Open Wire Transpositioning Systems
Open wire was the first of many media for the transmission of speech, data, signals and other communications elements. Later as aerial multi-pair copper cable (buried cable and underground cable) were installed, problems of a similar nature were encountered.
One of the first issues with wire lines (and why cable technology hit some rough spots along its developmental way) was with crosstalk. Along with the "skin effect," of line wires and the feature of "attenuation"; that is, differences of wet and dry conductors, insulators and pins upon the daily operation of an aerial wire circuit, "crosstalk" was a major initial problem.
Prior to the days of alternating current power distribution and transmission lines appearing across the landscape, open wire telephone was immune to the distortions and interference caused by this technology. Not until the early 1900s would large scale networks of a.c. power transmission and distribution tie cities and towns together and pose problems to open wire telephone.
However, open wire lines in the early days were susceptible to crosstalk. Even without neighboring power lines, open wire lines were typically large toll leads with many conductors. Inductive coupling between adjacent circuits on the same crossarm, or the crossarms below and above, produced this interference. This was unintelligible noise heard over a telephone receiver by both parties on a phone line. This electromagnetic noise would usually be faintly audible but not quite decipherable to the listener.
Crosstalk is essentially an errant current. Say you have pair one and two on the top arm of an early 1885 toll lead. These four wires occupy pin positions number one, two, three and four. The spacing between the pairs is 12 inches. Wires number two and three, adjacent to one another, will induce voltages (even though they are not physically connected) and a flow of current begins causing crosstalk to be heard.
Now, there are other issues with interference, such as tightness of wire splices, termination points, unequal spans or spacing between wire pairs and such, however, the earliest and most troubling problem in the early leads was that of crosstalk noise.
The first of two solutions readily became apparent to early telephone engineers. The design of the lines could incorporate transpositioning--whereby each pair was interchanged in a specifically engineered pattern along the line--to break up this magnetic coupling and reduce crosstalk noise. Or, wire spacing could be adjusted so that the wires of a pair wouldn't receive the same amount of induced voltage. When carrier systems began to appear in the early 1920s, telephone companies and railways, began to seek this solution--and implement more efficient transpositioning methods--to nearly end this distortion problem.
The first lines used point and drop transpositions of a simple nature. The point transposition, which is far more efficient, interchanged the wires of a pair at a "point." These points were created by using two- or one-piece transposition insulators on double arms at specifically located points along the line. Early transposition insulators were placed on transposition pins. These pins were slightly taller than standard AT&T standard pins. However, in most cases either glass or porcelain transposition pins used standard telephone-qualified locust, iron or steel pins with wooden cobs.
Each Bell or Independent System used a variation of the transposition scheme developed throughout the early years of telephony. By the early part of the 20th Century, AT&T, for example, had developed five favored schemes for transpositioning:
- Exposed Line Transpositioning for low voice frequency circuits which were very common on all leads up until the 1950s
- C1 Transpositioning scheme for both voice and some pioneering carrier systems in use up to 30 kc
- J5 Transposition style for low frequency voice channels and carrier up to the late 1930s technology of the time
- The Type O1 Transposition scheme for the 1955 advent of O-Carrier 16-channel systems up to 156 kc and,
- R1C (and its variations) which was applied to rural exchange open wire with a few extraneous carrier applications.
The drop type transposition required two spans of an open wire line to complete its complete twist and was sometimes called the "rolling" transposition method. Because the point-type can interchange conductors within less than a single foot of distance, was more efficient than the drop style. It was the primarly means to transpose high quality or carrier circuits. However, for voice frequency (very low frequencies), the rolling type drop bracket two span transposition was used. This type of transposition incorporated the drop and phantom bracket on aerial construction and was an application of the "O1 Transposition" scheme.
In Europe, use of the crossarm was sparing and tall poles would carry steel or iron brackets to maintain each conductor above the ground. There, wire interchanges were handled in a similar "rolling" manner, but callled "barrel" transpositions. These barrel transpositions were continuous; that is, each pole along the route was a transposition pole--unlike North American standards where ever other or several tangent structures were placed between "transposition" poles.
Phantom Loading Unit for Both Side and Phantom Circuits
A variation of the Drop Bracket (two span) transposition was the Phantom Transposition Bracket, utilizing a special elevated single insulator pin bracket above the arm and a second portio of the bracket containing a double drop on two insulator pins. The earliest forms of these "Butterfly Brackets," as the linemen called them, were two piece. Later they were modified to employ thick steel for installations at locations where extra strength was required: transpositions at tight line corners, river crossing poles, at severe elevation drops and the like.
