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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Dear Elimoel, There are some points I want to mention in reply to your ideas concerning Dual Frequency TDR for Coincident Measurement of water content & porosity. Of course, they are not meant to discourage you from testing new approaches. 1. From your previous inquiries on the mailing list I gathered that you were thinking about using different pulse generators for determining the soil permittivity at different frequencies. The problem with this approach is that it is very difficult to assign a specific frequency to a permittvity measured in the time domain. In fact, the frequency will change with the experimental setup (e.g. cable type and cable length) and the sample itself. 2. You might want to consider measuring permittivity directly in the frequency domain (e.g. network analyzer). However, in this case it will be difficult to measure at a high permittivity because of the strong attenuation of high frequencies. 3. Another important point is the measurement volume at different measurement frequencies. Because the rationale of your approach is to measure two different permittivity, you cannot expect to have the same measurement volume for both frequencies. 4. In order to get a good contrast in permittivity you need a high second frequency (>> 2 GHz). An interesting question is whether the same probe can be used for the two measurements. Regular TDR probes are not designed for high frequencies and some unexpected behavior could occur (e.g. non-TEM mode propagation). 5. There is the purdue test method of Drnevich et al. This method determines the bulk density by the measurement of permittivity in an undisturbed and a compacted soil sample. Currently they are working to get the technique fully recognized by the ASTM. See the prodeecings of the TDR 2001 conference for more information. Good Luck, Sander Huisman