l:G ¼ log Z 4p AB dBi ð2:38Þ2lwhere G is the gain and Z is the pyramidal horn’s efficiency, typically 50%.3-dB beamwidth in E-plane: y E ¼ 54 l Bdegð2:39Þ3-dB beamwidth in H-plane: y H ¼ 78 l Adegð2:40ÞIf it is necessary to have the shortest length possible, then by scalinga bL 1 ¼ L 12A 2BThe gain of the conical horn can be written asG ¼ 2 1:41 10 log D dBilð2:41Þð2:42ÞIf the aperture is uniformly illuminated, the 3-dB beamwidth of a conicalhorn can be approximated to the 3-dB of a circular aperture of equivalentillumination:y BW 58 l Ddegð2:43ÞThe advantage of using conical horns is the ability to exploit higher-modepropagation.Another variation of the horn feed often used in primary feeds,particularly for big earth stations, is a hybrid-mode horn. When annularcorrugations are placed on the inner wall of a circular waveguide, a hybridmodehorn is formed. If the annular corrugations are placed in such a way thatneither TE (transverse electric) or TM (transverse magnetic) modes can bepropagated, then a hybrid mode is generated. The hybrid-mode horn antennascan be used to achieve axially symmetric beamwidths and to improve crosspolarization and sidelobe performance.Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.