Reflection-cooefficient bridges have been around for the last ONE HUNDRED &
FIFTY YEARS.

They have always incorporated artificial lines, or line simulators, or real
lines in the standard or reference arm of the bridge.

The reflection-coefficient bridge was used to locate faults on the first
oceanic telegraph cables by comparing the faulty cable with an artificial
version maintained at the terminal station specially for the purpose. An
artificial fault was moved along the artificial cable until the bridge was
balanced. The wideband signal generator was a 100-volt wet battery and a
telegraph key. The bridge unbalance indicator was a mirror galvanometer
using a light beam 5 or 6 feet in length and a sensitivity measured in
nano-amps.

The equipment was mounted on a mahogany bench, housed in beautifully
polished mahogany cases. Electrical connections were made by copper bars
between brass screw terminals, all changes in direction of bars were at
90-degrees. All brass and copper surfaces not needed for electrical
connections were brightly polished and coated with a clear laquer. The
overall appearance of the test room was a work of art, produced by a master
of his electrical and mechanical skills, with a quiet pride in the knowledge
that no-one else could possibly better improve operating efficiency of the
station and the cables which radiated from it in various directions under
the
ever-restless waves.

The same arrangement was used to locate oceanic cable faults in the 1970's.
I designed a fault locating test equipment with 10:1 bridge ratio arms which
saved space in the artificial line rack. The artificial line matched the
real
line from 1/10th Hz to 50 Hz. Cables had amplifiers every 20 or 30 miles
which also had to be simulated in the articial line.

For a 100 years or more, new multipair phone and other cable types have been
acceptance tested with reflection-coefficient bridges. One pair in the cable
is exhaustively tested for everything the test engineer can think of to make
sure there's nothing wrong with it. The known good pair is then used as the
standard arm of the bridge and each of the other 1023 pairs in the cable is
compared with it in the other arm of the bridge. It is a very sensitive
method of detecting cable faults. Care must be taken to terminate each pair
with its Zo. If standing waves are present then a dry high-resistance
faulty soldered joint might not be detected if it is located at a current
minimum.

Pulse-echo cable-fault locating test sets use a network to simulate the very
wideband line input impedance Ro + jXo. It is essential to balance-out in a
bridge the high amplitude transmitted pulse which would otherwise paralyse
the echo receiver

And for many years amateurs have unknowingly used reflection-coefficient
bridges immediately at the output of their transmitters. They have been
incorrectly named by get-rich-quick salesmen as SWR, forward and reflected
power meters. These quantities exist only in the users' imaginations and the
meter doesn't actually measure any of them.

A more appropriate name for the instrument is a TLI. (Transmitter Loading
Indicator). A pair of red and green LEDs would suffice to answer the
question " Is the load on the transmitter near enough to 50 ohms resistive
or is it not near to 50 ohms resistive ? "
---
Reg, G4FGQ

==========================================

"Peter O. Brackett" wrote
Reg:
[snip]
The fixed standard arm of the rho bridge (instead of a 50-ohms resistor)
can
be just a very long length of transmission line of input impedance Zo =
Ro+jXo which, of course, varies with frequency in exactly the required
manner.

Or, as I often did 50 years back, make an artificial lumped-LCR line
simulating network to any required degree of accuracy.
----
Reg
[snip]

Caution... take care, the "reflection police" may get ya!

Roy and Dave took me to task on another thread for even suggesting just
such
an
approach. A semi-infinite line!!! Hmph... no way they were gonna let me
get away
with that. Roy wanted to know what "semi-infinite" was!!!

Dave even told me that my idea of having a lumped approximation to Zo was
impossible!
This was a completd surprise to me since over 300,000 units of an xDSL
transceiver I
recently designed for the commercial marketplace and which have all been
shipped
and installed by BellSouth, Verizon, SBC and other such unknowing folks
incorporates
just exactly that kind of circuitre!

Hmmmm... I guess I lucked out and none of those customers noticed I was
balancing \
a lumped approximation of Zo against a real distributed complex Zo!

:-)

--
Peter K1PO
Indialantic By-the-Sea, FL.

