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This discussion of TDR is very interesting...obviously the final chapter has
yet to be written!
Modifying the Campbell Scientific CS615-L probes by insulating them with
heat shrinkable tubing or encasing them in ceramic tubing would require
recalibrating the system for all measurements. The effects of the heat
shrink would be minimal either way. A ceramic tube surrounding both rods
would greatly desensitize the system, as the dielectric BETWEEN the two rods
has the majority effect on line impedance, signal loss, velocity factor, etc.
Just a note on semantics: A waveguide is a special case of a transmission
line, such as the cylindrical, rectangular or square waveguides used on
radars or UHF television transmitters or the fiberoptic lightguides used for
data and telephony transmission. Calling the unterminated balanced line
used on TDR probes a "waveguide" is like calling the 300 Ohm flat twinlead
from a TV antenna a waveguide. Let's call a probe a probe.
Despite what I said about a ceramic tube around the TDR probes, I am
proposing a terminated uStripline probe with a thin insulating dielectric
layer over the uStrip to a faculty member in the Environmental area. This
approach would require a more powerful RF pulse and/or more sensitive
detection circuitry, but it would eliminate the effects of displacement of
the nominally parallel rods by rocks, etc. in the sample and ions in saline
soil. If this crude diagram is not disrupted in transit, the uStripline
probe would look something like this:
soil sample
Insulating dielectric ////////////////////////////
Oscillator & 50 Ohm uStrip (~)________________________</
PC board //////////////////////////>/ 50 Ohm
Dielectric //////////////////////////</ Termination
Ground Plane -------------------------->/
Insulating dielectric /////////////////////////////
soil here NOT detected
As you can see, the uStripline probe would be directional--this might be
useful when trying to determine the direction of flow through a region (or
maybe not.) Probes might also be placed back-to-back for some measurements.