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Simple Baluns in Parallel Probes for Time Domain Reflectometry
Author(s) -
Spaans Egbert J. A.,
Baker John M.
Publication year - 1993
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1993.03615995005700030006x
Subject(s) - balun , reflectometry , transmission line , time domain , signal (programming language) , coaxial cable , electrical impedance , coaxial , materials science , finite difference time domain method , antenna (radio) , electrical engineering , optoelectronics , acoustics , electronic engineering , computer science , optics , engineering , physics , cable gland , computer vision , programming language
Time domain reflectometry (TDR) is used for the simultaneous determination of soil moisture content and electrical conductivity (EC). Limitations of parallel probes compared with coaxial probes have been signal losses in the antenna wire leading to the probes, and the required use of an expensive balun. In addition, signal attenuation in the balun hampered accurate EC determination. We describe a balun that avoids these limitations. It is a 1:1 balun, which means that it converts the electrical field from unbalanced to balanced, but no impedance transformation takes place. Consequently, this balun does not attenuate low‐frequency signals, which is inherent to commonly used commerical 1:4 baluns. The balun is inexpensive and easy to fabricate, and is attached directly to a parallel pair of waveguides. This permits a system with coaxial transmission line from the TDR to the parallel waveguides. For soil moisture content determinations, the new balun performs at least as well as a conventional balun. Furthermore, EC of NaCl solutions determined from the TDR trace showed excellent agreement with EC measured with a conductance meter.