Design and analysis of a low profile broadband wide scan array

Abstract

The development of a broadband, wide-scan, small and low profile phased array is the topic of this paper. A survey of the literature shows that there are a number of radiating elements that may be used in broadband phased arrays. The flared notch or Vivaldi element [1] has the required bandwidth and scan capability but it is not very compact, at least 0.5 – 0.75λ C tall, and has high cross-polar levels in the off principal planes. The thicker ridged parallel plate element [2, 3] has better cross-polar isolation performance but its height is about the same. Another possible low profile element is the high dielectric constant puck radiator with stacked discs [4]. It has about an octave bandwidth, wide scan capability and a height of 0.15λ C . The problem with this element is the consistency and uniformity of its fabrication to be used in an array. For the lower frequencies, the puck becomes rather large and heavy. The lowest weight radiator reported to-date is the long slot radiator with periodic feeding by connected dipoles [5] with a ground plane. In a small array implementation of such elements, 4 × 8 array, the mismatch loss is rather high. Such elements are purported to have a lower mismatch loss only for array sizes larger than 10λ × 10λ. Further, there is the balun loss to transform the 400-ohm array feed impedance back down to 50-ohm. Height of the radiator is approximately 0.29λ C . Quite clearly, this type of radiator is not particularly suitable for use in compact, small arrays. An interesting radiator is the cavity-backed patch with balanced feed probes used in a UHF array [7]. The size of the radiator is 0.5λ C × 0.5λ C with a height of 0.12λ C . This array, however, does not scan and has an operating bandwidth of 43%. The capacitive feed probes require a rather elaborate design to achieve the wide bandwidth. In the present development, we will show that much broader band operation can be obtained through the use of simple stepped probes feeding cavity-backed patches sized for wide scan without incurring grating lobes. The cavity is filled with rohacell foam and the patches are supported by layers of this foam. Such a radiator designed for a maximum scan of 45° in the E-plane is shown to have a frequency ratio greater than 2.42:1 or bandwidth > 83% and with an active scan VSWR of 2:1. The broadband property is the result of capacitive feeding of the patch with two large stepped probes in a balanced configuration. Further, the element is compact, with a height at less than 0.25λ C including the balun feed where λ C is the wavelength at mid-band and hence is ideally suited for use in small, low profile phased arrays.