George C. Barber

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. >

Finite-difference time-domain method for electromagnetic radiation, interference, and interaction with complex structures

The finite-difference-time-domain (FDTD) method is reviewed and then used to model and predict the radiation patterns of a monopole antenna mounted on the bottom of a perfectly conducting helicopter structure. The computed radiation patterns are compared with measurements to demonstrate the accuracy of the FDTD method. To study the effect of antenna interference, a second monopole antenna is mounted on the structure and the patterns are recalculated. The perfectly conducting helicopter model is then replaced by a partially composite/partially conducting material structure and the electromagnetic fields penetrating the structure from an incident plane wave are analyzed. A preprocessing geometry program, GEOM, is used to generate an FDTD geometry model, assuming solid surface helicopter structure. >

NEC and ESP codes: guidelines, limitations, and EMC applications

The computer modeling and simulation of EMC programs of complex structures are addressed, using the NEC and ESP moment method codes. The rules and guidelines of modeling using the two codes are summarized. In addition, visualization and automation of the modeling process are made possible through an interface computer program GEOM developed at Arizona State University. Finally, the interference between two wire antennas mounted on a complex helicopter model is given as an example of using these codes in EMC applications. >

Modeling and Simulation of a Helicopter-Mounted SATCOM Antenna Array

A seven-element helicopter-mounted microstrip-patch switched-beam antenna array for satellite communications (SATCOM) is described. The performance of the array antenna was simulated. A prototype was constructed, and measurements of the antennas patterns were made. The effects of the blades of the helicopter on the patterns of the antenna were investigated. Fluctuations of 10 dB or more in magnitude due to the effects of the blades were observed in the patterns.

Exact mutual impedance between sinusoidal electric and magnetic dipoles

Analytic expressions are presented for the exact mutual impedance of sinusoidal electric and magnetic monopoles in a homogeneous medium. Numerical computations are carried out and presented. It is shown that computed values of mutual impedances for both linear monopoles and surface-patch monopoles in the near-overlapping case exhibit an excellent convergence to the theoretical values when monopoles are physically overlapped. This demonstrates the robust accuracy of the presented expressions in the near-singular integration kernel for the mutual impedance computation. The expressions are also compatible with existing techniques. Minor modifications are needed to enable existing programs to compute such mutual impedances. >

Advanced Helicopter Electromagnetics: industry, government and university

The Advanced Helicopter Electromagnetics (AHE) was initiated at Arizona State University on January 1, 1990, to address research needs on developing analytical and measurement methods for helicopter EM applications. The AHE program is a coalition of research and education efforts supported by aerospace industry, federal government agencies and state government. The objective of the AHE program was to develop analytical methods and modelings of antennas on metallic and non-metallic bodies for advanced helicopter applications and to facilitate technology transfer through industry, government and university. Under the support of the AHE members and the joint effort of the research team, significant progress has been achieved. The research effort has focused on practical helicopter electromagnetic problems, such as antenna pattern modeling and prediction, composite materials, HF antennas, conformal antennas for UHF and possibly VHF, and scale model measurements. The paper provides a brief review of the AHE program through representative examples of some of the problems that were investigated.<<ETX>>

Three-dimensional automatic FDTD mesh generation on a PC

The authors present a program developed at Arizona State University to generate automatically three-dimensional surface FDTD (finite-difference time-domain) mesh for a complex object. The program is based on an efficient ray-tracing algorithm and has been tested in various numerical simulations. The mesh generator can be used for a complex object enclosed by conducting and thin-dielectric surfaces. The high efficiency of the algorithm allows the mesh generation to be performed on a special computer. As an example, the mesh generation for a scale helicopter is shown.<<ETX>>

Scale model helicopter antenna pattern measurements at ASU's EMAC Facility

The ElectroMagnetic Anechoic Chamber (EMAC) facility has been in operation at Arizona State University (ASU) since 1988. In addition to providing experimental verification for theoretical prediction of radiation and scattering targets, this multi-purpose facility acts as a testbed for a unique, semicompact antenna test range. Antenna patterns of a scale model helicopter have been measured at 500 MHz and compared to Numerical Electromagnetics Code (NEC) prediction. These measurements demonstrate that accurate antenna patterns can be measured at frequencies much lower than expected according to the conventional wisdom concerning rectangular anechoic chambers and compact ranges. Measurements made at EMAC of a scale model helicopter at UHF demonstrate remarkable agreement with theoretical prediction, particularly in the case of the electrically small reflector, absorber, and chamber.<<ETX>>

Low-loss material coating for horn antenna beam shaping

A comprehensive study of the coating on the inner walls of a horn antenna is very beneficial for minimizing unnecessary losses while maintaining the same beam shape. An analytic method to investigate ohmic losses is presented. Based on the analytic investigations, guidelines are provided for low-loss designs of lossy material coatings for horn antenna beam shaping. Experimental results in support of the theory are also presented.<<ETX>>

Modeling and simulation of a helicopter-mounted antenna using WIPL-D

The results obtained using WIPL-D for simulation of a seven element helicopter-mounted switched-beam antenna have been discussed. We have discussed the magnitude of the distortions caused in the antenna amplitude pattern for different rotor blade positions. We compared the results from these simulations to patterns when the main rotor and rotor hub are removed from the helicopter. This allows us to isolate the effects due to the blades and rotor from those of the rest of the helicopter airframe. As expected, the blades of the helicopter have a large impact on the pattern of the antenna. Fluctuations 10 dB in magnitude or more can be seen in the predicted patterns. Using a switching scheme that assigns a fixed coverage area to each patch element would not be the best choice because the gain performance will suffer when one of the elements experiences a drop in performance due to an obstacle. Based on the results shown, a more sophisticated switching method may be necessary since multipath degrades the performance of helicopter-mounted antennas. We have found that WIPL-D is a valuable tool throughout the process of modeling a helicopter-mounted antenna.