OT - The WT-4 Waveguide System:
A bit of history, lost to the ages.
Were it not for fiber...
Tim, Thread,
I thought that you would enjoy the message posted below. It relates to the Bell System WT-4 Waveguide System which you may recall from your own experience, or you may recall from my mentioning here, in the past.
It was posted on the Compuserve Telecommunications Forum by Mr. Albert LaFrance, and is reprinted here with his permission.
While this message speaks in an historic sense to the properties inherent in r-f waveguides, note the similarities (and differences) to today's optical modes of transmission.
In the end, it's my understanding that some of these guides were used as conduit to pull fiber through.
Regards, Frank Coluccio
=================
Matthew Sadler of the Cold War Communications e-mail
list did some research in Bell System publications and
located technical information on the WT-4 long-distance
waveguide systems.
A consolidated two-part message follows:
==================
"A transmission system operating over circular waveguide
has a potential for future use in long distance
communications because of the very wide bandwidth that
can be achieved. The attractiveness of the waveguide
medium stems from the low-loss transmission characteristics
of circular guide when excited in the TE01 mode. This mode
has the unusual property that, above its cutoff frequency, its
loss varies inversely with frequency to the 3/2 power.
Therefore, in theory, as low a loss as desired can be
obtained merely by using a sufficiently high operating
frequency. In practice, however, the required frequencies
are so high that hundreds of other modes are also
propagated. These unwanted modes are coupled to the
desired TE01 mode by geometrical imperfections in the
guide (caused by dimensional tolerances, curvature, etc.)
and eventually cause its loss to rise with the frequency and
give rise to transmission deviations. The mode conversion
process is dominated by the bends that are necessary to
follow any practical right-of-way alignment or hilly terrain.
Accordingly, the design of a circular waveguide medium
involves tradeoffs among waveguide structure, waveguide
size, operating frequency band, and the precision with which
the waveguide can be manufactured and installed.
"Two types of waveguide are expected to be employed to
reduce mode coupling and conversion. Helix waveguide
(picture shown) uses very fine copper wire backed by lossy
material as the lining of a steel tube. This design supports the
TE01 mode in a low-loss manner, while other modes, which
generally have a longitudinal component of wall current,
couple into the lossy dielectric around the outside of the
copper helix and thus are attenuated.
This high attenuation substantially reduces the spurious mode
energy coupling back into the TE01 mode and therefore
reduces transmission deviations. However, the intentional
bends needed to conform to a practical route tend to
introduce significant losses in helix waveguide, particularly at
the higher frequencies. For this reason, dielectric-lined
waveguide (picture shown), consisting of a steel tube
copper-plated on the inside and lined with a thin layer of
polyethylene, is expected to be used on 98 percent of a
waveguide route.
The dielectric lining is designed to minimize the mode
coupling that occurs in bends and thereby minimize the
losses. The addition of sections of helix waveguide at
periodic intervals will limit both the magnitude and frequency
of the transmission deviations that would accumulate if only
dielectric-lined waveguide were used.
"Both helix and dielectric-lined waveguides have been
designed with inside diameters of 60 millimeters. This value
was chosen to provide minimum loss across the frequency
band from 40 GHz to 110 GHz. Losses from 1 to 2 dB per
mile are anticipated, which, as discussed further in Section
10.3, will permit waveguide repeater spacings in the 25-mile
range, using currently available millimeter-wave power
sources and digital modulation techniques."
==========================
More on the Waveguide System:
10.3.4 WT4 MILLIMETER WAVEGUIDE SYSTEM
During the last decade, various ways of implementing the
nationwide telephone network in digital terms have been
examined, but none has proved practicable. There are
basically two reasons for this. First, attractive digital
technology for providing long intercity circuits was not
available; long-haul digital coaxial systems, forerunners of
the T4M system, were not economically competitive with L4
or L5.
Note that repeatered line costs are a very important factor in
long-haul systems, a different situation from short-haul
carrier systems in which digital systems have proved
attractive. Second, with analog space-division voiceband toll
switches, the potential economies of digital connections
between growing digital exchange area networks of T1
systems could not be realized. It now appears that these two
basic difficulties may gradually disappear.
