How to Control Surface Waves

We saw earlier that surface waves radiate and can lead to a significant increase in the backscattered field. It is therefore of great interest to investigate ways to control them. We recall from Fig. 4.5d as well as Fig. 4.12 that the surface waves basically were driven by two semi-infinite arrays located on each side of the finite array. Thus, if we could somehow introduce a barrier between the two semiinfinite arrays and the finite array, we would expect a weaker excitation of the surface waves in the finite array. One such possible practical arrangement could consist of a finite number of columns between the semi-infinite and finite arrays where the column currents had been reduced by insertion of load resistors in each element. Such an arrangement could also serve as absorbers of the two surface waves as well as the Floquet waves incident upon the edges of the finite array. 

in Fig. 4.17 we show three cases where the elements in the edge colnmns (one at each edge) of the array shown earlier in Fig. 4.15 have been loaded with 50, 100, and 150 ohms, respectively. We observe only minor changes at the low frequencies 2-6.2 GHz where surface waves do not exist. However, a significant reduction is obtained at the higher frequencies 6.3-12.0GHz, where surface waves are prevalent from 6.3 to 8.5 GHz (see Fig. 4.15). Note further that the greatest reduction is obtained when the load resistors are 100 ohms (see Fig. 4.17, middle). 

Furthermore, we show in Fig. 4.18 three cases where one, two, and three columns at each end of the finite array have been loaded with various resistors as indicated at the top of the figure. First, to the left we show the single column case, namely the optimum case shown earlier in Fig. 4.17, middle. Next follows the two- and three-column cases as shown in Fig. 4.18, middle and right, respectively. As we would expect, a steady improvement is observed as we increase the number of columns. 

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