Frequency Domain Reflectometry Method Research Articles

Improvement of FDR Methods for Identifying Defects of Jointed HV Cable

This paper considers frequency domain reflectometry (FDR) as an effective way to locate high voltage (HV) cable defects. The formula relating FDR to cable local equivalents impedance value is presented, and critical methods such as window function, frequency step, and bandwidth to process FDR data before Inverse fast fourier transform (IFFT) are also demonstrated. Finally, a multi-periods reflection analysis from the IFFT curve is introduced with illustrations from fields measurement to improve the identification of defects for cables with multiple joints.

A Novel Dampness Diagnosis Method for Distribution Power Cables Based on Time-Frequency Domain Conversion

Dampness invasion of power cable joints may lead to cable insulation degradation gradually. Once the cable joint is severely damp, it rapidly leads to breakdown failure for the power cable insulation system and finally results in catastrophic cable accidents. To avoid power outages, it is of great significance to diagnose cable joints with severe dampness defects. Existing methods have certain limitations. In this article, a novel time-frequency domain conversion method for determining cable joints with severe dampness defects is proposed. For this method, the cable joint is first located by applying the conventional frequency domain reflectometry (FDR) technology. Then, the time domain waveform at the cable joint is obtained based on the proposed time-frequency domain conversion algorithm. Finally, the cable joint with severe dampness defect can be distinguished by judging the polarities of the time domain spectrum at the cable joint. To verify the effectiveness of the proposed method, simulations and experiments are performed with comparisons with the conventional FDR method. Simulation and field detected results prove that the proposed method can effectively distinguish cable joints with severe dampness defects.

Time and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives

Electromagnetic (EM) methods for estimation of water content in porous media (e.g., soil and tree stems) are based on estimates of the media's bulk dielectric permittivity, which is sensitive to water content. Time domain reflectometry (TDR) and frequency domain reflectometry (FDR) are routinely used for rapid, automated and minimally destructive approach for routine measurement of soil water content (θsoil) and stem water content (θstem) in the laboratory and field. Applications and advances of TDR and FDR in soil science have been well-documented, but no attempt has been made to critically review the use of EM methods for measuring θstem. In this paper we review the principles, calibration, sensor configuration, and sensor installation of the TDR and FDR methods for the measurement of θstem. Perspectives are presented on new TDR calibration, the development of new sensors, and technologies for the simultaneous measurement of θstem, sap flow, bulk density/porosity, radial distribution of θstem, and electric conductivity. Also discussed were the effects of tree morphology and wounding on the accuracy of both methods. This review provides information for the novice and expert alike to guide them on the advantages, limitations, development, and application of the EM methods required to improve the reliability and validity of measured values of θstem.

Accurate measurement of optical fiber time delay based on frequency domain reflectometry

Optical fiber time delay system has been widely used in optical-controlled phased array antenna, radar distributed network, interferometric optical fiber hydrophone and high-speed photoelectric chip. These applications require high-accuracy and high-stability time delay generated by the system. Time delay measurement directly determines the precision and resolution of the system. Therefore, high-precision time delay measurement method is of great significance in developing the optical fiber delay system. In this paper, progress and problems of optical fiber time delay measurements are discussed. A new method of precisely measuring the time delay in optical fiber is proposed. We use the frequency domain reflectometry (FDR) to avoid the discrepancy between measuring range and measuring precision, which exists in both time-of-flight (TOF) method and phase discrimination approach. An intensity modulated 1064 nm laser signal is used as a light source. The modulation frequency is tuned from 10 MHz to 200 MHz in steps of 0.5 MHz. The spectrum of echo signal is obtained by measuring the amplitudes and phases of echo signals at different frequency points. The delay information is obtained via the inverse fast Fourier transform (IFFT). The precision of delay measurement in our method is determined by step size of frequency variation, and a higher-precision measurement is realized by using interpolation zero algorithm. Since our method is not to modulate the optical frequency, but to control the frequency of the modulation signal loaded on the electro-optic modulator, it is easy to achieve the high-precision and high-linearity frequency modulation. In this paper, theoretical analysis and time delay measurement are used to compare the FDR method with conventional TOF measurement method. The accurate measurement of 33–200 ps is realized, and measurement error is lower than 7 ps. We also design an incremental measurement experiment to study the resolution of the FDR method, which achieves a delay resolution of 3.3 ps. The influence of temperature jitter is analyzed to prove the reliability of experimental results. It proves that the FDR method has a higher measuring accuracy than the TOF method. The time delay measurement precision can be further improved by expanding the modulation bandwidth. Our method is to be applied to an optical fiber delay system to improve the precision and resolution of system delay.