No. 4 Electronic Switching Systems will provide digital toll
switching, and a long-haul digital transmission system, the
WT4 waveguide system, has been developed and may
become attractive for new construction when the demand
for circuits in buried facilities exceeds the capacity that can
be achieved on existing coaxial cable installations.
10.3.4.1 Channelizing Plan
The WT4 system is a long-haul (4000-mile), high-capacity
digital facility utilizing buried circular waveguide as the
transmission medium. In the system designation, WT4, the
"W" stands for waveguide and the "T4" indicates that it is a
transmission system channelized to carry the
274.176-megabit-per-second DS-4 bit rate on each
broadband channel. The transmission plan for the system
provides 60 digital channels in each direction of
transmission, with each channel using 2-phase DPSK
modulation. A fully loaded WT4 system can carry nearly
230,000 2-way PCM voice circuits.
10.3.4.2 Medium Fabrication and Placing
As noted in section 6.1.3.5, two types of waveguide
medium (dielectric-lined and helix) will be used on a WT4
route. Normally, about 98 percent of the waveguide installed
will be dielectric-lined. In both cases, the steel waveguide
tubes have a wall thickness of about 0.15 inch and are
drawn to very close dimensional tolerances on diameter,
roundness, and straightness to control mode conversion.
As shown in figure 10-17, the waveguide tubing itself is
supported by spring-mounted rollers inside a steel sheath
5-9/16 inches in diameter with 3/16-inch walls. The
manufactured lengths of sheath will be joined in the field by
welding and will be buried 4 feet under ground.
The waveguide lengths are then joined, also by welding, and
pushed into the completed sheath. Thus, the finished
structure will be extremely rugged and is expected to be
highly resistant to mechanical injury. Corrosion protection
will be maintained for the sheath, and a dry nitrogen
atmosphere will be maintained inside both waveguide and
sheath.
10.3.4.3 Right-of-Way and Repeater Spacing Requirements
Right-of-way requirements for waveguide are similar to
present coaxial cable requirements, except that there can be
no sharp bends. The minimum radius of curvature will be
about 250 feet, determined by the elastic limit of the steel
sheath. This condition may cause a curved right-of-way to
be required in some bends, but causes no serious limitations
in following elevation changes in most terrain.
Because of the low transmission loss, repeater station
spacings of about 25 miles are anticipated. It will thus be
possible in most instances to locate the repeater stations
near existing roads, providing easy access for commercial
power and for maintenance.
10.3.4.4 Repeater and Repeater Station Design
As shown in Figure 10-18, the received signals from the
waveguide are divided at each repeater station into 120
broadband channels (60 for each direction of transmission)
by means of a waveguide filter network. The DPSK signal in
each channel is then processed by its own repeater.
In each repeater, the received DPSK signal is shifted down
to an intermediate frequency (IF) centered at 1.36 GHz,
amplified, filtered, and equalized. It is then detected and
regenerated, after which the signal is a stream of on-off
baseband pulses at the 274-megabit-per-second rate,
identical to the originally transmitted digital signal.
Thus, the original signal is available at each repeater, and
broadband channels can be added or dropped or the
channel assignment of a given signal can be changed at any
repeater station. For continued transmission in the
waveguide, the baseband signal is used to drive a modulator
that modulates the phase of a millimeter-wave IMPATT
diode oscillator. The output millimeter-wave signals are then
recombined onto the waveguide in a filter network similar to
that used at the input.
The repeater buildings will be above ground with two
rooms. One room will contain the repeater electronics under
fairly close environmental control, and the other will contain
cooling equipment and diesel generator backup power.
10.3.4.5 Protection Switching
A fully loaded WT4 system will consist of 57 working
channels carrying service and 3 protection channels. Two of
the protection channels will be switched in automatically.
The third protection channel will be manually patched in by
interchanging cables at the baseband points of the
repeaters.
A protection switching span may contain up to 12 repeater
hops (each hop having a maximum spacing of 25 miles),
providing a maximum span length of 300 miles. Error
performance will be monitored at the receiving end of the
protection span. If the error rate exceeds a predetermined
level, the carried signal will be switched automatically from
the working channel to a protection channel.
When the failed hop is identified, the manual protection
channel will be used to replace the failed hop during repair,
and the automatic protection will be released. It is estimated
that an outage objective of 0.02 percent per 2-way
4000-mile connection will be met.
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