This paper introduces a moment-method formulation, which is capable of accurately modeling moderately thick cylindrical wire antennas. New algorithms are presented for the efficient computation of the cylindrical wire kernel and related impedance matrix integrals. These algorithms make use of exact series representations as well as efficient numerical procedures and lead to a significant reduction in overall computation time for thicker wires. Another major advantage of this moment-method technique is that it is no longer restricted by the segment length-to-radius ratio limitations inherent in past formulations, thereby making it possible to achieve solution convergence for a much wider class of wire antenna structures. Several examples illustrating the superior convergence properties of this new moment-method formulation are presented and discussed.

A method of moments approach for the efficient and accurate modeling of moderately thick cylindrical wire antennas

The authors demonstrate that the difficulties encountered when modelling anti-resonant antennas are primarily due to the conventional variable gap source model which is commonly employed in thin-wire moment method codes. An alternative source model is introduced, for use in analysing this class of antennas, where the essential feature is that the gap width of the feed region remains fixed. The fixed gap model is compared to the variable gap model for both a half-wave dipole, where the variable gap is still a valid source model, and for a full-wave anti-resonant dipole, where the fixed gap source is shown to be superior to the variable gap model.

Accurate modelling of anti-resonant dipole antennas using the method of moments

The static electric field distribution in the gap between two solid perfectly conducting semi-infinite cylinders is obtained in terms of a Fourier-Bessel eigenfunction series. For dipole antennas whose cylinder diameter 2a and gap length 2d are both much less than the operating wavelength \lambda , this field can serve as the quasistatic excitation field in the gap of the dipole. However, the Fourier-Bessel series is slowly convergent. It is transformed into a rapidly convergent series of ultrasphetical polynomials whose weighting function explicitly satisfies the Meixner edge condition. Numerical results are presented graphically for both the axial electric field on the gap surface and the associated potential distribution. Gap ratios of d/a from 0.01 to 10.0 are considered and it is shown that as d/a \rightarrow 0 the solution approaches the two-dimensional solution obtainable by conformal mapping.

The static electric field distribution between two semi-infinite circular cylinders: A model for the feed gap field of a dipole antenna

The analysis of a cylindrical post mounting structure in a rectangular waveguide is outlined. Theoretical results, which may be obtained directly from the theoretical expressions without the use of empirical factors, are shown to be in excellent agreement with experimental measurements. The theoretical results may also be used to deduce an equivalent circuit for the mount, and such an equivalent circuit is presented for a case of frequent interest, namely that where the TE10 mode is the only propagating waveguide mode.

Analysis and modelling of a single-post waveguide mounting structure

A center-fed, solid, circular cylindrical dipole of radius a with feed gap of width 2d radiating in a circular waveguide of radius b terminated in infinite ground planes is rigorously analyzed by applying both the conservation-of-complex-power technique and the multiple-reflections technique. The analysis begins by studying the dependence of the dipole admittance on its feed gap width and on its length, 2l, as well as on b, with ka (k=2 pi / lambda is the number) as a parameter. From the decreasing amplitudes of the almost periodic oscillations of these input admittances as b/ lambda is increased, the input admittances of dipoles radiating in free space (b to infinity ) are estimated using a variable-bound approach. The effect of gap width (d/a >

On the effect of gap width on the admittance of solid circular cylindrical dipoles

A numerical comparison is made between the admittance of an infinite cylindrical antenna excited by a realistic electric field across a finite gap, and the admittance found by assuming a constant electric field in the same gap. A simple approximate relation between the two is derived, while a second simple expression shows how the admittance changes as the gap width is varied.

J. Jacobsen

Papers

Admittance of an infinite cylindrical antenna with realistic gap field