A Sandwich Wire Aerial

Abstract

This paper describes the design and characteristics of a planar array made as a stack of sandwich wire aerials. The design is based on the sandwich wire aerial descrlbed by Rotman and Karas in the Microwave Journal of August, 1959, but it incorporates a number of improvements. Although this type of aerial was first reported ten years ago; it has received very little attention. We have found, however, that in its present form it has many attractive features. It gives excellent electrical performance, occupies a small volume and is light and rugged. The aerial is illustrated in Figure 1.The operation of the aerial can be explained by considering a transmission line consisting of a central conductor mounted between two ground planes, as shown in Figure 2. When -the central conductor is parallel to the ground planes, the energy propagates along the line in the guided strip line mode. No radiation takes place because the currents in the wires are travelling in opposite directions, at intervals of a half wavelength and thus the radiation from each half wavelength is in anti-phase and cancels out. Any tendency for incomplete cancellation is suppressed by extending the side wall-ground-planes in front of the centre conductor, so as to form a cut-off wave guide for waves whose polarized parallel to them. If, however, the line is undulated with respect to the side walls with a period of one wave length, as shown on the diagram, then the components of current perpenldicular to the side walls are all in the same direction and radiation results. Because this component of the currents is proportional to the sine of the angle of inclination, one would expect that the amplitude of radiation to be substantially independent of frequency over a wide band. This is confirmed by experiment.Since propagation is by a transverse electromagnetic mode rather than a waveguide mode the velocity of the wave is almost independent of the physical dimensions of the line. It follows that the accuracy of the trough in which the conductor lies, or the exact position of the conductor within it, is not critical and consequently the ground plane and side walls may be constructed by folding thin sheets of aluminium into troughs. For lightness and accuracy the undulating wires are supported on thin dielectric strips, and are made using printed circuit techniques.Several different forms of periodic structure can be employedfor example, triangular, sinusoidal, trapezoidal etc. A trapezoidal form has been used since it is easy to specify and construct and has its bends a quarter of a wavelength apart which assists the cancellation of reflections from them. Residual reflections from the bends and R. Grham and C. Dawson are with Research and Advanced Projects, Elliott-Automation Radar Systems Limited.