Experimental verification of the effect of aperture shape on prompt IRA response

This article presents experimental measurements verifying the relationship between aperture shape and prompt response in an impulse radiating antenna (IRA). Various aperture configurations were realized by constructing a lens IRA with focusing aperture significantly larger than the transverse electromagnetic (TEM) feed structure and then eliminating portions of the aperture with metal blocks. Virtually any prompt response can be realized by a suitable construction. Experimental results agree well with predictions made by evaluating the aperture integral of the TEM mode distribution.     

A description and the measured performance of three coaxial beam-rotating antenna prototypes

Many high-power microwave (HPM) sources utilize an azimuthally symmetric output mode, like the TM/sub 01/ circular waveguide or the coaxial TEM modes. If radiated directly, these modes produce a doughnut-shaped radiation pattern, with a boresight . Mode-conversion techniques for transforming the azimuthally symmetric mode to one with a more desirable radiated pattern are possible, but mode conversion is typically undesirable, due to inefficiencies and due to increases in system size and weight. Antenna designs have been explored that will radiate the azimuthally symmetric mode directly, but those considered to date tend to exhibit low gain, and do not radiate a boresight peak (along the longitudinal axis of the source). This article describes the measured performance of three prototype antennas, all of the coaxial beam-rotating antenna (COBRA) class. These accept directly an azimuthally symmetric mode, and radiate a high-gain, circularly polarized beam with a boresight peak. The antennas achieve this capability by varying the electrical length of a path from a focal point to the aperture plane as a function of the azimuthal angle of the aperture. A brief overview of the general theory of COBRA operation is first presented. Next, measured data, characteristic of the input impedance and far-field patterns of three COBRA prototypes, are given. The architectures of COBRA prototypes reviewed in this article include those utilizing (1) a single, stepped paraboloidal reflector; (2) a dual reflector; and (3) a dual reflector with a coaxial feed.     

Impulse-radiating antenna with an offset geometry

An offset impulse-radiating antenna (IRA) is numerically analyzed and compared with a typical centered IRA. In the typical centered IRA, the transverse electromagnetic (TEM) feed arms block the aperture because they are located at the center of the aperture. This blockage causes multiple reflections inside the antenna and, thus, ripples in the tail of the radiated waveform. In the offset IRA, the TEM feed arms are removed from the aperture, lowering the tail ripples caused by multiple reflections between the TEM feed arms and the reflector. The boresight gains and the impulse amplitudes are seen to be essentially the same for both IRAs. The monostatic radar cross section of the offset IRA is significantly lower than that of the centered IRA for the plane wave incident from the boresight direction because the wave incident to the offset IRA is diverted toward the focal point of the reflector, which is away from the boresight direction. The offset IRA has a shadow behind the reflector. This feature can be useful in bistatic radar applications because the antennas can be placed in the shadows of each other.     

分析宽频带、双极化、恒束宽四脊喇叭的混合方法

介绍了作为反射面天线馈源的宽频带、双极化、恒波束四脊喇叭，运用有限元和模式匹配相结合的混合法详细且计算了四脊喇叭的广义散射矩阵。然后运用口面积分计算了四脊喇叭的辐射远场，给出了四脊喇叭和用四脊喇叭做馈源的反射面天线的实测结果。     

Optimization of microstrip feed geometry for prime focus reflectorantennas

The radiation characteristics of a circular microstrip antenna are studied numerically. Surface integral equations are used to formulate the problem from the boundary conditions and moment methods are used to reduce the integral equations to a matrix equation. An analytic method is used to design a microstrip feed and to achieve symmetric radiation patterns with low cross polarization and backlobe levels. The backlobe level is reduced by adding a quarter-wavelength choke to the side wall or the ground plane of the antenna and the bandwidth is improved by stacking two layers. The performance of the feed with the reflector antenna is also considered. One of the feeds was fabricated and tested. Satisfactory agreement between the computed results and the measurement data was obtained. The microstrip feed has a very small size which should reduce its blockage of the reflector aperture     

Analysis of reflector antenna systems with arbitrary feed arraysusing primary field superposition

In contrast to secondary pattern superposition, where the fields reflected from the main reflector arising from each element are superimposed in the far field of the reflector, the approach presented here sums the primary fields at the reflector surface before the physical optics radiation integral is performed. The method allows each feed array element to have arbitrary position, orientation, pattern, and excitation (magnitude and phase). In addition, it is inherently efficient because evaluation of only one time-consuming radiation integral is required, rather than one per feed element as in secondary superposition. The method allows for accurate calculation of the power radiated from the feed, permitting the reflector gain and spillover efficiency to be determined within the context of a single computer program. The accuracies and characteristics of this method are demonstrated with several examples     

