K1TTT Technical Reference

Time Domain Reflectometer (TDR) on the cheap

 

Stan Griffiths wrote:

 

> Anyway, I have found through many years experience selling Tek TDR units to

> the communicatins industry that there are indeed times when you want to TDR

> a feed line WITH the antenna attached.  Each antenna type has a

> characteristic reflection that produces a unique pattern.  You can record

> these patterns for the antenna system without knowing what every wiggle in

> the trace actually means.  Later, when trouble develops, you can take

> another reading and compare the graphs.  Sometimes the problem will show up

> in the antenna and not in the feedline by looking at the recorded pattern

> from the antenna itself and noting that it has changed from the original

> picture.  Using a TDR, we found one antenna of a stacked TV

 

i use this method on my Beverage antennas, it has helped several times to

locate problems in the transformers vs the wires.(on4un 2-wire bi-directional

design with 2 transformers on a pair of wires)  i have 6 pairs of plots showing

them right after installation that i can compare when i suspect something is

bad and don't feel like walking (or skiing) back the lines to see if the wires

are down somewhere.

 

note, my tdr consists of a $20 tektronix 535a scope and a $20 e-h research labs

pulse generator.  for the beverages i marked the knobs with the settings, for

other uses i calibrate it with a known length of cable (my 160m 1/4 wave stub

makes a nice standard).  this setup has helped several times in locating damaged

feedlines, bad switches, and connectors.  its not much good within about 50' of

the shack(pulse rise time and scope bandwidth aren't that good), but

beyond that its pretty good(and i can always add 50' of good cable to

get past that limitation).

 

More recent notes:

I have now updated the scope to a Tektronix 465 which has a much better

bandwidth.  Using a digital camera makes it easy to save traces.