Time domain reflectrometry measurements using a movable obstacle for the determination of dielectric profiles

B Will,B Schiek,S Schultz,M Gerding

doi:10.5194/ars-6-1-2008

Abstract

Abstract. Microwave techniques for the measurement of the permittivity of soils including the water content of soils and other materials, especially TDR (time domain reflectometry), have become accepted as routine measurement techniques. This summary deals with an advanced use of the TDR principle for the determination of the water content of soil along a probe. The basis of the advanced TDR technique is a waveguide, which is inserted into the soil for obtaining measurements of the effective soil permittivity, from which the water content is estimated, and an obstacle, which can mechanically be moved along the probe and which acts as a reference reflection for the TDR system with an exactly known position. Based on the known mechanical position of the reference reflection, the measured electrical position can be used as a measure for the effective dielectric constant of the environment. Thus, it is possible to determine the effective dielectric constant with a spatial resolution given by the step size of the obstacle displacement. A conventional industrial TDR-system, operating in the baseband, is used for the signal generation and for the evaluation of the pulse delay time of the obstacle reflection. Thus, a cost effective method for the acquisition of the dielectric measurement data is available.

Highlights

The aim of this investigation is to obtain representative profile measurement results from a material under test
Time domain reflectometry (TDR) is known as a method to obtain a reliable estimation of the soil water content from a measurement of the real part ε as well as the imaginary part ε of the relative permittivity εr (Kupfer, 2005; Hoekstra, 1975)
In many cases TDR is used in order to get just one integral value for the relative permittivity of the material (Robinson, 2000)

Summary

Introduction

2theFuefnfecctitoivnealpperrminitctiipviltey along the analyzed probe length lmech can be determined: TrIenεhleaceftleofpdsr=eotdopawtgh∆aae∆vtteietmofgrfneeueafcissdtpieveee2sdtpru.eocrfmtueirlteetiscvtlriitokymeoiafngtnhcieertcpicuelnwaeratrwvaeatsevdeisgmudeiiddreeiucs(tm4ol)yr. CoOabxviaioluwslayvetghueidspeastiwalithreasorleultaiotinveopfetrhmeitstyivstietymεdr itrheectplyrodpea-gapteinodnsvoenlocthitey liesnggitvheninbteyr:val lmech and on the pulse delay time ∆tref corresponding to the obstacle displacement, rec sp=puelsc√etciε0vdreellya,.y. The higher time. If the the spatial resolution pulse delay time is in the shorter the order of tt(hh1ee) inherent time jitter ∆tjitter of the TDR system, the upper rewsohliuletiothnelipmroitpiasgraetaiochnevdealoncditcyanalboendgettheermpirnoebdeboyf: the test sttiievv∆tiiuttlyypmoεisn0hf ·ot=hεweefn12fd.ciieTn0l∆heFectitjgeri.iftcft1eemrcistii.xvdteeutpreeerrmomfinittheteidvpibteyyndethteresacterefidbfeemcsteitvdheieuppmee.rrmmT(5hiitt)e--.

This is a goo electrom and the

For these measurements the obstacle was fixed in position