Cavity perturbation technique

A method which circumvents many of the disadvantages of the transmission line and cavity perturbation technique was pioneered by Stuchley and Stuchley (1980). This technique calculates the dielectric parameters from the microwave characteristics of the reflected signal at the end of an open-ended coaxial line inserted into a sample to be measured. This technique has been commercialized by Hewlett Packard with their development of a user-friendly software package (Hewlett Packard 1991) to be used with their network analyzer (Hewlett Packard 1985). This technique is outstanding because of its simplicity of automated execution as well as the fact that it allows measurements to be made over the entire frequency spectrum from 0.3 MHz to 20 GHz. 

Bengtsson, N. and Risman, P. 1971. Dielectric properties of foods at 3 GHz as determined by a cavity perturbation technique. Measurement on food materials. Journal of Microwave Power. 6(2) 107-123. 

The application of microwave contactless techniques to the complex permittivity measurements of organic semiconductors is briefly discussed. Special attention is paid to the cavity perturbation technique of Buravov and Shchegolev and the dielectric resonance technique of Jaklevic and Saillant. 

Recently, Grigera et al. (28) performed dielectric measurements by the cavity perturbation technique in the region of... 

Ohlsson, T., N. E. Bengtsson, and P. O. Risman. 1974. The frequency and temperature dependence of dielectric food data as determined by a cavity perturbation technique. The Journal of Microwave Power 9 129-145. 

Cavity perturbation techniques are used for resonator-based sensing systems [7]. When the complex permittivity or permeability of a region within the resonator is changed, the resonant frequency is shifted and the shift. A/, is given by... 

The microwave fi equency conductivity and dielectric constant were measured using the cavity perturbation technique [90,114,142,143]. The resonant cavity used was cylindrical with a TMoio frequency of 6.5 GHz. The entire cavity is inserted into a dewar filled with He gas to provide a temperature range of 4.2-300 K. Alternatively, the microwave fi-equency conductivity and dielectric constant may be measured using a microwave impedance bridge [144]. 

The permittivity of Fe203 was measured using the cavity perturbation technique [17]. The main components of the measurement system include a resistive heating furnace and a cylindrical TMort) resonant mode cavity. The system measures the differences (frequency shift and change of quality factors) in the microwave cavity response between a cavity with an empty sample-holder and the same cavity with a sample-holder plus the sample at each specified temperature. These differences are recorded in a Hewlett Packard 8753B vector network analyzer and then used to calculate the permittivity. The details about this technique and apparatus used for the measurements can be found in the published literature [17,18]. 

In this study, the concept of microwave reflection loss is introduced to indicate the effect of sample dimension on microwave heating. The microwave reflection loss of ferric oxide has been studied in the temperature range fi-om 297 to -1400 K at 915 and 2450 MHz based on permittivity measurements using the cavity perturbation technique. It is demonstrated that the maximum microwave absorption with reflection loss of-38.46 and -35.97 dB can he obtained for ferric oxide having thicknesses of 0.03 and 0.01 m at 915 and 2450 MHz, respectively. The... 

Although details of measurement methodology are provided in a subsequent chapter, we may note briefly here that for conductivities at frequencies less than 1 MHz (and fixed temperature), a capacitance-conductance bridge suffices. An impedance analyzer can be used to extend this range to 1 GHz. Microwave cavity perturbation techniques must be used in the range 10 GHz to 100 GHz. Kramers-Kronig transformation of Infrared Reflectance data can provide data for frequencies > 100 GHz. 

As mentioned in an earlier chapter, two of the most well established test methods for EMI shielding effectiveness (EMI-SE) are the ASTM (American Society for Testing and Materials, Conshohocken, PA, USA) method ES 7-83 and the US military standard MIL-STD-462. Frequencies in the 1 KHz to 1 GHz region are most commonly used. However, the cavity perturbation technique and the network analyzer technique, both used for generic microwave measurements and discussed earlier in Chapter 12, can also be used to generate EMI-SE data. 

Naishadham [475] used the cavity perturbation technique to study the overall microwave characteristics, including EMI-SE, of the CPs poly(acetylene) (P(Ac)) and poly(p-phenylene-benzobisthiazole) (P(BT)), at 8.9 and 9.89 GHz, and continuously in the 2-20 GHz frequency region. Figs. 19-la.b.c summarize some of his results. 

A very sensitive and accurate technique for the determination of low loss sample properties is called the perturbation technique. This measurement utilizes both the change in frequency and absorption characteristics of a tuned resonant cavity. Full theory and design details are available as a standardized procedure published by the American Society for Testing and Materials (ASTM 1986). 

The techniques used for measurement of AC conductivity are varied, and depend on the frequencies of interest. For v < ca. 10 Hz, a capacitance-conductance bridge can be used [396]. EIS, which yields both real and imaginary components of the complex resistance, i.e. impedance, can be used up to ca. 1 GHz. The cavity perturbation and related techniques in microwave measurements have been used for frequencies up to ca. 30 GHz [397]. For higher frequencies, mathematical (Kramers-Kronig) transformation of IR Reflectance data must be used [398]. 

This NMR technique has been applied to a series of alkanediammonium ions, and the results (induced shifts of proton resonances) are summarized in Fig. 2. It may be seen that the shielding region extends for approximately 4.5 methylene units, or 6 A, which coincidentally is the interatomic distance axially spanning the cavity of cucurbituril. Similar induced chemical shift effects are found in CNMR spectra, and UV spectral perturbations are noted upon encapsulation of certain aromatic-ring bearing ammonium ions (particularly 4-methyl-benzylamine). Conclusive evidence for internal complexation with cucurbituril has been secured by crystallography [3]. 

On the basis of the evaluation of the proton affinity (860.6 kJmoP for hexa-methylbenzene and 845.6 kJmoP for tetramethylbenzene 148)), the possibility of obtaining hexamethylbenzene and tetramethylbenzene as carbocations in the pores of a zeolite had been excluded. However, Haw and co-workers 146) recently demonstrated by means of NMR spectroscopy that H-heptamethylbenzene may be formed in the cavities of a H(3 zeolite. H-hexamethylbenzene and H-tetramethyl-benzene ions have been observed in zeolite H(3 by a combination of IR and UV-visible spectroscopies 149,150). DRS UV-Vis- and FTIR spectroscopy proved to be techniques well suited to verify, under reaction conditions, the existence of stable H-hexamethylbenzene and H-tetramethylbenzene in the zeolite. Owing to the symmetry properties of H-hexamethylbenzene and H-tetramethylbenzene, characteristic changes of their vibrational features were observed when the aromatic system was perturbed upon protonation. In the same study it was found that the lower polymethylbenzene homologues, such as 1,3,5-trimethylbenzene (PA = 836.2 kJmol ), did not undergo appreciable protonation in H(3 zeolite. On the basis of these results, a proton affinity limit for hydrocarbons that form stable... 

Conventional preparation techniques result in non-uniform supported metal species, which are difficult to characterize structurally. Therefore, Gates developed a strategy [266,267] to prepare nearly uniform metal clusters, either on metal oxide support or encapsulated in zeolite cavities. Starting from supported metal carbonyl clusters as precursors, the synthesis proceeded with decarbonyla-tion aiming at a minimal perturbation of the metal frame. However, fragmentation or aggregation of metal clusters may occur. 

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