Forward radiation from axially symmetric reflector antennas

The fields radiated from focus-fed symmetric reflector antennas in the vicinity of forward axial direction are determined by a series representation of the physical optics integral. The considered feeds have cosine-tapered patterns with different tapers inE- andH-planes. The influence of feed pattern asymmetry, subreflector blockage, feed taper, focal distance to diameter (f/D) ratio and the reflector size on the performance parameters of paraboloidal, Cassegrain, near-field Cassegrain, and corresponding Gregorian antennas is investigated. Design curves are presented to predict the performance parameters of the considered antennas.     

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%.     

SABOR: description of the methods applied for a fast analysis ofhorn and reflector antennas

The program presented in this paper is intended to provide a valuable aid for teaching antennas to electrical engineers, and to provide fast and accurate pre-designs for professionals. This paper covers the theory and numerical techniques used in SABOR. In summary, this program computes the radiated field of an aperture antenna (horn or reflector), using a common engine based on the Gauss-Legendre quadrature method for evaluating the radiation integrals. For horn, the aperture fields are the usual dominant modes of the feed waveguide, with a quadratic phase correction. For reflectors, the aperture fields are computed using geometrical optics ray tracing from the feed horn. Also, equivalent-reflector concepts are applied for dual-reflector antennas. The paper includes some examples to demonstrate the most important features of the program     

Radiation from a dielectric coated hemispherical conductor fed by acoaxial transmission line

A dielectric-coated hemispherical conductor mounted on an infinite perfectly conducting ground plane and fed by a coaxial transmission line is investigated. Green's functions for the region above the ground line are derived with separated homogeneous and particular solution parts so as to be compatible with numerical analysis techniques. A magnetic field integral equation is constructed in terms of the unknown annular aperture tangential electric field and is solved by the method of moments. A comparison of the characteristics of the dielectric-coated hemispherical conductor and a flush-mounted coaxial line to an infinite homogeneous region above the ground plane is presented with respected to the tangential aperture electric field, with respect to the tangential aperture electric field, the coaxial line apparent input impedance, and the far radiated field     

The annular aperture antenna with a hemispherical center conductor extension

An annular aperture antenna mounted on an infinite ground plane and containing a hemispherical center conductor extension above the ground plane is investigated. A Green's function for the region above the ground plane is derived so as to be compatible with numerical solution techniques. A magnetic field integral equation is obtained in terms of the unknown tangential aperture electric field and is solved by the method of moments. A comparison between flush mounted and hemispherically extended annular aperture antennas is presented for the tangential aperture electric field, the coaxial line apparent input admittance, and the far radiated field.     

Hemispherically capped thick cylindrical monopole with a conical feed section

The axis boundary condition scalar potential integral equation is used to study the hemispherically capped thick cylindrical monopole with a conical feed section. The monopole is perfectly conducting and is fed by a coaxial line whose outer radius is connected to an infinite perfectly conducting ground plane. One approximation in the equation is that the field across the coaxial aperture is approximated by the fundamental TEM coaxial line mode. The integral equation is numerically solved by the moments method using entire domain basis functions with delta weights. A simple way for checking the convergence of the solution that needs no new integrations is given. The theoretical current distributions, input admittances, and radiation patterns are given for a thin, moderately thick, and thick monopole with and without a conical feed section. This feeding section simplifies the matching of the antenna to conventional signal generators and improves the high frequency characteristics of the monopole.     

Prompt aperture efficiencies of impulse radiating antennas witharrays as an application

A concept of prompt aperture efficiency is introduced for the purpose of comparing and optimizing the performance of impulse radiating antennas (IRAs). The aperture efficiencies of popular lens and reflector IRAs are computed as the ratios of peak radiated power densities on boresight compared with that produced by an ideal IRA with an aperture of equal area and equal total input power. Loss of aperture efficiency occurs through two distinct mechanisms: from power that falls outside the aperture and is lost and from nonuniform power and polarization distributions within the aperture. Both loss mechanisms are addressed, and means for increasing efficiencies are identified. The optimum aperture for a given feed structure is derived. Prompt aperture efficiencies approaching 100% are feasible in TEM-horn arrays and similar structures     

Analysis of the radiation patterns of reflector antennas

The development and application of a numerical technique for the rapid calculation of the far-field radiation patterns of a reflector antenna from either a measured or computed feed pattern are reported. The reflector is defined by the intersection of a cone with any surface of revolution or an offset sector of any surface of revolution. The feed is assumed to be linearly polarized and can have an arbitrary location. Both the copolarized and the cross polarized reflector radiation patterns are computed. Calculations using the technique compare closely with measured radiation patterns of a waveguide-fed offset parabolic reflector. The unique features of this technique are the freedom from restrictive feed assumptions and the numerical methods used in preparing the aperture plane electric field data for integration.     

