Reflection from the aperture of a longE-plane sectoral horn

The complex reflection coefficient of a longE-plane sectoral horn is derived using the geometrical theory of diffraction. Single diffraction alone is shown to be sufficient to describe the contribution from theE(widely separated) edges, while singly and doubly diffracted fields from theHedges are included by using an open waveguide result derived elsewhere. A relatively uncomplicated expression is obtained for the complex reflection coefficient of the horn which agrees well with experiment for horns with slant wall lengths greater than about3frac{1}{2}guide wavelengths.     

Analysis of finite parallel-plate waveguide arrays

The radiation and phase scanning properties of a planar array are investigated using wedge diffraction theory. The TEM and TE01radiation patterns are determined assuming that each guide is excited with equal amplitude but variable phase which is specified by the desired scan angle. Since the TE01pattern can be used directly as theH-plane radiation pattern of an equivalent rectangular waveguide array, the analysis for this instance is extended to consideration of aperture reflection coefficients     

An aperture-matched compact range feed horn design

The moment method and the uniform geometrical theory of diffraction are used to obtain two separate solutions for theE-plane far field pattern of an aperture-matched horn antenna. This particular horn antenna consists of a standard pyramidal horn with the following modifications: a rolled edge section attached to the aperture edges and a curved throat section. The resulting geometry provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The moment method is used to calculate theE-plane pattern and voltage standing-wave ratio (VSWR) of the antenna. However, at higher frequencies, the moment method requires large amounts of computation time. On the other hand, the uniform geometrical theory of diffraction provides a quick and efficient high frequency solution for theE-plane field pattern. In fact, the uniform geometrical theory of diffraction may be used to initially design the antenna; then the moment method may be applied to "fine tune" it. In both methods, a two-dimensionalE-plane model of the antenna is used, but these two-dimensional solutions yield excellent agreement with measured data of the actual three-dimensional antenna. This procedure has been successfully applied to design a compact range feed horn.     

a broad-band constant beamwidth corrugated rectangular horn

Using aperture field theory to predict the far-field radiation from a rectangular horn, a horn with substantially constantE- andH-plane beamwidths of17degand10degrespectively, is designed to operate over a 2.4:1 bandwidth. Assuming a cosine aperture field distribution in both theE- andH-planes of the horn, the general conclusion is first made that an aperture phase error of 0.37 wavelengths at the lowest operating frequency produces the least beamwidth variation (3 dB) over the band. A general design curve showing "constant" beamwidth as a function of horn throat length is produced, furthering the design of such horns with beamwidths in the range approximately9degto27deg. TheE-plane walls of the horn are corrugated, and to cover the bandwidth corrugations are comprised of "T-section" slots which are designed from a simple transmission line model. To realize the specified beamwidths, a compound horn configuration is adopted. Some experimental results obtained from a prototype horn are given.     

A short horn with high E-plane directivity

TheE-plane beamwidth of a small pyramidal horn can be substantially reduced by the addition of paxabolic cylinder flanges of the proper focal length. Analysis of this antenna involves both geometrical diffraction theory and conventional aperture techniques. In the numerical and experimental examples chosen the flanges halve the half-power beamwidth without severe bandwidth reduction. Consequently a lightweight compact horn is achieved.     

A new multimode rectangular horn antenna generating a circularly polarized elliptical beam

A multimode circularly polarized rectangular horn antenna generating an elliptical shaped beam is described. This antenna operates in two orthogonal mode sets, namely the TE10+ TE/TM12and TE01+ TE/TM21modes. By virtue of the higher order TE/TM modes, the apertureE-field distribution can be tapered such that the effectiveE-plane far-field beam width is approximately equal to theH-plane beam width of the other orthogonal set of modes, resulting in low off-axis polarization axial ratio. Because of the tapered aperture distribution, the radiation patterns also have low sidelobes. The elliptical cross section beam is a direct result of the rectangular shaped aperture. This antenna, used in conjunction with a spacecraft to illuminate an elliptical zone on the earth surface, offers high edge-of-coverage gain, low sidelobes, low edge-of-coverage (EOC) axial ratio, less RF sensitivity to the space environment, and low cost. The performance of this antenna has been evaluated experimentally.     

