An equivalent network for square and rectangular shaped microstrip radiating elements is derived. In order to simplify the problem the radiating element is considered as two slots separated by a transmission line of low characteristic impedance. The slots are characterized by their radiation pattern, directivity, and equivalent admittance. A design procedure for open circuit halfwave resonators and for arrays of such resonators is given. Finally, some antennas in the X band are designed and measured.

Linearly polarized microstrip antennas

An alternative method is presented for computing far-field antenna patterns from near-field measurements. The method utilizes the near-field data to determine equivalent magnetic current sources over a fictitious planar surface that encompasses the antenna, and these currents are used to ascertain the far fields. Under certain approximations, the currents should produce the correct far fields in all regions in front of the antenna regardless of the geometry over which the near-field measurements are made. An electric field integral equation (EFIE) is developed to relate the near fields to the equivalent magnetic currents. The method of moments is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method, and in the case of a rectangular matrix, a least-squares solution for the currents is found without explicitly computing the normal form of the equation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions. Numerical results are presented for several antenna configurations. >

Planar near-field to far-field transformation using an equivalent magnetic current approach

Presented here is a method for computing near- and far-field patterns of an antenna from its near-field measurements taken over an arbitrarily shaped geometry. This method utilizes near-field data to determine an equivalent electric current source over a fictitious surface which encompasses the antenna. This electric current, once determined, can be used to ascertain the near and the far field. This method demonstrates the concept of analytic continuity, i.e., once the value of the electric field is known for one region in space, from a theoretical perspective, its value for any other region can be extrapolated. It is shown that the equivalent electric current produces the correct fields in the regions in front of the antenna regardless of the geometry over which the near-field measurements are made. In this approach, the measured data need not satisfy the Nyquist sampling criteria. An electric field integral equation is developed to relate the near field to the equivalent electric current. A moment method procedure is employed to solve the integral equation by transforming it into a matrix equation. A least-squares solution via singular value decomposition is used to solve the matrix equation. Computations with both synthetic and experimental data, where the near field of several antenna configurations are measured over various geometrical surfaces, illustrate the accuracy of this method.

Near-field to near/far-field transformation for arbitrary near-field geometry utilizing an equivalent electric current and MoM

Approximate boundary conditions are a means for simulating material and surface effects in scattering and propagation. A number of conditions are discussed, and criteria are given for their validity.

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Approximate boundary conditions

A millimeter-wave dual frequency circularly polarized feed for an offset reflector antenna is being developed for a portable satellite ground terminal. The two frequency bands are Q - and K -bands for transmit and receive, respectively. A compact feed design consisting of a single corrugated horn with two circular waveguide concentric openings at the horn throat is described. Good aperture efficiency and low sidelobes in both of the frequency bands are achieved.

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A compact Q-K-band dual frequency feed horn

An analysis is presented which uses the uniform geometrical theory of diffraction for determining the near fields diffracted by a paraboloid either with or without a conical flange attached to its circular rim when an axially propagated plane wave is incident on the concave or convex portion of the paraboloidal reflector. The field correlation theorem is used to determine the power coupled by a prime focus feed associated with the paraboloid which is being illuminated by a uniform plane wave. Based on this analysis, the front-to-back ratio of unflanged and flanged paraboloids is computed. Computed results show satisfactory agreement with the available measured as well as computed results based on alternative procedures. The variation in the on-axis gain on a prime-focus reflector when the feed is displaced from the focus is studied. Typical computed results are presented and compared with the available measured data. Computed results on the front-to-back ratio of paraboloids (flanged or unflanged) illuminated by a PFF whose radiated field exhibits phase variation over a constant radius are also presented. >

Front-to-back ratio of paraboloidal reflectors

A technique based on a recent paper [3] for designing shaped dual reflector antennas that appears to be significantly better than the traditional method [1], [2] is presented. The main advantages derived are extreme computational simplicity and the ability to utilize very accurate feed pattern expressions. A typical practical problem is solved using [1] and our method to bring out these features.

A note on the shaping of dual reflector antennas

The importance of corrugated conical horns in space-fed illumination of phased arrays is indicated. Design details pertaining to corrugated conical horns of optimum proportions for uniform illumination of a phased array with a prescribed area are also presented.

Corrugated conical horn as a space feed for phased-array illumination

A correction to the available radiation formula [1] for E -plane sectoral horn has been proposed. Calculated radiation patterns based on the corrected formula, are shown to be in better agreement with the measured radiation patterns for several values of fiare-angles of the E -plane sectoral horns.

A correction to the available radiation formula for E -plane sectoral horns

A multipole expansion technique to study radiation from aperture antennas in the form of an open-ended waveguide-horn is presented. Particular attention is paid to dual-mode and corrugated circular horns. The radiation pattern of these feeds, derived in terms of multipole moments, is in the form of an algebraic series which converges very rapidly. Further, based on the multipole-expansion technique presented, the far-zone fields of an axially-symmetric aperture-fed paraboloid are obtained in closed form. It is demonstrated that there is a drastic reduction in computer time by using this technique instead of the conventional numerical integration procedure for calculating the far-zone fields of the paraboloid.

M. Narasimhan

Papers

Front-to-back ratio of paraboloidal reflectors

A note on the shaping of dual reflector antennas

Corrugated conical horn as a space feed for phased-array illumination

A correction to the available radiation formula for E -plane sectoral horns

Radiation from aperture-fed paraboloids employing source-multipole expansions