Horn Aperture Research Articles

“Phase Centers” of Far Infrared Multi-Moded Horn Antennas

Multi-moded horn antennas can be used as high efficiency feeds for bolometers when diffraction limited resolution is not required. For example, such horns are proposed for the PLANCK Surveyor, a satellite telescope due to be launched in 2007 to make definite measurements of the Cosmic Microwave Background. In a previous paper we described an accurate approach involving electromagnetic modelling using a rigorous mode matching technique to obtain both the horn aperture fields and the corresponding far field radiation patterns. In this paper we extend this description to determine the “phase center” of such horns when used on large reflecting telescopes. The “phase center” is ill defined as the individual spatially coherent fields making up the far field pattern all appear to come from different phase centers. The best average phase center location is therefore redefined in terms of the virtual beam waist position behind the horn aperture at which the focus of the telescope should be located in order to optimise angular resolution and on-axis gain for the beam on the sky. A number of alternative techniques to locating the phase center are discussed in detail in thes paper.

High-frequency reflection from the aperture of a long E-plane sectoral horn

The result of Yee et al. (1968) for the parallel-plane TE/sub 10/-mode reflection coefficient, used in Jull's (1972) formulation for the reflection coefficient of a long E-plane sectoral horn, is replaced by the Boersma's (1974) corresponding result. At longer wavelengths, the expression that emerges displays a much better match with experiments than Jull's original expressions.

A mode matching-integral equation technique for the analysis and design of corrugated horns

A hybrid mode matching-integral equation method is presented for the analysis and design of corrugated horns. By applying a particular formulation of the equivalence principle, which implies covering the horn aperture with a metallic plate, the original issue is divided into an exterior and an interior problem. The former is formulated by using a combined field integral equation, the latter by employing a modal expansion. The separation of the two problems allows running an optimization procedure to meet all the requirements needed in high performance radiating systems. The technique has been used for the design of a small flare angle and a discrete-section profiled horn. The theoretical predictions remarkably agree with experimental data.

Characterization of corrugated feed horns at 216 and 300 GHz

We present measurements of the near-field beam profiles of two corrugated feed horns with center frequencies of 216 and 300 GHz. Both co-polar and cross-polar components of the electric field have been probed across the operation band of the horns. A full-wave numerical near-field transformation, based on a vector Kirchhoff integral, is implemented to transform the measured field to the horn aperture for a comparison with the ideal aperture field. From the overlap integral of the transformed and the ideal field, we conclude that the field radiated by these horns is the HE11 mode, to a purity of better than 99%.

The hexagonal horn as an efficient Gaussian beam launcher

A new type of feed horn-the hexagonal horn-is proposed and investigated by means of Gaussian-Hermite beam-mode analysis. The electric-field distribution at the horn aperture plane is approximated. The polarization efficiency is 88.66%. The fractional power in the fundamental beam mode is about 85%. The near- and far-field radiation patterns are calculated. The high fractional power in the fundamental beam mode of the horn indicates that it can be used as an efficient Gaussian beam launcher in quasi-optical systems.

Experimental variable beamwidth horn

An H-plane sectoral horn contains ferrite powder dispersed in a solid low-loss dielectric medium. The permeability of the medium is varied by an externally applied magnetic field to alter the phase distribution across the horn aperture to produce a variable beamwidth horn.

Calculation of the field radiated by horn-antennas using the mode-matching method

Methods employed for evaluating fields radiated by horn antennas, like the aperture field method and the spherical wave expansion, have not taken into account the contribution by fields existing outside the horn aperture (e.g., along the outer walls of the horn). The use of the mode-matching technique for analyzing the transition from horn antenna to free space makes it possible to include in the analysis even those fields which exist outside (e.g., behind) the horn aperture when determining the far as well as the near field. Furthermore, the effects of the finite wall thickness and the discontinuity caused by the transition from waveguide to free space are also taken into account. A suitable development of the fields at the transition of the conical/corrugated horn to free space using the mode-matching technique is presented. Two different convergence diagrams, depending on the kind of modes excited in the horn antenna, are given. They show the results from extensive convergence investigations. With the help of these diagrams, a reliable evaluation of the fields radiated by horn antennas is guaranteed. Practical measurements of the fields radiated by horn antennas confirm the field distributions gained through analysis. >

The phase center position of a microstrip horn radiating in an infinite parallel-plate waveguide

In this paper the field radiated by a flared microstrip line in an infinite parallel-plate waveguide with separation between plates of less than half a wavelength is studied. The field at any point of the parallel-plate waveguide is expressed as a summation of modal solutions of the Helmholtz equation. The phase center of the microstrip horn is calculated for uniform amplitude distributions along the horn aperture. The theory may be applied to obtain the phase and amplitude distributions along the output ports of microstrip bootlace lenses. >

