Surface waves passive arrays

From a practical point of view, the question is of course whether these surface waves can hurt the performance of a periodic structure when used either passively as an FSS or actively as a phased array. And if so, what can be done about it. 

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. 

Much of the spotlight in this chapter has been focused upon surface waves on passive periodic structures. It appears that at this point in time we have two distinct groups, one of which is associated with the presence of a stratified medium placed in the immediate neighborhood of the periodic structure. It always requires a stratified medium to exist but is independent upon whether the structure is finite or infinite. It readily shows up in programs based on the infinite array approach like, for example, the PMM program. 

The second problem is actually more complex. When considering an infinite array, the terminal impedance will be the same from element to element in accordance with Floquet s Theorem. However, when the array is finite, it is well known that the terminal impedance will differ from element to element in an oscillating way around the infinite array value (sometimes denoted as jitter). We postulated that this phenomenon was related to the presence of surface waves of the same type as encountered in Chapter 4. However, there is a significant difference in amplitude of these surface waves in the passive and active cases. This is due to the fact that the elements in the former case in general are loaded with pure reactances (if any), while the elements in the latter case are (or should be) connected to individual amplifiers or generators containing substantial resistive components (as encountered when conjugate matched). 

Automated droplet-based manipulation methods described in this article include electrowetting, dielectrophoretic, thermocapillary, surface acoustic wave (SAW), and pressure-driven channel-based droplet systems. Fabrication of arrays of elements to control these droplet manipulation methods typically involves the use of photolithography. The methods of addressing of the elements have become increasingly sophisticated with several efforts to utilize passive- and active-matrix control strategies. Trends and issues associated with each method are described. 

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