Reg wrote:
"For 100 years or more, new multipair phone and other cable types have
been acceptance tested with reflection coefficient bridges. One pair in
the cable is exhaustively tested for everything the test engineer can
think of to make sure there`s nothing wrong with it.."

Why bridge test a cable pair that has continuity and accessible
terminals?

I would rather measure the transmission characteristics that I might
use.

The impedance of a 2-wire circuit may be of interest for balancing a
term-set, but that is usually accomplished by adjusting the balance
network by trial and error for the best balance or for most transhybrid
loss. Another option is to accept a compromise fallback network which
gives whatever hybrid balance results, good or bad.

One can locate a line fault by using:
wavelength = V / f

Where multiple repeaters are in a chain, as in Reg`s undersea cables,
each repeater can generate its own unique pilot tone. One can check the
tones to determine where the chain is broken. I`ve done that with
terrestrial microwave systems and recorded the tone interruptions on a
multichannel event recorder with synchronized timing marks. Whenever an
outage occurs, time, location, and duration are charted.

For a rough check on local telephone loops in the swirtched telephone
system here, the phone company had a dial-up tone oscillator in its
central offices. More significantly, other subscribers can be dialed up
to determine the quality of the connections that can be made.

Data circuits often have a loop-back capability in data modems, used to
determine error rate. This is another way to evaluate circuits.

For broadcast program lines, and other leased circuits, the phone
company will treat the line to meet specifications. The customer then
tests his own circuits to make sure he is getting what he pays for.

There are "silent" test systems for multipair cables which test with
tones outside the audible range. These can evaluate attenuation and
cross-talk and these can be related to the similar values in the audible
range.

SWR is a function of reflection strength. I see no problem in labeling a
reflection strength as SWR, even though there may not be enough cable
for a standing wave pattern. I think TLI would be a fine meter name too.

Best regards, Richard Harrison, KB5WZI

I would rather measure the transmission characteristics that I might
use.

===============================

You've never acceptance tested a 20-mile long phone cable, 542-pairs,
88mH-loaded every 2000 yards.

There are so many things which can go wrong with it you can't believe it.

For example, it is a waste of time measuring line attenuation (loss) on all
542 pairs as a means of detecting a possible imperfection in any one pair.

Very serious defects, sufficient to disrupt normal service, can be entirely
overlooked if attenuation is measured just at one or two frequencies as a
check to see if loss is between specified performance limits. Loss is so
small on transmission lines it is very difficult to measure accurately. It
can get lost in temperature changes especially on overhead lines.

I know - I've done it !

It is obvious the most sensitive of ALL measuring instruments is a bridge
used to compare one value with another, good with bad. The bad sticks out
like a sore thumb even if it is only a teeny bit bad.
---
Reg.

Where multiple repeaters are in a chain, as in Reg`s undersea cables,
each repeater can generate its own unique pilot tone. One can check the
tones to determine where the chain is broken.

==============================

How does each repeater generate its unique pilot tone when a trawler or
earthquake breaks the inner conductor. Or do you have another way of
powering repeaters at the bottom of mid-atlantic?

Reg, G4FGQ

One can locate a line fault by using:
wavelength = V / f

=====================

How do you manage at the lower frequencies when velocity is a function of
frequency ?
---
Reg

On Tue, 16 Sep 2003 01:02:49 +0000 (UTC), "Reg Edwards"
wrote:

One can locate a line fault by using:
wavelength = V / f

=====================

How do you manage at the lower frequencies when velocity is a function of
frequency ?
---
Reg


Inventing new problems? Old wine in new bottles more like it ;-)

The velocity to the nearest geo-synchronous satellite is close enough
to constant that it doesn't matter. One repeating station and it is
quite obvious when it is dead (solves the parking problem for the next
one to replace it too).

GEOS too far away? Use LEOS instead and talk around the dead one
(it's going to fall into the sea/Australia/China/Canada anyway).

And for those still in love with wire are promises from nanotechnology
to tether satellites to earth in the future (power generation for
cheap - life expectancy for the guy that throws the switch is nil
however).

73's
Richard Clark, KB7QHC