A comparison among 1-, 3-, and 7-horn feeds for a 37-beam MBA

A very common multiple beam antenna (MBA) configuration consists of a collimating device illuminated by an array of feeds. The collimating device is usually a reflector or a lens. The feeds are usually horn antennas with a circular aperture. The reflector is usually offset-fed to eliminate aperture blockage; the lens is center fed. The antenna's feeds are excited to produce a finite number of beams, so as to provide contiguous coverage of the field of view. The designer is forced to minimize the angular spacing between adjacent beams in order to maximize the minimum gain over the antenna's field of view. On the other hand, the feed horn's aperture gain is maximized when the feed horn spacing and its aperture diameter are equal. This results in antenna efficiency of the order of 30% when a single feed horn is excited to produce a beam. When a cluster of 3 or 7 adjacent feed horns are excited to produce a single beam, antenna efficiency can be increased to 50%. When it is tolerable, several identical antenna apertures can be used to replace a single aperture configuration. In this case, each of M apertures produces approximately N/M beams of an MBA that produces N beams. Horns producing adjacent beams do not illuminate the same aperture. This permits the use of a much larger horn aperture for a given beam spacing. This results in reduced spillover, higher gain of each beam, and increased antenna efficiency of each aperture. This paper investigates the maximization of gain for several lens antennas. It shows that antenna gain is increased as its focal length is decreased. That is, a focal length-to-diameter ratio (F/D) less than 1 is preferred. >

An E‐plane sectoral horn fitted with side‐lobe suppression devices

AbstractA finite difference method is used to design a new type of sectoral horn antenna with corner channel inserts which produces a tapered aperture amplitude distribution in the E‐plane. The channel inserts, which present a simple modification to the horn aperture geometry, act as effective side‐lobe suppression devices. The efficiency of the suppression technique was successfully tested on an experimental horn whose performance is compared to that of a horn with similar geometrical dimensions but with no channel sections.

The electrical characteristics of a conical horn-reflector antenna employing a corrugated horn

The electrical properties of a conical horn-reflector antenna of 2-3 degrees beamwidth have been investigated over the frequency range of 12-18 GHz. It is shown that the employment of a corrugated horn reduces the characteristic spillover side lobe of the conical horn-reflector antenna by 15 dB and yields excellent main beam circularity over the full Ku band. This results from the taper of the field amplitude toward the edges of the horn aperture in both the E-plane and the H-plane illumination. The performance of the antenna makes it suitable for use in closely packed interferometers for radio astronomy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of diagonal horns through Gaussian-Hermite modes

The radiation characteristics of diagonal horns are investigated by means of Gaussian-Hermite modes. It is shown that, for reasonably long horns, the beamwidths in the principal and 45 degrees planes are equal to within 10%, and all sidelobes are below -15 dB. It is also demonstrated that the phase center of a diffraction-limited horn is close to the aperture, whereas the phase center of a constant-beamwidth horn is behind the throat. The maximum coupling to the lowest order copolar Gaussian mode is 84%, and the total amount of power coupled into the cross-polarized lobes is 9.5%. More significantly, the aperture efficiency of a Cassegrain antenna fed by a diagonal horn has a maximum value of 81%, which compares with 87% for a corrugated horn. The maximum efficiency is achieved when the aperture of a diffraction-limited horn is placed at a confocal tertiary focus, although a secondary focus gives an aperture efficiency that is only 10% lower, suggesting that diagonal horns are suitable for focal-plane arrays. >

802GHz integrated horn antennas imaging array

Pattern measurements at 802GHz of a single element in 256-element integrated horn imaging array are presented. The integrated-horn antenna consists of a dipole-antenna suspended on a 1-micron dielectric membrane inside a pyramidal cavity etched in silicon. The theoretical far-field patterns, calculated using reciprocity and Floquet-modes representation of the free-space field, agree well with the measured far-field patterns at 802GHz. The associated directivity for a 1.40 lambda horn aperture, calculated from the measured E and H-plane patterns is 12.3dB + or - 0.2dB. This work demonstrates that high-efficiency integrated-horn antennas are easily scalable to terahertz frequencies and could be used for radio-astronomical and plasma-diagnostic applications.

