Antenna Systems & Technology - Fall 2016 - 16

FEATURE ARTICLE New Loop and Adcock Array Technology for Improved Radio Direction Finding Performance Ralph Prigge, Product Manager - Alarnis Antennas Introduction The incessant efforts to reduce the size of antennas while sacrificing little or no performance, often driven by the particular requirements for tactical applications, resulted in the development of a new antenna technology which finds application in radio Direction Finding (DF). A topology for a stabilised anti-phase dual-fed loop antenna is presented that reduces sensitivity to cross-polaried wave incidence, increases gain at higher frequencies and provides improved pattern stability when compared to traditional crossedloop antenna designs. This loop topology provides the desirable qualities of Adcock arrays at high frequencies whilst substantially improving the low frequency sensitivity of electrically small Adcock arrays. Background The Watson-Watt method of radio direction finding (DF) is traditionally associated with the use of two different types of antenna arrays. The more popular is the Adcock array, often comprised of two radiating elements, typically crossed monopoles or dipoles, fed in anti-phase. A crossed pair of Adcock arrays (e.g. shown in Figure 1) is used in conjunction with the Watson-Watt DF method, resulting in a four-element array. The second type of antenna array is the crossed loop array (Figure 2) where two loop antennas are positioned with their axes perpendicular to each other and parallel to the ground. Figure 1. Crossed dipole pair Adcock array Figure 2. Crossed loop array Relative Merits of Loop and Adcock Arrays Loop-based Watson-Watt arrays are generally perceived to be inferior to Adcock arrays, mainly due to the inherent sensitivity of loop-based radiators to Horizontal Polarization (HP), which causes unavoidable estimation errors (Figure 3) when the incoming wave is not purely vertically polarized (VP), and secondly, due to inaccuracy when waves are incident from high angles of elevation. These problems often come to the fore at low frequencies (in the HF band) with sky wave propagation but not ground wave propagation. At higher frequencies, however, ground waves support appreciable HP signal propagation and loops then suffer accuracy degradation due to cross-polarization, making them unpopular for use in the upper HF and VHF regions and above. Figure 3. RMS DF error when incident wave has cross-polar (ϕ-oriented) content 16 Antenna Systems & Technology Fall 2016 Figure 4. Gain of an Adcock Array www.AntennasOnline.com http://www.AntennasOnline.com

Table of Contents for the Digital Edition of Antenna Systems & Technology - Fall 2016