Antenna Systems & Technology - Fall 2016 - 17

FEATURE ARTICLE Adcock arrays however, suffer a disadvantage over loop elements from a sensitivity point of view when the arrays are small in size as shown in Figure 4, which makes Adcock arrays less desirable for use in small tactical systems. New Loop Topology A new method has been invented and patented that overcomes to a large extent the shortcomings that loops have when used in a DF array. This stabilised anti-phase dual-fed loop significantly reduces the sensitivity of the loop to cross-polarized signals which allows the loop to be used in a wider range of applications to perform DF. The new topology also overcomes the pattern breakup observed in normal loop topologies when the antenna starts becoming large. This allows the new loop design to Figure 5. Comparison of loops for DF estimation be used over a much wider frequency range and to start approaching conventional Adcock Array performance at higher frequencies (approaching λ/5). Figure 5 shows the improved DF accuracy achievable when using the new loop topology in environments with both VP and HP incident waves. Measured Performance Comparative measurements of a prototype of the new loop topology were conducted against a benchmark top-fed loop of the same size and shape. Figure 6 shows sample patterns obtained when the source is at a 45° angle (providing mixed HP and VP incoming waves) for the two antennas. Figure 8 shows the measured gains of the two loops, revealing an additional gain benefit of the proposed topology over a top-fed loop. Use in a DF System Figure 7 shows the typical DF error obtained by a DF array consisting of a pair of crossed stabilized anti-phase dual-fed loops. Measurements are done with a purely vertical source and one with a 35° slant. Note that the noisy region below 40 MHz arises from measurement problems with low SNR, rather than a problem with the array. Figure 6. Measured antenna patterns for top-fed loop and stabalized loop with source at 45° slant Figure 7. Measured accuracy of a DF array utilizing stabilized anti-phase dual-fed loops Hybrid Adcock-Loop Element Figure 8 (page 18) shows the gain of an Adcock array of similar size to the loop discussed in the previous section. We see by way of this example that a negative feature of the Adcock array is a lower gain, when electrically small, compared to a physically equivalently sized loop antenna. For this reason, when implementing compact DF systems such as for use in man portable applications, it can be preferred to resort to loop antennas, for higher DF sensitivity, despite their known cross-polarization problems. Article continued on page 18 www.AntennasOnline.com Fall 2016 Antenna Systems & Technology 17 http://www.AntennasOnline.com

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