Location attempt of a degraded portion in a long polymer-insulated cable

The ability of frequency domain reflectometry (FDR) for locating a portion in a cable longer than 1.0 km, where either degradation or abnormality occurred in its insulation, was evaluated by conducting two experiments. First, a 72-m coaxial cable insulated with low-density polyethylene (LDPE) was locally degraded concurrently by heat at 100 °C with gamma irradiation to 312 kGy. Then, this cable was connected to two 1000-m coaxial LDPE cables at either end. Furthermore, a 2-m coaxial lead LDPE cable was connected to a network analyzer, forming a cable of a total length of about 2074 m. The FDR method successfully located the degraded portion and also the connecting points in this long cable. It has also become clear that the FDR method can locate a 30-cm part in a 1000-m coaxial LDPE cable at a position about 900 m from its end when it was heated temporarily using a ribbon heater.

Wire Fault Diagnosis in the Frequency Domain by Impedance Spectroscopy

The typical methods for the wire fault location are the time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR). In TDR, attenuation and dispersion of the reflected signal limit the reachable accuracy for wire faults location. Because time of flight is evaluated, the wire faults with small impedance changes are difficult to locate. The FDR method has better accuracy than the TDR method, because power signals are used as incident signals and Fourier transform is applied to locate wire faults. However, it is difficult to identify the types of wire faults. A new technique is proposed to detect and locate the wire fault using impedance spectroscopy (IS). IS is applied to measure the input impedance of the wiring system. The load impedance of the wire fault can be abstracted from the input impedance in order to identify the type of wire faults. The input impedance is a period function of the frequency and the period of this function is linearly proportional to the distance to the fault. The fast Fourier transform of the input impedance can give a single spike to locate the wire fault. This novel method enables the location of hard (open and short circuits) and soft (wire faults with small impedance changes) faults. The input impedance of the wire faults in the defined low-frequency range can be applied to identify the types of wire faults. The capacitance and inductance of the cable system can be applied to detect the wire fault if it is open or short circuit. The measurement deviation of the wire fault location in this paper is maximal 40 cm and is independent of the distance to wire faults.

Dielectric Constant of Sand Using TDR and FDR Measurements and Prediction Models

The effects of soil dry density and water content are being examined through experimental time-domain-reflectometry (TDR) and frequency-domain reflectometry (FDR) methods in correlation with electromagnetic simulations. The infiltration rate (hydraulic conductivity) of water in sand is exceptionally high, resulting in heterogeneous moisture distribution through the soil. The effective dielectric constant of the soil/water/air mixture is dependent on the soil's dry density and moisture content. Both TDR and FDR methods are performed on a coaxial transmission line filled with a soil/water/air mixture. The flow of the water through the soil creates a dynamic situation in which the soil/water/air electrical impedance changes over time. The resulting soil has heterogeneous water content, creating varying electrical impedance values along the length of the coaxial line. The soil compaction, i.e., dense or loose, has significant impact on the heterogeneity of the moisture content through the soil and the dry density of the soil. In each case, the effective dielectric constant is determined from the data collected from TDR and FDR experiments, and the values are compared with the predictions using established empirical models by Topp, Hilhorst, and Hendrickx. With the exception of the data represented as a function of the degree of saturation, the Hendrickx model appears to best represent the measured dielectric constants since it falls within two standard deviations of the measured data. A computer simulation technology (CST) Microwave Studio is used to supplement experimental observations of various soil moisture contents in a coaxial cell. Simulations confirm that the change in the dielectric constant through the soil is a result of the heterogeneous moisture distribution. It was found that the soil moisture content has a major impact on the resulting dielectric constant from measurements or modeling. In the coaxial-cell device, soil moisture migration during the testing period results in a heterogeneous moisture regime and a temporal dielectric constant. This is particularly exaggerated for high-hydraulic-conductivity soils such as sand.

