Craig R. Birtcher

Higher-order finite-difference schemes for electromagnetic radiation, scattering, and penetration .2. Applications

For pt.1 see ibid., vol.44, no.1, p.134-42 (2002). Higher-order schemes for the finite-difference time-domain (FDTD) method - in particular, a second-order-in-time, fourth-order-in-space method, FDTD(2,4) - are applied to a number of problems. The problems include array analysis, cavity resonances, antenna coupling, and shielding effectiveness case studies. The latter includes a simplified model of a commercial airliner, with a personal electronic device operating in the vicinity of the aircraft. The FDTD computations are also compared to measured data for this case. Incorporating PEC and other types of material boundaries into higher-order FDTD is problematic; a hybrid approach using the standard FDTD method in the proximity of the boundary is proposed, and shown to perform well.

HIRF penetration through apertures: FDTD versus measurements

The penetration of high-intensity radiated fields (HIRF) into conducting enclosures via apertures is an EMI issue that is relevant to all aviation. The stories are numerous, of disrupted communications, disabled navigation equipment, etc., due to the effects of EM sources external to the aircraft. Here, the FDTD method is used to predict the shielding effectiveness of conducting enclosures with apertures, and the numerical results are compared with measurements. Several issues related to the FDTD analysis of highly resonant and high-quality factor (high-Q) structures, such as windowing and acceleration techniques, are examined and discussed.

Folded loop antenna for mobile hand-held units

The vertical folded loop antenna, modeled as wire and printed radiating element mounted on a conducting box, simulating a cellular telephone with and without dielectric coating, is analyzed. The finite-difference time-domain (FDTD) method is used to calculate radiation patterns and input impedance. The results are compared with measurements and with NEC data. Very good agreement is obtained in all cases. Parasitic loading is used to enhance the bandwidth of the printed element. The antenna meets the design requirements for existing and future mobile communication systems.

Checkerboard EBG Surfaces for Wideband Radar Cross Section Reduction

Electromagnetic band-gap (EBG) structures have noteworthy electromagnetic characteristics that include their reflection phase variations with frequency. This paper applies this unique reflection phase property to alter the direction of the fields scattered by a radar target to reduce its radar cross section (RCS). This redirecting of the scattered fields occurs when a surface is covered with a checkerboard of alternating EBG structures, and results in a wider frequency band RCS reduction. RCS reduction compared to a PEC surface of 10 dB can be realized over 60% frequency bandwidth. Simulations of monostatic and bistatic RCSs of two dual EBG checkerboard surfaces, square and hexagonal, are compared with those of equal-sized PEC ground planes. The simulated monostatic RCS is also compared with measurements. Both TEz and TMz polarizations for oblique incidence are considered. Excellent agreement is obtained between simulated and measured patterns, for both the square and the hexagonal EBG checkerboard surfaces. An approximate analytical expression is provided as a guideline for a 10-dB RCS reduction of a dual EBG checkerboard surface compared to that of a PEC.

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. >

Design, Simulation, Fabrication and Testing of Flexible Bow-Tie Antennas

Design, simulation, fabrication and measurement of two different novel flexible bow-tie antennas, a conventional and a modified bow-tie antenna with reduced metallization, are reported in this paper. The antennas are mounted on a flexible substrate fabricated at the Flexible Display Center (FDC) of Arizona State University (ASU). The substrate is heat stabilized polyethylene naphthalate (PEN) which allows the antennas to be flexible. The antennas are fed by a microstrip-to-coplanar feed network balun. The reduction of the metallization is based on the observation that the majority of the current density is confined towards the edges of the regular bow-tie antenna. Hence, the centers of the triangular parts of the conventional bow-tie antenna are removed without compromising significantly its performance. The return losses and radiation patterns of the antennas are simulated with HFSS and the results are compared with measurements, for bow-tie elements mounted on flat and curved surfaces. The comparisons show that there is an excellent agreement between the simulations and measurements for both cases. Furthermore, the radiation performance of the modified bow-tie antenna is verified, by simulations and measurements, to be very close to the conventional bow-tie.

Mutual Coupling Compensation in UCAs: Simulations and Experiment

In a beamforming system, mutual coupling among the elements can significantly degrade the system performance. However, the mutual coupling effects can be compensated if an accurate model of mutual coupling is available. This paper utilizes a mutual coupling matrix model to compensate mutual coupling in the beamforming of a uniform circular array. In addition, a circular array of dipoles was built and measurements were performed. The predictions are compared with measurements, and verified with results from full-wave simulations

Analytical evaluation of the asymptotic impedance matrix of a grounded dielectric slab with roof-top functions

An analytical technique is derived to solve the asymptotic part of impedance matrix elements for printed circuit structures using roof-top subdomain expansions. The key to this problem is the analytical transformation from an infinite double integral to a suitable finite one-dimensional (1-D) integral. The newly developed formula is applied to the monostatic radar cross section (RCS) of a microstrip patch. Comparisons are made with measurements and conventional method of moments predictions.

Evaluation of input impedance and radar cross section of probe-fed microstrip patch elements using an accurate feed model

Input impedance and radar cross section results for probe-fed single rectangular and circular microstrip patch elements obtained using an accurate and efficient numerical model are presented. The model uses a full-wave Greens function/Galerkin solution in which the connection of a vertical probe feed to the patch is rigorously included in the analysis by use of a special basis function called the attachment mode, which is derived from the corresponding cavity model solution. Comparisons with measured input impedance and monostatic radar cross section data demonstrate the efficacy of the theory. This model accurately predicts the performance of probe-fed patches printed on thick and/or high dielectric constant substrates and patch elements with more than one probe feed, cases where other less rigorous models fail. >

HIRF penetration and PED coupling analysis for scaled fuselage models using a hybrid subgrid FDTD(2,2)/FDTD(2,4) method

A hybrid method of subgrid FDTD(2,2) with FDTD(2,4) is presented. Both the standard FDTD(2,2) as well as the hybrid technique are applied to shielding effectiveness analysis of a scaled model of a Boeing 757. Also, analysis of EMI generated by personal electronic devices is performed on the same scaled fuselage model.