Phased Array Case

Several features are worth observing. First of all the fluctuations from element to element have been greatly reduced but obviously not completely eradicated. Second, the Floqnet cnrrents in Fig. 1.3c have been reduced from 0.045 mA to 0.032 in Fig. 1.5a—that is, a reduction of approximately 0.032/0.045 = 0.71. 

This reduction is easy to explain by inspection of the equivalent circuit shown in Fig. 1.6a. 

Here the voltage generator is connected in series with its generator impedance Zq and the scan impedance Z. The ratio between the currents 

We emphasize that this reduction is by no means embarrassing. It is in basic agreement with the conjugate matched case where the current ratio would be 0.50 and the efficiency 50%. See also the discussion in Appendix B.9. 

But how do we explain the much stronger reduction of the ripples associated with the surface waves Well, we shall later in Chapter 4 investigate surface waves in much more detail. It will there be shown that the terminal impedance associated with the surface waves is quite low, say of the order of Zsarj 10 ohms for each of the two surface waves. Thus, by the same reasoning as for the Floquet currents above, we find for each surface wave a reduction equal to 10/(10 + 100) = 0.091. This is of course an average value but explains the strong ripple reduction observed in Fig. 1.5a. 

---

In this chapter we shall smdy the FSS case in more detail and in particular how to condol the surface waves. The phased array case will be discussed in Chapter 5. 

We also indicated that this type of surface wave could be controlled in various ways. One approach is to load each element resistively. If used as an FSS, the resistors should have a low value in order not to significantly attenuate the reflected signal. In case of phased arrays a resistive loading could be obtained by simply feeding the elements from constant voltage generators with realistic generator impedances. 

One might well ask the question. Why not jnst operate in a frequency range between the two types of surface waves WeU, in the case of an FSS it has been demonstrated numerous times that stability with angle of incidence can be obtained only for small interelement spacings (see, for example, reference 34). And basically the same is true for phased arrays in particular if designed for broad bandwidth. See Chapter 6 for details. 

It is by now well known that the variation with angle of incidence of the scan impedance of phased arrays as well as the bandwidth of hybrid radomes can be reduced by using dielectric slabs placed between free space and the device in question. To be sure, the dielectric constant should in general be less than 2 (for a single slab) and the thickness should be somewhat thicker thau A./4 in the dielectric. An example of applying this technique is shown in Fig. C.15. Compared to the uncompensated case in Fig. C.13, we observe some improvement... 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

The alternation of temperature in a hydrothermal reaction was demonstrated to be crucial in changing the crystal phases or morphologies of nanomaterials. In the case of K2Cr207, when the reaction temperature was increased to 180 °C, K-OMS-2 microspheres consisting of nanoneedles were synthesized in contrast to the nanoduster arrays composed of tetragonal prism nanorods synthesized at 120 °C (Figure 8.1a-c). In the case of Na2Cr207, when the reaction temperature was 100 °C, Na-OMS-2 phase was formed. However, when the reaction temperature was... 

Vibration Diagram Method. In actuality the last cases above are not described accurately by this dipole array model because actual phases of the electric fields are significantly altered from those of linear waves. (A more realistic, but complex model is to consider amplitude and phase characteristics of the oscillating vertically polarized component of electric field resulting from rotation of a line of transverse dipoles of equal magnitude but rotated relative to each other along the line such that their vertical components at some reference time are depicted by Figure 2.) For this reason and to handle details of focused laser beams one must resort to a more mathematically based description. Fortunately, numerical... 

---