Further modifications in the 1940s and early 1950s, incorporated a single piece Phantom Bracket. This was the last major alteration of its design until they were phased out completely in the 1950s. According to a GTE Outside Plant Engineer with whom I worked, the Phantom Bracket was essentially found to be--after millions were installed nation-wire by Bell, the Independents and the railway companies--to be nearly completely ineffectual at breaking mutual induction so as to prevent crosstalk. Thus, the drop and point-type brackets remained the most used in modern open wire construction.
This drawing portrays a 50-wire toll lead pole head. Each pin is numbered from the most important circuits (top arm) in descending order to the fourth arm end (number 40). Each arm numbered beyond "40" is considered "exchanage" or containing regional/local circuits. There are exceptions to this rule, but this typically follows the AT&T scheme.
The pole head is modeled after the 1885 style of arm: 10A. Note the 12-inch separation between each pair, with the exception of the "pole pairs," whose distance and separation is 16-inches on either side of the pole, and are treated differently, both electrically and mechanically.
This 50-wire lead could, with phantom circuits included, had a capacity of 77 circuits.
Phantom brackets were required to be installed in a specific way as were the drop brackets, on each arm. This was standard practice by the Bell Companies, the Independent Telcos and by the Railway Systems' signals division.
These drawings show the most traditional placement of brackets on tangent structures. When the grades of elevation changed, hence, the line geometry might cause placement of brackets to be altered. Sometimes, guy wires interfered and pins would have to be moved and holes re-drilled; brackets flipped. Occasionally, due to easement restrictions, highway and railway bridges, these brackets might be relocated. This was, of course, with the prior permission of installation and maintenance supervisory personnel.
The earliest transpositions were with transposition insulators or forms of drop brackets, such as the P- and S-Brackets. Above are four "Types" of phantom transpositions as used by telephone companies and railway systems. Note the arrows which denote "from C. O."
The "F" Tandem Transposition Bracket was a unique device, similar to the floating Case Point Type 6" bracket. It was used by various communications companies, although I do not recall its use by Northwestern Bell, CONTEL, Centel or various Midwestern telephone companies.
However, in various locations around the United States and Canada, this unique character did make some appearances. Let's discuss the unit and how it was used.
Like the Case Point Type unit, it "floated" or was suspended at a transposition point required by the proper transposition scheme being used on the particular lead. Detail plans had to specify this specific location and its use was confined to places on the pair where spans denoting its use but preventing a pole with traditional crossarm tandem brackets to be used. These uses might be dictated by placement over a river, a dry gulch, a multi-lane highway crossings or due to special right of way issues, preventing a regular pole to be placed.
The unit came as parts: two wet process brown porcelain insulators, two 3/8ths by 4 1/2-inch bolts, washers and nuts for the unit. The base was a tubular piece of aluminum or fiberglass the same length as a typical crossarm-mounted tandem bracket: about a foot long. Sometimes a "guard wire" would be added to the unit. This prevented possible contact with above pairs in wind or ice, or simply poor span spacing where wires might droop onto pairs below owing to the distance between supports.
The unit was installed without tie wires, just like the other Case Point Bracket. It was installed thus: slide the left end of the bracket between the two pairs. Now the line wires would be on either side of the initial insulator. Make certain the groves carrying each pair were secure and tight on both sides. The left wire of the pair needed to be carried on the top groove; the right in the bottom groove of the first insulator.
Now, raise the unit so the second spool insulator contacts the right wire at the bottom groove. Take the bracket base and push it so that the left wire is placed in the top groove of the second insulator. Now, tighten the span and attach ties at the insulators wedded to the crossarm of the next physically ground-supported pole.
If a "guard wire" was added, the common nature of the bolt diameter would easily qualify any machine nut and extra washer on top of the unit. You would take some spare 109 line wire turn the left end of the wire into a clockwise loop and the far right end of the wire to a counterclockwise loop. Insert wire and attach to the bolt protruding above each insulator washer and bolt. Attach washers and nuts. Now, you have created a fail-safe unit so if a line wire of the pair being tandem transposed broke, or a spool insulator be broken by a thrown rock or gunfire, the guard wire would not drop the transposition arrangement. Fiberglass was often used because, unlike aluminum, the base would perform as a dielectric in case of failure and contact with other working wire.
Inductive Coordination of Telephone Plant, Electric Utilities and Railway System Lines
Here we need to add a little caveat to our discussion on aerial wire transpositioning. It would be a perfect world if it were CAD-drafted, however, with the differences in geographic conditions, weather and climatological issues, we also have the intrusion of other electrically-energized facilities: neighboring electric utility power transmission and distribution systems and apparatus. This also includes the operation of railway open wire with its myriad centralized track control a.c. systems, signal circuits and their own communications media.
Above we have a diagram taken from an early treatise on transposition design for a railway system at the turn-of-the-last-century. We have an EAST to WEST/From C. O. ---> diagram above detailing a non-phantomed voice frequency openwire lead which is expected to be exposed to nearby signal (both d.c. and a.c.) railway lines as well as paralllel and crossing electric power distribution and transmission lines.
