M. Narasimhan

Bio: M. Narasimhan is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Radiation pattern & Conical surface. The author has an hindex of 10, co-authored 45 publications receiving 296 citations.

Radiation from wide-flare corrugated E-plane sectoral horns

Abstract: An analytical and experimental investigation of the radiation properties of wide-flare corrugated E -plane sectoral horns ( 10\deg , where \alpha_{0} is the half-flare angle) excited in the HE_{11} mode [1] with improved E -plane radiation patterns and suppressed side-lobe levels over a broad bandwidth is described in this paper. Given the flare length and half-flare angle of the horn, the far-field radiation pattern is determined by two methods, one of which involves aperture-field integrations based on vector diffraction formula over a constant phase surface at the mouth of the horn and the other of which employs an expansion of the aperture field in terms of free-space cylindrical TE and TM wave functions. Good agreement is obtained between the two methods with measured radiation patterns for several values of \alpha_{0} . Given the flare lengthy variation of the on-axis gain with \alpha_{0} of the horn is analytically investigated, and the design procedure for determining optimum \alpha{0} , in order to obtain maximum on-axis gain, is also indicated. Furthermore, design and experimental studies on wide-flare horns under consideration are also summarized.

Synthesis of near-field patterns of a nonuniformly spaced array

Abstract: A technique of synthesis of near-field patterns of a nonuniformly spaced linear array of point dipoles with identical direction of current flow for each array element, or a uniformly spaced array of point dipoles with variation in direction of current flow for each dipole is presented. Further, it is described how one should prescribe the near-field (NF) pattern and how one should sample the same, while performing the NF pattern synthesis. Also discussed is how NF pattern synthesis should be performed so that the synthesized NF amplitude pattern closely follows a prescribed far-field amplitude pattern of the same array. Numerical computations are performed to demonstrate the validity of the physical concepts made use of in the technique proposed for performing the NF pattern synthesis successfully.

GTD analysis of the near-field patterns of conical and corrugated conical horns

Abstract: A novel technique for the analysis of the principal plane near-field patterns of conical and corrugated conical horns excited in the dominant mode based on the uniform geometrical theory of diffraction (UGTD) [1], [2] is presented. Numerical computations of the near-field amplitude patterns of a few typical conical and corrugated conical horns based on the GTD technique detailed here correlate well with the measured patterns, justifying the validity of the analysis presented.

A new method of analysis of the near and far fields of paraboloidal reflectors

Abstract: An analytical technique for predicting accurately the near (electric and magnetic) fields as well as the far fields of a reflector antenna with a pencil beam is presented. The technique proposed involves the near-field geometrical theory of diffraction (GTD) analysis of reflector antennas developed earlier and spherical vector mode functions. The proposed technique does not place any restriction on the range of polar angles or radial distances of the observation point. It is demonstrated that the technique proposed can predict the fields radiated by the reflector with greater accuracy by comparing the calculated results with the available measured results. A few important applications of the analysis proposed are also highlighted.

GTD analysis of the radiation patterns of conical horns

Abstract: The far-field radiation patterns of conical horns of arbitrary flare angles excited in the TE_{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 the HE_{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.

Linearly polarized microstrip antennas

Abstract: 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.

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

Abstract: 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. >

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

Abstract: 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.

Approximate boundary conditions

Abstract: 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.

A compact Q-K-band dual frequency feed horn

Abstract: 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.