wrote in message
...
spamhog wrote:

A good time-domain reflectometer

Thank you Barry! Would TV/FM transmitter and cell BTS installers
normally have such a device? I don't know any ham who has one but
these days I am talking to professional antenna people. And, by the
way, if one thing I am working on pans out, they'll come to install a
BTS right atop my condo. (The challenge will be to convince them to
let me hang HF wires from the same tower :-)

If you have an oscilloscope, a pulse generator and a calculator you
have
a TDR.

If you don't have a pulse generator it takes about 2 IC's to build one.


--
Jim Pennino


You would need an extremely fast rise-time pulse generator and an
exceptionally fast oscilloscope to match a typical commercial TDR. The
Tek 1502 unit I used when at Eastman's Research Labs had a pulse
rise-time of 140 picoseconds, and the internal display was a sampling
scope with a bandwidth of over
3 GHz. I now have a Tek 1S2 plug-in for my Tek 585A oscilloscope at
home. It's fine for amateur use, but don't expect to see the tiny
impedance "burbles" of BNC connectors with it. {And carrying around a
1502 is _FAR_ easier than the 585!}

Quoting from a Tektronix application note on TDR resolution:
“the resolution limit … wherein two discontinuities or changes on the
transmission line … begin to merge together … Per this definition, the
resolution limit is: half the … 10% to 90% risetime or 90% to 10% fall
time (depending on whether the TDR response is calibrated with a short or
open circuit).”
To convert this to distance, you need to know the velocity factor for the
cable you are testing and the speed of light.

Your installers may not always carry a TDR with them, but I would bet
they have access to one.

73, Barry WA4VZQ