GTD analysis of the radiation patterns of conical horns

The far-field radiation patterns of conical horns of arbitrary flare angles excited in theTE_{11}mode are obtained employing the geometric theory of diffraction (GTD) based on the theory of Kouyoumjian and Pathak [3] and the slope diffraction technique [4]. The analysis presented enables one to predict accurately radiation patterns over the main beam, near and far sidelobes, and the becklobe of the horn. Validity of the analysis is established by satisfactory agreement between the calculated and measured patterns of an experimental conical horn. The radiation patterns of wide-flare corrugated conical horns excited in theHE_{11}mode of operation have also been calculated over the main beam, which contains most of the radiated energy (up to -40 dB with respect to boresight field), employing slope diffraction technique, and a good agreement is noticed between the calculated and measured radiation patterns.     

Slope diffraction and its application to horns

The first order geometrical theory of diffraction (GTD) predicts vanishing fields along the surface of a conducting wedge for the incident electric field polarized parallel to the diffracting edge. The slope diffraction coefficient is a valid correction term for incidence angles removed from the shadow boundary. A new slope diffraction function for the half plane is presented along with applications. This new form of slope diffraction coefficient for the half plane is valid through the shadow region. Reciprocity is invoked to find the far-fields for a source on the surface of the conducting wedge. In addition to applying the two-dimensional slope diffraction analysis to practical problems, the equivalent current concepts have been extended to include equivalent slope currents for the analysis of either finite or curved edges. This new form of the slope diffraction function has been successfully used to provide anH-plane horn pattern analysis that is considerably less tedious than previously possible with GTD. Both pure GTD solutions and hybrid solutions using conventional aperture integration for the main beam region and GTD for the far-out side and back lobes are compared with experimental results.     

On the fields in a conical horn having an arbitrary wall impedance

The electromagnetic fields propagating up a cone having an arbitrary wall impedance are found using an asymptotic solution. Three special cases are then considered: the smooth-metal wall, the corrugated wall, and the metal wall with a lossy-dielectric lining. The last case, in the form of an absorber-lining is then shown to behave like a corrugated horn since it too provides a highly taperedE-plane andH-plane aperture distribution. Furthermore, it does this over a much larger bandwidth, over 3:1, with negligible gain drop.     

A low-blockage dipole array reflector antenna feed for the lower microwave frequencies

An antenna is described which was devised as a feed for a small axisymmetrical paraboloid reflector at an operational frequency of just over 1 GHz. The antenna consists of a broadside array of two dipoles on a printed circuit board (PCB), joined by a common transmission line. Each dipole is backed by a small strip reflector. With this simple array, which has one central feed point, theH-plane radiation pattern can be varied independently of theE-plane pattern. In addition, the aperture blockage of the feed is small.     

Analysis of pyramidal horn antennas using moment methods

A hybrid numerical technique is developed for electrically large pyramidal horn antennas radiating in free space. A stepped-waveguide method is used to analyze the interior surfaces of the horn transition. The electric field integral equation (EFIE) is employed on the outer surfaces of the pyramidal horn including the radiating aperture. Meanwhile, the magnetic field integral equation (MFIE) is used on the aperture to relate the aperture fields and those in the horn transition The resultant hybrid field integral equation (HFIE) is solved numerically by the method of moments. This formulation is both accurate and numerically stable so that high-gain microwave pyramidal horns can be analyzed rigorously. Far-field radiation patterns, both computed and measured, are presented for three electrically-large X-band horn antennas. The comparisons demonstrate that this method is accurate enough to predict the fine pattern structure at wide angles and in the back region. Computed far-field patterns and aperture field distributions of two smaller X-band horns are also presented along with a discussion on the validity of the approximate aperture field distributions routinely used in the analysis and design of pyramidal horns     

Dual-polarized fan-beam feed horn

A simple dual-polarized feed horn that radiates fan-shaped primary patterns is described. The radiated patterns for the two orthogonal polarizations are essentially identical and are suitable for illuminating cut paraboloidal reflectors withf/Dratios of about 0.3. The horn is similar to conventional flared waveguide feed horns except that the corners have been modified to yield an octagonally shaped aperture. The horn has the added advantage of suppressedE-plane sidelobes, thus reducing spillover radiation.     

A GTD analysis of the far-out sidelobes of cassegrain antennas

Using recent developments of the geometrical theory of diffraction (GTD), the completeE- andH-plane patterns of Cassegrain antennas are computed. The pattern in the main beam region is computed by conventional aperture integration methods and the GTD analysis is used to evaluate the far-out lobes caused by spillover and by various edge diffraction processes. The effects of the edge curvature and surface curvature are included in the computations.     