Millimeter-wave integrated-horn antenna. II. Experiment

For pt.I see ibid., vol.39, no.11, p.1575-81 (1991). The impedance and radiation patterns of a dipole-fed horn antenna in a ground plane are experimentally investigated at microwave and millimeter-wave frequencies. The agreement with the full-wave analysis technique presented in part I is good. The results indicate that for a 70 degrees flare-angle horn, horn apertures from 1.0 lambda -square to 1.5 lambda -square with dipole positions between 0.36 and 0.55 lambda yield good radiation patterns with a gain of 10-13 dB a cross-polarization level lower than -20 dB, and resonant dipole impedances between 40 Omega and 120 Omega . It is also found that the impedance measurements can be safely used for 2-D horn arrays, but the radiation patterns differ because of the Floquet modes associated with the array environment. The integrated horn antenna is a high-efficiency antenna suitable for applications in millimeter-wave imaging systems, remote-sensing, and radioastronomy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the performance of metallic-wall conical horns with centrally-loaded dielectric material

A known asymptotic-analysis technique is extended to determine the HE11 mode electromagnetic fields which can exist inside and across the aperture of a metallic-wall conical horn which is centrally-loaded with a concentric dielectric material. Two horns of special interest are then examined: the foam-core air-lining (FCAL) horn and the dielectric rod-loaded (DRL) horn because their radiation patterns (especially those of the FCAL) are close to those from an ideal corrugated horn (CH) of the same size, which the subject asymptotic-analysis technique can also treat. Copolar and 45°plane crosspolar pattern computations and gain, along with corresponding measurements, are given for several horns of the above types and show very good agreement. However, the limitations of this asymptotic technique, using only the single HE11 model, in predicting extremely good 45° plane crosspolar patterns are described.

High gain variable beam feed for satellite communications

The radiation behaviour is presented of a high gain variable beam antenna system consisting of a homogeneous dielectric sphere mounted at an appropriate position in front of the aperture of a dielectric-coated conical horn. The fields in the dielectric-coated conical horn are analysed using spherical hybrid modes. The condition for low cross polarization balanced hybrid modes is derived. Then radiated far field components of the feed system are first analysed by treating the dielectric sphere as a scatterer of electromagnetic waves. In a second approach, the dielectric sphere is treated as a microwave lens illuminated by an aperture source. Theoretical results are compared with experimental measurements, and good overall agreement is observed. The feed system has improved radiation characteristics with increased directivity and on-axis gain reduced sidelobe and low cross polarization levels. These features make the new horn an attractive feed for satellite communications and radio astronomy.

Monolithic millimeter-wave two-dimensional horn imaging arrays

A monolithic two-dimensional horn imaging array has been fabricated for millimeter wavelengths. In this configuration, a dipole is suspended in an etched pyramidal cavity on a 1- mu m silicon-oxynitride membrane. This approach leaves room for low-frequency connections and processing electronics. The theoretical pattern is calculated by approximating the horn structure by a cascade of rectangular-waveguide sections. The boundary conditions are matched at each of the waveguide sections and at the aperture of the horn. Patterns at 93 and 242 GHz agree well with theory. Horn aperture efficiencies of 44+or-4%, including mismatch and resistive losses, have been measured. A detailed breakdown of the losses is presented. The coupling efficiency to various f-number imaging systems is investigated, and a coupling efficiency of 24% for an f0.7 imaging system (including spillover, taper, mismatch and resistive losses) has been measured. Possible application areas include imaging arrays for remote sensing, plasma diagnostics, radiometry and superconducting tunnel-junction receivers for radio astronomy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis and Design of Wideband Constant Beamwidth Sectoral Horns

An accurate analysis of planar aperture sectoral horns is presented. It is based on a simple adaptation of the mode matching technique, which allows for the inclusion of the aperture effect in the analysis without resorting to the conventional integral equations formulation. This technique provides accurate results, which are confronted with experiments and then used in a parametric study of the sectoral horns, aimed at achieving a constant beamwidth over an octave bandwidth or more. It is concluded that for the narrow E- plane aperture horn this objective is hard to attain, hence a different, convex aperture antenna is used. Experimental design of this configuration leads to the desired performance of a nearly constant beamwidth over more than an octave.

Soft and hard horn antennas

Horn antennas with soft and hard boundaries are analyzed. A soft boundary which exists in classical hybrid-mode horns gives zero field intensity at the wall. A hard boundary corresponds to a uniform field distribution over the horn aperture. Soft and hard horn antennas are compared with respect to directivity, sidelobes, and beamwidth. The dependency between the edge taper directivity, and sidelobes is also calculated based on the solution to the spherical hybrid modes in a conical horn with arbitrary wall impedances. This makes it possible to study how to compromise between directivity and sidelobes. Also discussed is how the different wall impedances may be realized, and some preliminary experimental work on hard horns is presented. >

Imaging in a Gregorian antenna from 12 to 30 GHz

A Gregorian antenna with the main reflector illuminated by magnified image of a small horn aperture was built and tested from 12 to 30 GHz. The image is approximately frequency-independent, and the main reflector is illuminated with negligible spillover. Polarization distortion caused by aberration is very small, in excellent agreement with a simple expression derived previously by the author (ibid., March 1987). Spatial filtering by the subreflector causes the far-field sidelobes in the principal plane orthogonal to the symmetry plane to be very low, about 80 dB below the main beam at 16.5 GHz for angles from the axis that are greater than 20 degrees .< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>