Reflectometry, time-domain

An incident wave (Ejnc) propagates through the transmission line up to a point where the dielectric behavior is changing (end of the line). This yields reflection and the reflected wave (Eref) travels back. 

because the direction of both waves is opposite, the currents also (Iinc Iref) have opposite direction. Therefore, the current is 

It should be noted at this point that this simple approaeh is only valid for MUT with no spatial distribution. If the ehamber does not have negligible geometrical length or the measurement aims in the assessment of higher frequencies, one has to account for the field distribution within the chamber. For more information, refer to Cole et al. (1989). 

There are three special cases—the open line, the short circuit, and the termination with a matching resistor. 

In case of an open line, no energy can be dissipated at the end of the line, thus all energy is reflected and the reflected voltage equals the voltage of the incident wave Vref = Vinc-The reflection factor is r(Z oo) = = 1. 

---

Melting temperature Time domain reflectometry Tumor-inducing Time-of-flight... 

More sophisticated methods that actually measure volumetric water content can also be used, such as time domain reflectometry (TDR). In Figure 14, an example of TDR results is presented. Both the calculated and measured (i.e., TDR) volumetric water contents provide a similar picture of the profile water status by depth with time. Proper soil characterization data, such as those shown in Table 6, are necessary for these calculations and improve understanding of the test system. The determination of water-holding capacity (WHC) at 0.03 MPa field capacity (FC) and 1.5 MPa... 

Performance data Two moisture monitoring systems were installed, one at Disposal Area A and one at Disposal Area AB plus in May and November 1999, respectively. Each monitoring system has two stacks of time domain reflectometry probes that measure soil moisture at 24-in. intervals to a maximum depth of 78 in., and a station for collecting weather data. Based on nearly 3 years of data, there is generally <5% change in the relative volumetric... 

Field methods of measuring soil water are primarily designed to measure water in the range of -10 to -1500 kPa of pressure. However, different instruments have different ranges as shown in Table 5.1. Tensiometers, porous blocks, and thermocouple psychrometers are usually installed in the field and measurements are taken on a regular basis. Neutron probe and time domain reflectometry (TDR) equipment are usually carried to the field each time a... 

Time-domain reflectometry (TDR) involves the use of two or more substantial metal rods inserted into soil. The rods are parallel and are attached to a signal generator that sends an electrical input down the rods. The time it takes the signal to travel down the rods is dependent on the soil s apparent dielectric constant, which, in turn, is proportional to the amount of water in the soil. Upon reaching the end of the rods, the signal is dissipated and the amount of... 

Other near-IR applications which use similar pulse and phase instrumentation as used in lifetime measurements include optical time-domain reflectometry(25) and photon migration in tissue. 26 ... 

G. Ripamonti and S. Cova, Optical time-domain reflectometry with centimeter resolution at fW sensitivity, Sec. Lett. 22, 818-819(1986). 

Dirksen, C. and Dasberg, S. (1993) Improved calibration of time domain reflectometry soil water content measurements, Soil Science Society of America Journal 57, 660-667... 

Or, D. and Wraith, J.M. (1999) Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry A physical model, Water Resources Research 35 Suppl. 2, 371-383... 

Acoustic time domain reflectometry operates on the principle that the velocity of sound and ultrasound increases in gases and decreases in liq-... 

Hart, G. L., Lowery, B., McSweeney, K., and Fermanich, K. J. (1994). In situ characterization of hydrologic properties of Sparta sand Relation to solute travel using time domain reflectometry. 

Time Domain Dielectric Spectroscopy Time Domain Reflectometry... 

Several comprehensive reviews on the BDS measurement technique and its application have been published recently [3,4,95,98], and the details of experimental tools, sample holders for solids, powders, thin films, and liquids were described there. Note that in the frequency range 10 6-3 x 1010 Hz the complex dielectric permittivity e (co) can be also evaluated from time-domain measurements of the dielectric relaxation function (t) which is related to ( ) by (14). In the frequency range 10-6-105 Hz the experimental approach is simple and less time-consuming than measurement in the frequency domain [3,99-102], However, the evaluation of complex dielectric permittivity in the frequency domain requires the Fourier transform. The details of this technique and different approaches including electrical modulus M oo) = 1/ ( ) measurements in the low-frequency range were presented recently in a very detailed review [3]. Here we will concentrate more on the time-domain measurements in the high-frequency range 105—3 x 1010, usually called time-domain reflectometry (TDR) methods. These will still be called TDS methods. 

Studies of the dielectric constant of solutions and the relaxation times of water in the presence of ions have been refined since the 1980 s and indeed difficulties do turn up if one looks at data from measurements over large frequency ranges. The variation of the dielectric constant with frequency has been studied particularly by Winsor and Cole, who used the Fourier transform of time domain reflectometry to obtain dielectric constants of aqueous solutions and the relaxation times in them. Their frequency ranges from over 50 MHz to 9 GHz. 

Figure 10.6. (A) Experimental set-up used for membrane compaction studies of a high-pressure separation system by ultrasonic time-domain reflectometry (B) Scheme of the separation cell showing the externally mounted transducer and the primary reflections identified as a, b and c, which correspond to the top plate-feed solution interface, feed solution-top membrane surface interface and bottom membrane surface-support plate interface, respectively (C) Change of the arrival time which translates into changes in membrane thickness during compaction. (Reproduced with permission of Elsevier, Ref [63].)...

Hawkes and Pethig (1988) identified a weak dielectric loss in the KHz region and explained it in terms of vibrational motions of the polypeptide backbone. Using time-domain reflectometry. Bone (1987) observed for chymotrypsin a dielectric dispersion that was centered at 12 MHz and increased with increasing hydration. The dispersion was attributed to relaxation of polar elements of the protein. 

Fig. 9.4 Soil water content, leaf area, plant water stress, flammability and daily rainfall of mature forest, secondary forest and cattle pasture during the severe 1992 dry season. As deep soil water was depleted during this measurement period (a, b), severe drought stress developed in some trees of the mature forest (d), but the loss of green leaf area was lower in the mature forest than in the secondary forest and cattle pasture (c). Because of this capacity to retain leaves despite severe water stress, the mature forest is rarely susceptible to fire even during a severe dry season such as this (e). Plant-available soil water was measured from 0 to 2 m depth (a) and from 2 to 8 m depth (b) using Time Domain Reflectometry sensors imbedded in the walls of deep soil shafts (Nepstad et al. 1994, Jipp et al. 1998).

Comegna, V., Coppola, A., and Sommella, A. Nonreactive solute transport in variously structured soil materials as determined by laboratory-based time domain reflectometry (TDR). Geoderma 92[3 1], 167-184. 1999. 

Vanclooster, M., Mallants, D., Vanderborght, J., Diels, J., Van Orshoven, J., and Feyen, J. (1995) Monitoring solute transport in a multi-layered sandy lysimeter using time domain reflectometry. Soil Science Society of America Journal 59, 337-344. 

FOTDR frequency optical time domain reflectometry... 

Look BG, Reeves IN, Williams DJ (1994) Application of time domain reflectometry in the design and construction of road embankments. Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, IL... 

Alternatively, as seen for the low-frequency range, high-frequency measurements can be performed in the time domain (time domain reflectometry methods [19]) that can be extended to 20 GHz, but with less accuracyrelative to network analysis [14]. 

---