Analysis of spherical hybrid modes in a corrugated conical horn

An approximate analysis of fields in a corrugated horn is considered using spherical hybrid modes. Under the special condition where the slots are about a quarter-wavelength deep, the approximations are slight in the far-field region of the horn. Solutions are presented for the lowest-order spherical hybrid modes, and a procedure is described for the determination of the horn radiation pattern. A first step in the procedure involves the calculation of the hybrid-mode field in the horn aperture. Because diffraction will be minimal in wide-angle corrugated horns, the aperture and radiation patterns should be similar when the far-field approximation applies. This feature is verified for a 120° horn making use of experimental results due to Kay.     

E-plane analysis of a modified horn antenna withsuppressed far-out sidelobe level

It is shown that the far-out sidelobes of a horn antenna can be suppressed by curving the edges in the aperture. A three-dimensional uniform geometrical theory of diffraction (UTD) analysis of the E -plane radiation pattern of the modified horn antenna was performed and compared with straight edges. This analysis indicates that the curvature of the edges does indeed influence the sidelobes to the effect that the modified horn antenna has significantly lower far-out sidelobes. The radiation patterns for modified horns were measured and compared with the calculated patterns and were found to be in good agreement     

The corrugated horn as an antenna range standard

The corrugated horn discussed here is a valuable tool for use in microwave pattern ranges. It has the properties of concentrated energy in the main beam, low backlobes, high efficiency and almost monotonic amplitude, and phase radiation patterns which make the corrugated horn useful for a source antenna in a pattern range and also as a possible standard antenna for calibration purposes. A sample of the references that have dealt with the corrugated horn is given for the benefit of the reader [1]-[7].     

Wide-angle sectoral horns using leaky-wave wall structures

A lensless method of phase correction for wide-angle sectoral horns is presented. It consists of supplying the narrow walls of the horn with a leaky-wave interface corresponding to a fast phase velocity. An analytical expression of the radiated field is derived by operating the Fourier transform of the theoretical distribution, and a set of theoretical radiation patterns are presented. The limitations of this type of correction, as far as bandwidth is concerned, are also examined theoretically. Two horn antennas illustrating this method of correction are described: anE-plane sectoral horn and anH-plane sectoral horn. Both have a flare angle of120degand an aperture of6lambda. TheH-plane horn uses a longitudinalU-shaped section, and theE-plane horn a transversely slotted rectangular waveguide. Both of them are measured with and without correction, and the experimental results are compared with the theoretical predictions.     

Microwave holographic metrology of large reflector antennas

A microwave holographic technique for the determination of amplitude and phase of the principal and cross-polarized aperture fields of large reflector antennas is described. The hologram formation process utilizes the elevation over azimuth scanning system normally associated with these antennas, and, in this respect, appears to be unique among other proposed methods of field probing. The present work describes the means used to obtain vital information on the antenna structure such asE- andH-plane phase centers of the feed, and rms values of the reflector surface profile errors. Accurate prediction ofE- andH-plane radiation patterns in the near- and far-field is also demonstrated.     

Errors in the predicted gain of pyramidal horns

The concepts of the geometrical theory of diffraction are used to derive the on-axis gain of two-dimensionalEplane sectoral horns. Geometrical optics and single (noninteraction) diffraction by the aperture edges yield essentially the Kirchhoff result-a monotonic gain versus wavelength curve. Reflection of diffracted fields from the horn interior and double diffraction at the aperture add an oscillation to this curve which is not significantly altered by further diffraction for moderate to large horns. Including these results approximately in Schelkunoff's equation for the pyramidal horn explains the gain variations observed in microwave gain standards and provides an error estimate in their predicted gain.     

Radiation performances of a modified rhombic dielectric plate antenna

An antenna consisting of the modified rhombic dielectric plate with double tapers in theH-plane excited by the open end of a transverse electric (TE_{10}) rectangular waveguide is proposed, and some antenna geometrical factors and radiation properties are given experimentally. It is found that these types of antennas have relatively narrow beamwidth and suppressed sidelobe in theH-plane, fairly flat main lobe in theE-plane, and medium performances of about 11.5 dB gain and 1.25 voltage standing-wave ratio (VSWR). It is also found that the radiation performances are virtually independent of frequency over a ten percent bandwidth about 10 GHz.