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).
--
David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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