Surface waves from groundplane

So far we have considered surface waves only on finite periodic structures without a groundplane. When a groundplane is added to an array of dipoles, it is usually driven actively. This case is in practice somewhat different from the passive case considered above by the fact that aU elements are connected to generators or amplifiers with impedances comparable to the scan impedances. As explained in Chapter 5, this leads to a highly desirable attenuation of any potential surface waves. 

From a physical point of view we note that the field associated with the surface wave on the latter is composed of an assembly of evanescent waves that is attenuated as we move away from the periodic structure. In fact these evanescent waves have fallen to rather low values at a distance of a quarter wavelength. Since this is about the typical spacing between adjacent slotted surfaces in many applications, we may conclude that the surface wave frequency for a single periodic surface is only lightly affected by the presence of a groundplane or a second slotted surface. 

The fundamental problem is now that all finite periodic structures may exhibit strong presence of surface waves at least at some frequencies as discussed in Chapter 4. We may envision that the finite FSS groundplane alone shows surface waves in one frequency band and the active array possibly in another. However, when the active array is placed adjacent to the FSS groundplane, we would expect both of these frequency bands to change and, possibly, to degenerate into a single frequency band. From a practical point of view, it is of course the surface waves on the combined structure that are most important. 

It was already pointed out in Chapter 2, Section 2.6.2, that a large array with groundplane, uniform illumination, and physical area A also has a receiving area equal to A. Thus, if the array is designed to have no grating lobes and no significant surface waves, we may conclude from one of the most fundamental laws in antenna theory that the directivity D of the array is... 

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