A spiral-grating array

A problem associated with a two-dimensional spiral doublet array is concerned with the fact that use of the spiral doublet as an element to obtain linear polarization leads to spacing difficulties, since the distance between elements is always twice as much in one dimension as in the other. Thus, the allowable scan is seriously limited in one principal plane. A method is proposed for obtaining linear polarization from a single spiral antenna by using it in combination with a polarization grating and a ground plane. Experimental results were obtained by combining eight of these elements in a one-dimensional array. For the broadside position of the main beam, the field was found to be linear, with the cross-polarized component of the main beam being 40 db down from the principal polarization. As the beam was scanned out to
\pm 45\deg
, the linearity decreased quite rapidly (to approximately 10 db at
45\deg
) and the plane of the linear polarization was rotated; sidelobes were higher than predicted. Use of this array rather than an array of doublets would involve a compromise concerning specifications, dependent upon the particular application of the array.