Shielding near-field

Dual chamber test method Measures near field shielding effectiveness by indicating the signal attenuation caused by passage through test material. 

In this paper, we provided the rationale and definition of a benchmark test called BMTl to look at the implications of THM couplings on safety parameters in the near field of a hypothetical repository. This hypothetical repository possesses composite features since it is based on a Japanese design, with a Japanese bentonite used as buffer material and the heat output characteristics of Japanese spent fuel. However, the permeability and strength characteristics of the rock mass are based on typical properties of granites of the Canadian Shield. 

Useful shielding effectiveness values for commercial applications in electronic housings are in excess of 40 dB at frequencies of 1 GHz. For military applications and for near-field shielding requirements, even better performance, in the region of 80-100 dB, is required. 

When discussing shielding effectiveness it is necessary to consider two radiation zones the near-field and the far-field (or smooth wave) zones. The distinction between them lies in the distance from the. source of radia tion. If the distance from source to shielding is less thai. one-sixth of the free path wavelength of the radiation to be shielded, the radiation is dominated by the smaller multipole components of the source field and is described as within the near-field zone." Above this zone it is in the far-field zone. A more detailed consideration of the problem can be found in Ref. 98. 

For near-field shielding, just as for far-field shielding, it is possible to approximate a general expression by means of the limits of the electrically thin and electrically thick samples. At the limit of an electrically thick shielding (c//5 > 1 or oj > oje), near-field shielding effectiveness can be approximated as... 

Also shown is a comparison of the results of the impacts of a near field and a far field on the same sample. It will be seen that, as already outlined in the theoretical consideration, the shielding effectiveness is higher for the near-field radiation. The shallow curve that approximates to the results of the near-field measurements is a... 

Fig. 19.76 Theoretically expected values (curves) of far-field and near-field shielding compared with actually measured values (points) of sample 4. (After Ref. 98.)...

Figure 19.77 illustrates that the measured conductivity ranges from 0.3 to 8 S/cm. The data generally fit the combined semiempirical expressions. The tendency to display a sizable anomaly at 300 MHz is evident. There is likewise a tendency to show a lower value than expected at 1 GHz. At low frequencies (30 and 100 MHz) the value of the measured near-field shielding lies closer... 

Table 19.7 Near-Field Shielding Effectiveness (SEn) of Various PVC/ICP Blends 3.2 mm thick. Measured at the Four ASTM Standard Frequencies (30, 100, 300 MHz, and 1 GHz) Using the Two-Chamber Box Method and Compared with Far-Field Shielding Effectiveness (SEf) at 1 GHz...

Fig. 19.77 Near-field shielding effectiveness data for samples 4, 7, and 9, as theoretically predicted for the ranges o> > Wc and (o < (o and as measured in the two-chamber box.

Near-field data of various ICP blends are summarized in Table 19.7. The near-field shielding values display the expected behavior. The anomaly of a relatively high figure at the frequency of 300 MHz is an exception. This, however, is not confined to our data alone. This effect is well known from the literature on measurements with the two-chamber box with various materials. It may be assumed that this is an effect that is due to the specific choice of dimensions of the standard two-chamber box used. 

An electromagnetic wave can be in near-field or far-field, depending on its distance from the source. The transition region of one to another is at a distance of XHti from the source, where is a wavelength of EMI radiation. When the operating frequency of an electronic device increases, far-field shielding becomes predominant. 

Makela et al. [875] carried out detailed studies of the EMI-SE properties of 1 to 30 /im thick camphor-sulfonic-acid-doped P(ANi) films having conductivities in the 10 to 100 S/cm region. Measurements were carried out in the near-field with a dual chamber, and in the far-field using a transmission line method, in 0.1 MHz to 1 GHz region. A strong correlation with surface film resistivity was found. Multi-layered structures were found to enhance shielding considerably, up to 40 dB at 100 MHz to 1 GHz. Fig. 19-3 summarizes some of their results in the near and far field. 

Fig. 19-3 Near-field (at 10 MHz) and far-field shielding as a function of surface resistance R for thin P(ANi)/CSA films. After Reference [875], reproduced with permission.

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Section 20 21 Acyl chlorides anhydrides esters and amides all show a strong band for C=0 stretching m the infrared The range extends from about 1820 cm (acyl chlorides) to 1690 cm (amides) Their NMR spectra are characterized by a peak near 8 180 for the carbonyl carbon H NMR spectroscopy is useful for distinguishing between the groups R and R m esters (RCO2R ) The protons on the carbon bonded to O m R appear at lower field (less shielded) than those on the carbon bonded to C=0... 

Z Arrangement was also ascribed to the isomer absorbing at higher field in the case of the ethyl compounds. CH and CH2 protons near the ring nitrogen are shielded by the aromatic ring in the Z compound. The protons at the ring carbon absorb at lower field (near 5.2 p.p.m.) in the Z compounds than in the E compounds (4.50-4.70 p.p.m.). The chemical shift of this proton may be used for E-Z discrimination in further substances. 

For a near-total shielding of the field produced by the main cotiductors (i.e. for - /. to be very low), it is essential to have the thickness of the enclosure is netir to 5p as possible. But this m iy prove to be a costly proposition. In addition, ti higher induced current in the enclosure will also tnean higher losses. This hits been established by computing the cost of the enclosure and capitalizing the cost of losses for minimum losses in the eticlosure... 

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