Impedance stick

Steric hindrance When a nonionic surfactant adsorbs on an interface between oil and water, the hydrophobic part of the molecule wUl orient itself towards the oily phase, while the hydrophUic part will stick out into the aqueous phase. One can envisage the interface, therefore, as having a coat of hydrophihc chains sticking out of the interface. When two oil droplets approach each other, the two coats would first make contact. The only way that the two droplets can coalesce is when the nonionic surfactant molecules move away from the contact point. However, they are strongly adsorbed and therefore impede the coalescence. Hence, when two droplets with such a layer approach each other, the coats will repel each other. Thus, the droplets will move apart again, and coalescence is prevented. 

Much of the remainder of the pool was covered with the SAPER-T tiles. These were comparatively cheap ( 50/sq.ft.) but contributed only 4 to 5 dB ER, as shown in figure 9. These tiles were used in this application primarily as an inexpensive low frequency reverberation reduction coating. (The reduced performance measured for these tiles in this application is a consequence of the unfavorable backing impedance presented by the thick concrete walls and method of attachment which used double stick tape instead of a rigid adhesive bond.)... 

Viral Inhibitors. A multivalency approach was used to inhibit influenza virus in-fectivity. Several sialic acid-based polymers have been synthesized that inhibit flu virus receptor-binding activity, which in turn impedes flu viruses from sticking to cell surfaces and subsequent viral infection of cells (see also Section 2.1.4) (155,156). 

Mechanisms These drugs are inhibitors of neuraminidase produced by influenza A and B. This viral enzyme cleaves sialic acid residues from viral proteins and surface proteins of infected cells, preventing clumping of newly released virions and sticking to cells that are already infected. By preventing these actions, neuraminidase inhibitors impede viral spread. Decreased susceptibility to the drugs is associated with mutations in viral neuraminidase. 

The area of the skin wetted or in contact with the electrolyte solution is called the effective electrode area of the electrode. EEA is a dominating factor determining electrode/skin impedance. EEA may he much larger than the metal area in contact with the solution (EA), which determines the polarization impedance. The electrode is fixed with a tape ring outside the EEA. Electrolyte penetrating the tape area increases EEA, hut reduces the tape sticking area. Pressure on the electrode does not squeeze electrolyte out on the skin surface because of the rigid container construction. 

Finally, the mutual impedance between a stick array with an external element is defined as... 

When all the mutual and the self-impedances of an assembly of stick arrays as shown in Fig. 3.8 has been determined, it is a relatively simple matter to find the currents on the elements. In fact, it was already discussed in my earlier book [62] and need therefore only to be restated here for easy reference. 

It is also of interest to consider the case where we excite a single column or row with identical voltages while all the other elements are terminated in loads like before. The impedance measured at the terminals of the column of elements in question is often called the embedded stick impedance Zgmb stk- We shall see that it often makes more sense to consider the embedded stick impedance rather than the embedded impedance of a single element. 

Consider Fig. D.l, where we show two infinite stick arrays q and q. We denote the mutual impedance between the reference element in array q and element m in array q by The mutual impedance between the reference element in... 

The embedded stick impedance is perhaps best illustrated by considering just three stick arrays as shown in Fig. D.4. Each element in the center column is fed by voltage generators with voltages V° ", where m refers to the row number. The two outer stick arrays are not fed but only loaded with identical load impedances Zl. 

Fig. D.4 The embedded stick impedance Zemb stk is the terminal impedance obsen/ed in the center stick array when all terminals in the center array are fed with voltages and all other stick arrays are loaded with identical load impedances and merely excited parasitically.

Substituting (D.9) into (D.8) finally yields for the embedded stick impedance... 

Inspection of (D.IO) shows that the embedded stick impedance Zemb stk consists of the stick self impedance Z° ° for the center array plus two terms representing the overcoupled impedances from the two outer arrays. 

It can be shown that the embedded stick impedance for any size finite array is structured the same way—that is, as a sum of the stick self-impedance Z° ° plus overcoupled terms associated with all the other parasitically excited stick arrays. 

We have calculated the stick self-impedance Z for an array without ground-plane with dimensions as used in Chapter 6 as obtained from the SPLAT program. This program cannot handle dielectric slabs however, it will approximate the effect of the dielectric underwear [136] by placing cylindrical dielectric shells around each element. The thickness of these dielectric coatings should be approximately equal to the thickness of the underwear. ... 

Thus, we show in Fig. D.5 an example of the stick self-impedance Z° ° with dimensions as shown in the insert and as obtained from the SPLAT program (includes a matching transmission hne). 

Furthermore, we show in Fig. D.6 the embedded stick impedance Zemb stk as given by (D.IO) as also obtained from the SPLAT program and with dimensions as given in the insert (includes matching transmission line). 

It is therefore of interest to investigate just a single stick array when we feed only a single pair of terminals while the rest are loaded with the same load impedances Z. We have denoted the terminal impedance for this case for the embedded element stick impedance Zemb eie stk- Examples are shown in Fig. D.7. The array has the same dimensions as nsed in the previons section (see insert). The calcnlations were obtained from the method of moment program ESP [137]. Similar to the SPLAT program nsed to obtain the resnlts in Figs. D.5 and D.6, it uses dielecttic cylinders placed aronnd each element. 

Finally we show in Fig. D.8 the self-impedance of a single dipole without capacitors or any other elements present but includes matching sections (see text earlier). As we would expect, the real part is low and the reactance is capacitive and high. Obviously, this is what sticks in peoples minds when they say that the type of array discussed in Chapter 6 will never have a chance for yielding a broad bandwidth (no pun intended). Note how the impedance shrinks as more elements are added and excited as observed in Fig. D.7. 

Fig. D.5 The stick self-impedance 7P ° for an array without a groundplane. Eiement dimensions identical to the case in Fig. D.3 (see insert). Obtained from the SPLAT program. The dieiectric underwear is rrxxleled by placing dielectric shells around the elements. Diameter approximately equal to total thickness of the underwear, includes a matching transformer (see text).

Fig. D.6 The embedded stick impedance Zemb stk as given by. 10) (no groundpiane). The center stick array is driven whiie the two outer stick arrays are Just haded with Zl = 100 ohms. Array dimensions as in Figs. D.3 and D.4 (see insert). From the SPLAT program, inciudes matching transformer (see text).

Fig. D.7 The embedded element stick impedance Zemb eie atk for element dimensions identical to Fig. D.6. From the ESP program. Dielectric "undenvear modeled by dielectric cylinder around the elements, (e With one parasitically excited dipole on top and bottom. Loaded with Z = 100 ohms, (b) With two parasitically excited dipoles on the top and two on the bottom. Loaded with Zl = 100 ohms. Includes matching section (see text), (c) With three parasitically excited dipoles on the top and bottom. Includes matching section (see text).

Thus, we have plotted all impedances in Smith charts normalized to the same impedance, namely 100 ohms. We can then compare the scan impedance Za as given by (D.5) in Fig. D.3 with the stick self-impedance as shown in Fig. D.5. Although some similarities are found at the middle to higher frequencies, the difference at the lower frequencies is simply enormous. One could then wonder whether that difference could be reconciled if we instead of the stick self-impedance Z would consider the embedded stick impedance Zen stk as... 

To summarize Using the embedded stick impedance Zemb stk instead of the stick self-impedance Z may lead to even greater deviation from the scan impedance Z at the middle frequency range. At the lower frequencies, Z and Zemb stk are similar but both differ substantially from the scan impedance Z. ... 

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