A multiradius, reciprocal implementation of the thin-wire momentmethod

An implementation of the moment method, entitled the multiradius bridge-current (MBC) moment method, for electromagnetic analysis of multiradius thin-wire structures (including multiwire, multiradius junctions) is presented. It is extension of the authors' uniradius bridge-current reformulation (see ibid., vol.37, p.1224-34, Oct. 1989) of Richmond's uniradius thin-wire theory (1974). The method features an exactly symmetric mutual impedance matrix ensuring reciprocity between sources, it is unconstrained with respect to both the length ratio and the radius ratio of adjoining segments (provided that the wires are electrically thin), and it permits the self-consistent inclusion of coaxial cable sections in the configurations under analysis. The method is validated through comparison with transmission-line theory for a two-wire line and a coaxial cable, and through comparison with measurements on a sleeve monopole antenna and a log-periodic dipole antenna. The MBC moment method program is shown to surpass the Numerical Electromagnetics Code (NEC) in terms of reciprocity and convergence for both an AM broadcast tower detuning stub problem and a bent two-wire transmission-line problem     

Resonance phenomena of log-periodic antennas: characteristic-modeanalysis

The method of characteristic modes is implemented in the form of a general-purpose computer program based on an established moment-method program using piecewise sinusoids. Application of the method to the log-periodic dipole antenna (LPDA) yields characteristic modes that are insensitive to changes in moment-method segmentation and are revealing with regard to parasitic resonance phenomena. A study of the modes on the LPDAs shows that the asymmetry resonance is dominated by a mode that is not restricted to one cell, although the single-resonant-cell postulate remains a good first-order explanation. On the other hand, the symmetric termination resonance is shown to involve more than one mode. A numerical study of various antenna deformations shows that only length extensions cause strong asymmetry resonances. The E-plane array is analyzed, and the results are shown to compare favorably with experiment     

Medium Frequency Re-Radiation from an Unstrung Steel Power Line Tower

Computations of AM broadcast re-radiation from a power line tower without phase conductor wires or overhead ground wires are compared with measurements. The tower is a 500 kV type located near Toronto. Computations of re-radiation using a detailed tower model including ground effects due to the footings, compared with measurements done in the near field of the tower, show good agreement in terms of frequency response and variation with distance from the tower. Two simplified computational models are compared with the detailed one in the far field and found to be accurate enough to be of practical use. Methods are given for deriving the models, predicting re-radiation from any tower within certain specifications, and extrapolating near-field measurements to far-field values.     

The cocurrent downflow contactor–a novel reactor for gas-liquid-solid catalysed reactions

The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold-up (E_g) values of 0.5–0.75, interfacial areas in the range 1000–6000 m²m^?3 liquid and k_La values in the range of 0.15–1.55 s^?1 for absorption using the O₂/H₂O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface-reaction rate controlled, due to the very efficient mass transfer (k_La up to 11.75 s^?1 under reaction conditions) and application of swirl flow-enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale-up can be undertaken without loss of performance efficiency.     

Colon extended physiologically based extraction test (CE-PBET) increases bioaccessibility of soil-bound PAH

Assessment of the risk to human health posed by contaminated land may be seriously overestimated if reliant on total pollutant concentration. In vitro extraction tests, such as the physiologically based extraction test (PBET), imitate the physicochemical conditions of the human gastro-intestinal tract and offer a more practicable alternative for routine testing purposes. However, even though passage through the colon accounts for approximately 80% of the transit time through the human digestive tract and the typical contents of the colon in vivo are a carbohydrate-rich aqueous medium with the potential to promote desorption of organic pollutants, PBET comprises stomach and small intestine compartments only. Through addition of an eight-hour colon compartment to PBET and use of a carbohydrate-rich fed-state medium we demonstrated that colon-extended PBET (CE-PBET) increased assessments of soil-bound PAH bioaccessibility by up to 50% in laboratory soils and a factor of 4 in field soils. We attribute this increased bioaccessibility to a combination of the additional extraction time and the presence of carbohydrates in the colon compartment, both of which favor PAH desorption from soil. We propose that future assessments of the bioaccessibility of organic pollutants in soils using physiologically based extraction tests should have a colon compartment as in CE-PBET.     

On the suppression of asymmetric artifacts arising in animplementation of the thin-wire method of moments

The original version of the thin-wire frequency-domain moment-method program developed by J.H. Richmond (1974) has been modified to suppress the computation of nonphysical asymmetric fields. Richmond's implementation uses piecewise sinusoidal expansion and testing functions, along with filamentary current approximations. The modified version, termed the bridge-current version, is described. The original program and the modified version are compared with each other and with simplified theory, where applicable, on the following symmetrical structures: a rectangular wire loop, a two-wire transmission line, and a log-periodic dipole antenna. The bridge-current version is shown to eliminate the computation of nonphysical asymmetric fields, to be essentially invariant with respect to variations in segmentation for the above-mentioned structures, and to produce results that compare well with simplified theories where applicable. It is noted that the bridge-current version is particularly advantageous for structures that include close-spaced parallel wires connected by short wire segments     

Medium Frequency Reradiation from a Steel Tower Power Line with and without a Detuner

Measurements and computations of field strengths and tower base currents are presented for a power line section consisting of 5 towers strung with skywires and illuminated by a nearby monopole antenna. This is done first without detuning stubs, and then with detuning stubs on one tower to reduce the re-radiation. Good agreement is obtained between moment-method computations and full-scale measurements. Computational modelling techniques to represent the power line, the detuning stubs and ground effects are given and validated. The beneficial effect of increased stub spacing from the tower leg is identified.