CORRU_PRIMER

Topics: Antenna, Frequency, Horn Pages: 41 (4016 words) Published: October 12, 2013
Tom Milligan
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Design of Corrugated Horns: A Primer
Christophe Granet and Graeme L. James
CSIRO ICT Centre, Electromagnetics & Antennas
PO Box 76, Epping 1710, NSW, Australia
E-mail: Christophe.Granet@csiro.au
Keywords: Corrugated horn antennas; antenna feeds; reflector antenna feeds

Slots

1. Introduction

0

ver the past 40 years, corrugated horns supporting so-called hybrid modes have become well established as feeds for
reflector antennas, and even as direct radiators. It is not difficult to trace the popularity of the corrugated hom, given the ability of certain hybrid modes to produce radiation patterns having
extremely good beam symmetry with low cross-polarization levels, a high beam efficiency with very low sidelobes, and the potential for wide-bandwidth performance [ l , 21. Why they are called “corrugated” is clear from the typical example of a horn shown in Figure 1, where the inside wall is manufactured in a succession of slots and “teeth.” The purpose of the corrugated surface is to provide the means to support the propagation of hybrid modes within the horn. Hybrid modes are basically a combination of TE and TM

is

Mode Converter

Figure la. A cut-away view of a typical corrugated horn.
76

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rkq

pzl

@
.
g

m

Id

m.......

...

Teeth
Figure lb. The details of the corrugated inside wall of a typical corrugated horn.
/€€€Antennas and Propagation Magazine, Vol. 47, No. 2, April 2005

modes. For this combination to propagate as a single entity with a common propagating velocity, the horn or waveguide must have anisotropic surface-reactance properties: properties that are satisfied by the corrugated surfacc. It is worth pointing out that hybrid modes can also be supported by other means, such as waveguides or horns partially filled with dielectric [l].However, these alternative possibilities are outside the scope of this design note. There have been many papers relating to the theoretical

description of corrugated horns, but relatively few giving tips on how to easily come up with a good, basic design that can then be optimized, either by trial and error or directly through an automated optimization algorithm. In an attempt to fill this gap, we provide here some basic information for the inexperienced. horn designer to get started in designing their first corrugated horn. The starting procedure outlined here is based on our combined experience in designing high-efficiency corrugated horns for numerous

Table 1. Corrugated-hornparameters.

~

Length
Total number of slots
Number of slots in the mode converter
Slot pitch
Slot width
Slot pitch-to-width ratio
Width of the slot teeth
Depth of thejth slot

L
N
NMC

p =L/N
W

6 = w/p
( P - W )= ( 1 - 4 P
d j where 15 j < N

applications over many years. Other experienced horn designers have, undoubtedly, different design starting points, but the following tips have always worked well for us. We begin with some practical considerations. The bandwidth

of a horn is usually defined by the frequency range over which the hom is required to have a suitable beamwidth and beam symmetry for a return lossusually better than 15 to 18 dB, and with a crosspolarization maximum better than -20 to -25 dB. These values are typical, but many high-performance applications have much tighter specifications. Remember that the following procedure is only designed to provide a good starting point.

Figure 2a. The geometrical parameters of a corrugated horn
with a variable-depth mode converter.

IC

L

=
I

i :
L%
,

...... .

A

a.

Figure 2b. The geometrical parameters of a corrugated horn
with a ring-loaded slot mode converter.

c

L

7

Figure 2c. The geometrical parameters of a corrugated horn
with a variable-pitch-to-width mode converter....

References: Microwave Horns and Feeds, London, IEE Electromagnetic Wave
Series 39, 1994, ISBN 0 85296 809 4.
tions on Microwave Theory and Techniques, MTT-29, 1981, pp.
IO. C. Granet, “Profile Options for Feed Horn Design,” Proceedings of the Asia Pacific Microwave Conference, Vol. I, 2000, pp.
10, October 1984, pp. 1134-1138.
Techniques for Reflector Antennas, 1983, pp. 211-216.
ISAP’96, Chiba, Japan, pp. 1133-1136.
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