Waveguide-fed pyramidal horn

Quick Summary

Background

The rectangular waveguide horn is one of the simplest microwave antennas. Its existence and early use dates back to the late 1800’s, and it is

widely used throughout the world as a feed element for large radio astronomy, satellite tracking, and communication dishes. In addition to its utility

as a feed for reflectors and lenses, it can serve as an element of phased arrays.

It is commonly used as a universal standard for calibration and gain measurements of other antennas. As such, it is referred to as the standard gain

horn, e.g. [Mayhew-Ridgers, G.].

The design approach is for an optimum horn. This is one that approaches maximum gain at the shortest possible length. The first rectangular horn

of the pyramidal variety was used by J.C. Bose in 1897, which he termed a “collecting funnel”.

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Quantity Typical Minimum Maximum

Polarisation Linear - -

Radiation pattern Pencil beam - -

Gain 15 dBi 11 dBi 24 dBi

Performance bandwidth 1.5:1 (40%) - -

Complexity Medium - -

Balun None required - -

Beamwidth 40 ° 20 ° 60 °

Popular application /industry

Calibration standard - -

Waveguide-fed pyramidal horn 2

The most widely used horn is one which is flared in both directions. It is widely referred to as a pyramidal horn, and its radiation characteristics are

essentially a combination of the E- and H-plane sectoral horns.

When both the E- and H-plane flare angles are zero, it reduces to an open-ended rectangular waveguide, which is often used for near-field antenna

measurements [Yaghjian, A. D.].

Physical Description

The horn consists of a rectangular waveguide section which is flared to reach a specified aperture size.

Feed Method

The antenna is fed using a rectangular waveguide, or a pin-fed coaxial to waveguide adapter.

Operation Mechanism

Horn antennas form a transition for propagating waves between a waveguide and free-space.

Performance

The bandwidth is limited by the frequency range of the waveguide section, i.e. by the cut-off frequencies of the fundamental and next higher order

mode in the rectangular waveguide.

The plots shown here are for a horn with a gain of 18 dBi.

Impedance Characteristics

The reflection coefficient is generally low but can be significant for low gain cases.

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Waveguide-fed pyramidal horn 3

Typical reflection coefficient

Radiation Characteristics

The familiar monotonic increase in gain with frequency is found when the flare length is fairly long in comparison with aperture dimensions.

Typical total gain radiation pattern at the centre frequency

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Waveguide-fed pyramidal horn 4

Typical co-polarised radiation pattern

Typical gain versus frequency plot

References

C. A. Balanis, Antenna Theory Analysis and Design, 2nd Ed., John Wiley, 1997, pp.682–695.

G. Mayhew-Ridgers, J. W. Odendaal, and J. Joubert, “On primary incident wave models for pyramidal horn gain calculations,” IEEE Trans. Antennas

and Propagation, vol. 48, no. 8, August 2000, pp. 1246–1252.

A. D. Yaghjian, “Approximate formulas for the far field and gain of open-ended rectangular waveguides,” IEEE Trans. Antennas and Propagation,

vol. 32, no. 4, April 1984, pp. 378–384.

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Waveguide-fed pyramidal horn 5

Model Information (FEKO) Model 1

A PEC model fed using a waveguide port

This model uses infinitely thin PEC sheets and is fed using a waveguide port which is excited with the fundamental TE10 mode. By default, the model

is solved using MoM with two symmetry planes. To reduce simulation times, MLFMM should be used for larger (higher gain) horns.

Model Information (CST MICROWAVE STUDIO) Model 1

A PEC model fed using a waveguide port

This PEC model is fed using a waveguide port which is excited with the fundamental (TE10) mode. Two symmetry planes are used to reduce

simulation times.

Model Validation

The radiation patterns and gain versus frequency characteristics were validated against measurements given in Balanis, C. A.

Each export model has been validated to give the expected results for several parameter variations in the design space.

Magus Analysis

The internal performance estimation is expected to be similar to a full 3D-EM analysis. Expect:

Small frequency offsets (-3% to +3%)

Possibly inaccurate reflection coefficients below -15 dB

Radiation patterns outside of the main beam may be inaccurate

Design Guidelines

The Antenna Magus design for gain produces an optimum gain horn, i.e. it produces maximum gain for the shortest possible flare length.

To increase the gain, increase the aperture dimensions and the flare length. The flare length should be increased by a larger factor than the

aperture dimensions.

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Waveguide-fed pyramidal horn 6

To E-plane beamwidth may be increased (decreased) by decreasing (increasing) the aperture height; while the H-plane beamwidth may be

increased (decreased) by decreasing (increasing) the aperture width.

Adjustments to the flare length with result in a trade-off between size and performance of the horn.

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