Research of Aircraft Cable Fault Detecting Method Based on Hilbert Huang Transformation

Aircraft cable fault diagnosing is considered to be most important for engineering maintenance. Several methods for cables testing have been developed, such as TDR, FDR and TFDR. Time Domain Reflectometry (TDR) relays much on impedance changes on the fault position, which is hard to using in detecting high resistance defects, intermittent defects; Time Frequency Domain Reflectometry (TFDR) method is used to locate intermittent faults, continuous faults and cross-connection faults aircraft wire, however, the algorithm of TFDR is complex. To the "Hard Fault"(short circuit and open circuit), the Hilbert-Huang Transform method is used in determining the optimal bandwidth of the incident reference signal and analyzing the phase and amplitude difference of superimposed signal which from the incident signal and the reflected signal on defects. To the "Fray Fault", Time and Frequency Domain Reflectometry method can be used with the signal processing method with Hilbert-Huang Transform. The experimental results indicate that this method effectively detect all types of aircraft cable fault, particularly for short lengths of cable.

Spatio-temporal soil moisture variability in Southwest Germany observed with a new monitoring network within the COPS domain

Within the ‘Convective and Orographically-induced Precipitation Study’ (COPS) 2007 in Southwest Germany and Northeast France a soil moisture monitoring network was installed. The aim of the network is to identify the interaction between the temporal and spatial variability of the soil moisture field and its influence on the energy balance and the moisture availability in the planetary boundary layer. The network is comprised of a large number of newly developed low-cost soil moisture sensors based on the frequency-domain reflectometry method (FDR). In total 47 soil moisture stations within the COPS domain were each equipped with two to four sensors simultaneously measuring vertical profiles of soil moisture and soil temperature down to 50 cm depth. This contribution describes the soil moisture network, its installation procedure and the calibration of the sensor output signal. Furthermore we discuss the soil texture distribution within the study area and present first analyses of the spatio-temporal soil moisture variability during a 13 month period from June 2007 till June 2008 based on regional differences and site specific properties (altitude and soil texture). Results show that the altitude plays a key role for the overall soil moisture pattern relative to the area mean due to the direct linkage to precipitation patterns. Soil texture controls the vertical soil moisture gradient relative to the near surface soil moisture, as their properties control water storage and drainage characteristics. Both factors significantly influence regional soil moisture patterns in Southwest Germany. Zusammenfassung –

Detection and Location of Multiple Wiring Faults via Time–Frequency-Domain Reflectometry

In this paper, we propose a high-resolution time-frequency-domain reflectometry technique as a methodology of detection and estimation of faults on a wire. This method adopts the time-frequency cross-correlation characteristics of the observed signal in both the time and frequency domains simultaneously. The accuracy of the proposed method is verified with experiments using a radio-guide-type coaxial cable and comparing it with traditional time-domain as well as frequency-domain reflectometry methods. It is clearly shown here that the proposed algorithm produces excellent results compared to the conventional methods for single as well as multiple fault cables.

Mixed-signal reflectometer for location of faults on aging wiring

Location of faults on aging cables is of great interest to maintainers of aircraft, cars, power distribution systems, communication systems, etc. One class of methods for locating faults is frequency domain reflectometry (FDR), using sine waves as the forcing function. A new frequency domain method called mixed-signal reflectometry (MSR) is described in this paper and compared to data from phase detection FDR (PDFDR) methods. The MSR is less expensive and smaller than the PDFDR and has very similar performance. A prototype system using the 100-200-MHz bandwidth with 256 40-kHz steps is shown to have a resolution of about 10 cm, very similar to a PDFDR in the same frequency band.