Equivalence of surface current and aperture field integrations for reflector antennas

The "extinction theorem" is used to prove that the fields of reflector antennas determined by integration of the current on the illuminated surface of the reflector are identical to the fields determined by aperture field integration with the Kottler-Franz formulas over any surfaceS_{a}that caps the reflector. As a corollary to this equivalence theorem, the fields predicted by integration of the physical optics (PO) surface currents and the Kottler-Franz integration of the geometrical optics (GO) aperture fields onS_{a}agree to within the locally plane-wave approximation inherent in PO and GO. Moreover, within the region of accuracy of the fields predicted by PO current or GO aperture field integration, the far fields predicted by the Kottler-Franz aperture integration are closely approximated by the far fields obtained from aperture integration of the tangential electric or magnetic field alone. In particular, discrepancies in symmetry between the far fields of offset reflector antennas obtained from PO current and GO aperture field integrations disappear when the aperture of integration is chosen to cap (or nearly cap) the reflector.     

An offset-fed reflector antenna with an axially symmetric main reflector

A design method for an offset-fed, dual reflector antenna (Cassegrain type or Gregorian type) system with an axisymmetric main reflector is presented. Geometrical optics (GO) and the geometrical theory of diffraction (GTD) are used to find the surface-current density on the main reflector. A modified Jacobi-Bessel series (JBS) method is used to find the far-field pattern for the physical optics (PO) integral. In the defocused mode of operation, a new technique is developed to find the reflection point on the subreflector corresponding to the defocused feed and a general field point on the main reflector. Two sample systems are designed.     

New approach to compact measurements on reflector antennas

The letter describes a novel scheme for very compact measurements on reflector antennas. The measurement configuration involves a plane reflector located just beyond (and parallel to) the aperture plane, so that radiated power is reflected and refocused back into the feed. Measurement of return loss can then give information on antenna efficiency and gain. The basic concept is described and its performance confirmed by means of experiments on a 10 ft (3.048 m) diameter reflector. Possible methods of extending the performance of the scheme are briefly discussed.     

Some examples of generalized cassegrainian and gregorian antennas

Some theoretical generalizations are given of two-reflector, rotationally symmetric microwave antennas fed by a plane wave across the feed aperture. So far as geometrical optics apply, the proposed designs offer 1) no reflection of energy back into the feed and 2) arbitrary illumination over the antenna aperture. The general solution is exhibited in terms of quadratures, and the reflector shapes for some simple cases are worked out in detail and plotted. Two families of antennas are found which image the feed aperture onto the secondary reflector. These antennas might be expected to have low spillover losses due to diffraction, but none of them is of practical proportions. It is shown in general that such imaging is incompatible with efficient illumination of the secondary aperture.     

The physical optics of focused scatterers using source multipole expansions

A multipole-expansion technique is developed which permits the two-dimensional scattering integral for reflector antennas to be replaced by a finite sum of simple algebraic terms. The series, which accurately represents the radiated field over the main antenna beam and first few sidelobes, converges rapidly because the source dimensions are small. It is demonstrated that the reciprocity theorem is satisfied by the boresight physical-optics field of a paraboloid illuminated by a generalized linear electric current feed centered at the focus.     

A Theoretical Analysis of Discrete Reflecting Beam Waveguide with Parabolic Cylindrical Reflectors

A discrete reflecting beam waveguide with parabolic cylindrical reflectors, proposed by M. Kamimura, is theoretically analyzed. Electric field elementary waves on the reflector and the exciting primary electric field from the launcher are represented in the elliptic cylindrical coordinate system, and boundary conditions on the reflector are introduced to derive simultaneous integral equations regarding the reflector current. By solving these integral equations approximately, the integral representation of the secondary electric field in the beam waveguide is obtained, and poles and residues of the integrand are calculated to obtain the propagation constant of the beam waveguide and beam waveguide modes. The beam waveguide mode reflected toward the transmitting side when an obstacle is placed in the beam waveguide is obtained.