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.