�From:�	Johan Smit ("smit.js")
�To:�		SOWACS@aqua.ccwr.ac.za
�Date:�	Tuesday, May 7, 1996  9:24 pm
�Subject:�	Re: TDR vs Radar vs shockwave reflection?

My understanging of the terms are:

TDR Time domain reflrctometry. The transmitted energy can beeither 
in the form of 
1) A single rectangular or otherwise shaped pulse. This can be 
repeated, eg., to provide readable displays, but any one pulse 
contains all the information. The pulse length must be longer than 
the time required for the last required reflection to be returned,so 
that the transmitted energy is constant during the time the 
reflections arrive.This is the normal TDR method for short lengthof
cable measurements.
2) A series of sine waves, the envelope of which may be shaped.The 
pulse time is normally short compared to the reflection time, but 
still contains a number of cycles of the carrier frequency. Thisis 
the method used in radar, and sonar. The transmitted energy is 
normally zero at the time the reflection arrives.
3) A variation of 2) where a very narrow pulse is transmitted. The 
transmitter rings, and the medium shapes the pulse. Thereflections 
therefore contain most of their energy at the frequency of minimum 
attenuation through the medium.
Shock waves fall in this category?

FDR Frequency domain reflectometry. The frequency of thetransmitted 
energy is swept under control, and the reflections add to the 
transmitted energy, in or out of phase. The transmitted energy is 
thus constant, there is no pulsing. The information required is 
obtained from the reflection versus frequency graph, or
Voltage Standing Wave Ratio. 
 
The principles are the same, the applied energy  varies. To get 
reflections in water, you have to apply the energy at thefrequency 
of least attenuation through water. 

The measurement of capacitance, and indirectly thereforedielectric 
constant, by means of a free running oscillator the outputfrequency 
of which varies depending on the capacitance, to my mind cannot be 
called FDR.

Greetings to all.
Johan Smit