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This forum is very interesting and educational. While my background in RF, signal processing and control systems for high power TV transmitters is helpful, the technology for measuring soil water content is all new to me. The feedback on my comments is also appreciated. There has been a lot of attention focused on Campbell Scientific's TDR equipment here lately. It looks like excellent equipment and I can't wait to begin using it. However, like other manufacturers, Campbell is not entirely forthcoming with details about its technology. If a company has a new and valid way to do something, they should patent it. Patents are in the public domain, but intellectual property is fully protected--I have a problem with proprietary technology when it is being used for basic research. (Again, I'm a newbie here and maybe I am concerned about nothing.) >I wander why would you terminate your strip line with a 50 ohms resistance. >TDR works as long as there is reflection. If your termination matches the >line impedance, you will not get any reflection. According to the Campbell Scientific sales literature, the big reflection from the end of the unterminated balanced line is used to trigger a bistable multivibrator for timing purposes. ANY change from the nominal line impedance also causes reflections. The change you are looking for is caused by the high dielectric constant of water relative to air or soil materials. Timing can be done in other ways (see todays announcement re "velocity domain" technology) properly and, in my opinion, a resistive RF termination equal to the characteristic impedance of the transmission line (50 Ohms for uStripline, 300[?--Campbell doesn't specify] Ohms for balanced line) would be a more elegant approach. According to the model for applying TDR to soil water content measurements, "the dielectric constant of water is generally a factor of twenty greater than that of other soil materials." (Campbell Scientific brochure.) Air has a dielectric constant of 1.0, while polyethylene is 2.3, quartz is 3.8 and Teflon is 2.1. (ARRL Handbook, 1978. P. 23.) REFERENCE DATA FOR ENGINEERS: RADIO, COMPUTER, AND COMMUNICATIONS, Seventh Ed., Jordan, ed., 1985. P. 33-5), tabulates the dielectric constants for water and various soils: Sea water 80 Fresh water 80 Dry, sandy, flat coastal land 10 Marshy, forested flat land 12 Rich agricultural land, low hills 15 Pastoral land, medium hills and forestation 13 Rocky land, steep hills 10 Mountainous (hills up to 3000 ft) 5 No doubt these broad categories, oriented to the US, will bring smiles to the faces of contemporary soil scientists and environmental researchers; however, they do confirm the large difference between the dielectric constant of water and everything else. They should also calm concerns about the affect saline soils will have on measurements. >If you want to make a TDR probe more rugged, you can short circuit it at the >end. You will then get as much reflection as if it was open but the signal >will be inverted. Obviously, the circuit must be able to deal with this >inversion and I don't know if Campbell's does. The basic principle of RF tuning stubs, radar choke joints, etc. is that a quarter wavelength away from an open is a short and a quarter wavelength away from a short is an open. Wavelengths decrease with increasing frequency. The Campbell sales literature indicates their probes have a square wave output, not a sine wave output. With the low-to-high frequency components of a square wave, there is no single wavelength--chances are the probe would still work, but would need to be recalibrated. More importantly, except for a metallic or capacitively coupled short around the two rods, the majority of the measurement is made BETWEEN the rods of the TDR probe. Connecting the ends would make the probe stronger, but might disturb a soil sample so that water would accumulate in the space created and give an erroneous reading. Connecting the rods at the ends would reduce the probability that the spacing between rods would be affected by rocks and/or roots in the soil. Any changes would require recalibration of the system. For these reasons and the fact that I am just plain interested, I would like to experiment with a terminated uStripline TDR probe. Dave Barnett UMass CEE Dept >Romain Gagnon Eng > > >Smart Rain Corporation inc >1505, Place de l'Hotel de Ville, suite 102 >St-Bruno, Quebec, Canada --- J3V-5Y6 >E-mail: Info@SmartRain.com >Phone: 1-514-441-4289 >FAX: 1-514-441-2147 >WEB: http://www.SmartRain.com