A superdirective array of normal mode helical dipoles

The principal characteristics of moderately superdirective normal mode helical dipoles are described both theoretically and experimentally. Use of a thin helix approximation permits integration of an assumed sinusoidal current distribution along a linear, foreshortened structure and yields a simple formula for the radiation field. Graphical displays of experimental and theoretical results illustrate that beamwidths of the order of 45\deg with 22 db sidelobe suppression are available in dipoles which have a total length of only 0.6 wavelengths. In the theoretical analysis of helical dipoles, a linear relationship is assumed between the axial phase constant and frequency. The experimentally determined resonant length and mode of the structure define the proportionality constant, s, between phase velocity and free space velocity such that the linearized k-\beta relationship used in radiation pattern analysis is \beta = k/s . A method for determining the resonant length of tape helices from theoretical k-\beta diagrams is described. The feasibility of utilizing helical dipoles as array elements, in applications where efficiency is secondary to size and directivity, is illustrated. The pattern multiplication technique is used in deriving an expression for the E -field for a two-element array. It is shown that with proper phasing of the fixed length elements, beamwidths in the range 30-40 degrees with front-to-back ratios in excess of 20 db over a pattern bandwidth representing 8% of the resonant frequency of an individual element are commonly available. A description and experimental data are provided for the tunable dipoles developed in order to achieve broadband operation. A hybrid is introduced in the array feed network so as to maintain a high front-to-back ratio over a broad range of frequencies. Experimental radiation patterns are displayed for a case in which the element phasing yields a front-to-back ratio which is independent of frequency. The overall dimensions of such moderately superdirective arrays are typically less than 0.2 \times 0.8 wavelengths and can readily be used over a broad frequency range when tunable (variable length) dipoles and a hybrid junction are employed.