PERFORMANCE OF FREQUENCY-DOMAIN REFLECTOMETER, CAPACITANCE, AND PSUEDO-TRANSIT TIME-BASED SOIL WATER CONTENT PROBES IN FOUR COARSE-TEXTURED SOILS

Frequency-domain reflectometry (FDR), capacitance (C), and psuedo-transit time-based (PTT) soil water content(SWC) probes have gained a broad acceptance for measurement of volumetric SWC (.v) in agricultural fields and otherapplications. The .v of typical coarse-textured soils can range from as low as 0.02 m3 m-3 at permanent wilting point to0.1 m3 m-3 at field capacity. Thus, a small change in .v would result in a change of a large portion of the total available water.Therefore, the SWC probes used to estimate .v in coarse-textured soils must be accurate and sensitive enough to detect smallchanges in .v. Few data that analyze the performance of these probes in coarse-textured soils (>90% sand) exist. Theobjectives of this research were to: (i) evaluate the validity of the calibration curves provided by the manufacturers for threeSWC probes (CS615-FDR., TRIME-PTT., and PR1-C.) for estimating .v in four coarse-textured soils, and (ii) identifywhether the performance of the probes is sensitive to soil type. Soil drying cycle experiments were conducted using threefine-sandy soils (Astatula, Ellzey, and Arredondo) and a fine sandy-loam soil (Dothan). Measurements began atnear-saturation and continued until the .v in each soil was near wilting point. The probe-estimated .v data were statisticallycompared with measurements of .v by the gravimetric method (.v-grv). Significant variations were observed in probeperformance between soils. The CS615-FDR probe provided the most accurate .v estimates. Performance of the TRIME-PTTprobe was lower than that of the CS615-FDR in all soils and its estimates were significantly different than the measured .v-grvvalues in all soils. The PR1-C probe provided the least accurate estimates overall and the root mean squared error (RMSE)ranged from 0.04 m3 m-3 in Ellzey soil to 0.05 m3 m-3 in other soils. Its estimates were significantly larger than the measured.v-grv in all soils, except Dothan. The CS615-FDR and TRIME-PTT probes produced their least accurate estimates in Dothansoil, which had the largest clay content (22.7%) among all soils. The variations in the performance of the probes betweenthe soils suggest, in part, that the FDR and PTT-based methods are not completely independent of the soil type and arenegatively affected by the clay content of the soil. All probes should be calibrated for an individual soil under considerationto obtain accurate.v estimations in coarse-textured soils. The calibration curves developed in this study should provide usefultechnical information to users when working with similar SWC probes and soils that have similar physical characteristicsto those used in this research.

FDR法による土壌・地下水汚染のモニタリング手法に関する基礎的研究

FDR法による土壌・地下水汚染のモニタリング手法に関する基礎的研究

Use of sub-hourly soil water content measured with a frequency-domain reflectometer to schedule irrigation of cabbages

Laboratory and field measurements of soil water content were obtained using gravimetric and Theta-Probe methods, the latter a frequency-domain reflectometry method. We obtained real-time in situ measurements of soil water content at depths of 30, 80, 160, 240 and 350 mm to evaluate irrigation practice. A datalogger recording the change in soil water content (and cumulative irrigation) at 20-min intervals was used, with appropriate calculations and graphical presentation, to predict the time and amount of irrigation water required for soil water content to reach field capacity. Measurements at three instead of five depths yielded a different depth-averaged soil water content under relatively dry conditions (less than 0.273 m3 m−3), and predicted a later start for irrigation and early crop water stress. Investing in additional sensores for scheduling irrigation would be compensated by financial resources saved through avoiding excess or deficit irrigation, and the associated application cost, loss of nutrients and soil due to deep percolation and erosion, and loss of crop production.