Active arrays

A. Nehorai A. Dogandzic. Cramer-Rao bounds for estimating range, velocity and direction with an active array. IEEE transactions on Signal Processing, 49(6) 1122—1137, June 2001. 

Spatial arraying has been used traditionally for the assaying of historic compound collections. As the identity of the molecule is correlated to position in the array, this approach carries the least challenge for stmctural assigmnent for active array positions. As an additional aspect to ease the implementation of arrayed libraries, arrayed libraries are commonly handled as solutions. As most biological assays are based on homogeneous test systems, providing the libraries in solution is the obvious... 

Solid CgQ is a redox-active array, with relatively weak intermolecular forces (comparable to interlayer forces in graphite). Therefore, it is a potential host for intercalation chemistry, like graphite or the transition metal sulfides. 

Another difference between the passive and active cases is worth mentioning. The first typically is comprised of FSS arrays with either slot and/or wire elements. They may be located in a stratified medium, but they will in general not directly contain a groundplane. In contrast, the active array will usually consist of a single array of either the wire or slot type, and they may also be located in a stratified medium but they are almost always provided with a groundplane. 

The groundplane serves essentially two purposes. First of all it ensures that we have only a single mainbeam, not two. Second, as discussed in Chapter 2, the groundplane can lead to a significant reduction of the RCS of an active array. However, as discussed in Section 2.9, the area of the groundplane relative to the area of the active dipoles is crucial from an RCS point of view. Thus, the exact modeling of the finite groundplane becomes important. We shall discuss this issue in the next section. 

We shall next study surface waves on active arrays with a finite FSS ground-plane. Our model will be similar to the one used in the previous section—except that in order to properly study surface waves, the model must be considerably wider. 

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. 

Distance from Array to Ground Plane 0.68 cm = 235 Ci on All Active Array Elements... 

Distance from Anay to (kound Plane > 0.68 cm 235 A on Al Active Array Elements... 

These resistive components cause significant attenuation of potential surface waves along the structure. In fact, they will in general be so weak that the surface wave radiation from active arrays can be ignored in contrast to the FSS case discussed in Chapter 4. However, they may be strong enough to produce jitter of the terminal impedance. 

If we are working with an active array, the good news is that in the future each element will most likely be connected to its own amplifier or generator having impedances with substantial resistive components. That is one of the most effective ways to eradicate potential Type n snrface waves. An additional light edge treatment could be useful. 

However, in cases where we are working with FSS s, the element loads, if there are any, will in general be entirely reactive that is, no attenuation of a potential surface wave will take place. Nor is it a good idea to place even small resistors in each element since that would lead to reflection and transmission loss of the principal mode. In that case only a small number of edge columns should be resistively loaded. This is not as effective a way to control surface waves as resistors placed in each element, but then again FSS s are more forgiving in that respect than are the active arrays. 

As defined here and used in the radar range equation, gain is strictly due to passive components and does not include active component (amplifier) gain associated with the transmitter subsystem. This distinction is important in estimating the performance of solid-state active arrays. 

The process of estimating dynamic range and system noise temperate is illustrated in Fig. 17.16. The process is delineated in terms of an active array but can be readily adapted to other radar configurations. To estimate the performance of a conventional passive array, G2 and NF2 would be set to imity in magnitude. The case of N = 1 corresponds to a conventional mechanically scanned antenna. This particular case was... 

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