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.