SIMPLE BALUNS IN PARALLEL PROBES FOR TIME-DOMAIN REFLECTOMETRY

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 commercial 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.