[TowerTalk] Hardline Fault

[Jim Brown]

On 8/21/2012 4:59 PM, Jim W7RY wrote:
> All you have to do is run a distance to fault measurement with a precision
> load on the far end. Using a wide frequency range (100 to 1000 MHz or the
> maximum frequency range that will allow you to "see" 500 feet with your
> particular cable) you can see all of the imperfections along the cable.

That's generally good advice, BUT -- TDRs are implemented in several 
different ways, and the limitations of one analyzer may be different 
from those of another.

I use the DG8SAQ VNWA, which does a sweep and then an inverse transform 
of the sweep to get the time response.  Using this method, any sweep 
bandwidth and be transformed to view any length of line. Small 
imperfections that don't show up at low frequencies will often show 
clearly at high frequencies. That's why taking the sweep to higher 
frequencies shows smaller faults. Also, with a linear sweep from 1 MHz 
to 1 GHz, for example, half of the energy will be in the top octave 
(that is, between 500 MHz and 1 GHz).  The transform method works quite 
well even with an unterminated line, a line feeding an antenna, or a 
matched load, and under all of those conditions can show a lot of small 
perturbations on the line.

With the inverse transform method, the manner in which the data is 
windowed can make a VERY large difference in the visibility of faults. 
When I'm doing TDR, I often look at the data with several windows and 
filter types.

73, Jim K9YC