In the photo above, we see a two-ten-pin 20 wire lead (with O-Carrier pairs) crossing under a Bureau of Reclamation 115-kV H-Frame transmission line. If we were to transpose this line (beginning at the C. O. or Terminal Pole), this is one way of doing it on crossarms one through four.
If a line were to utilize phantom groups, then the exposed telephone lead would incorporate these complicated arrays of drop and phantom bracket types. Again, the C. O./EAST would be to the left and the final destination (to field) WEST.
With the advent of "J" Carrier (12-channels for open wire construction; "K" Carrier for 12-channel cable pairs), spacing not only between the pins was much greater, but the clearances between crossarm placement also changed: 36 inches instead of 24 inches with traditional 10A and 10B arms. The pins of each pair were eight inches apart and the transposition systems utilized the most efficient point type break-iron and single piece transposition brackets.
The Defense Backbone Line in Nevada, California and Oregon, utilized this configuration.
A 32-wire non-phantomed line (with the addition of a fourth crossarm to the above structure) could provide 256 telephone circuits. A great improvement over the 1885 design above. Consider this 1938 in light of optical network fiber systems. But, in 1938, such improvements helped fight a major world war and offered benefits greatly exceeding the technology just ten years prior.
The wire spacings which came about in the 1929-1938 period, significantly reduced mutual coupling and nearly eliminated crosstalk. Along with new highly efficient point-type transposition schemes, hardware and the advent of high strength conductor adjusted within a fraction of an inch to each span, brought efficiencies to a higher order than before.
Insulator design also took advantage of the new tempered glass, with its lower attenuation qualities, such as the CS, CSA, and CSC single deep grove style manufactured by Corning, Hemingray, Armstrong and Kerr. On the eight and six inch pin separation designs below, the insulators were screwed upon steel pins electrically bonded at each pole. This improved transmission stability requirements and with the use of rolled steel joints at each wire splice, a physically strong and electrically sound connection could be made, eliminating echo and interference. Along with the new insulators came a new tie wire. Instead of using conventional round conductor similar in diameter size to the original conductor, flat tie wires were used on all insulators except at transposition points--where the very taught nature of the construction negated their use. These were sinewy leads, bulging with physical muscle, largely withstanding nature's forces and commanding technical respect.
American Railway Association/Communications Section
The picture above was taken looking south at the Gainesville, Texas train station when the above pole line was in operation. It has since been linewrecked in the mid-2000s. You will note the unique underslung application of the Alcoa Case 6-inch transposition bracket on the top arm. The lower arms sport the single bolt drop brackets. Scroll down further and detailed images illustrate their use. These were prone to torque loading and subject to being distorted if used in unbalanced conditions--angles, long spans and unequal arm spacings.
ARA Accepted Transposition Arrangements & Brackets
Beyond the Brackets: Transposition Schemes in all Their Rich Complexity
Looking at the Myriad of Transposition Methods
Such a simple task: twisted wires. However, the mathematical complexity of this process taxes the imagination.
The idea behind it is simple to combat the unintended interchange of conversations by way of "coupling" on a single or multiple pairs of communications conductors.
The solution is far more complex and troublesome. First, let's open this discussion to the question of "coupling." which will be considered for our open wire aerial circuit example (cable, with wires in close proximity to one another, opened an entirely more complicated bag of worms).
We've strung a mile of aerial copper wire to complete two pairs on a four-pin crossarm between the cable terminal and the last line's subscriber's terminal This will make up part of our local loop. These two uninsulated conductors are in close arrangement to one another throughout their little trip across fields, a couple of ravines and a stream. There is a 48-volt d.c. current flowing in them. Because of their unchanged condition being so close to one another, the adjoining pair induces a voltage on the neighboring pair. This is called electromagnetic induction and we determine the amount of e.i. by looking at their mutual inductance and measuring it at the ends of these wires.
Mutual inductance is a remarkable state of physics. It can work for you or against you. In our case, we want a trouble-free circuit arrangement in both pairs. What I say on the phone needs to be heard interference-free on the other end and for the subscribers to also hear without problem. If we do not break up this coupling, then it is fair to say, the subscribers carrying on a conversation might not hear each other well because of the induced interference and might very well slightly make out some partially intelligible conversation on the opposing pair of wires by means of "crosstalk." Because voice frequency telecom, unlike high frequency carrier, is of low frequencies, we have inductive coupling.
For high frequency carrier systems on open wire, such as Type "J" or "O," we have the issue of capacitive coupling.
Now, don't get me wrong here. There's a place for "intentional" mutual induction. We take advantage of it every day when we speak over the phone using analog systems where loading coils are applied to buried, underground or aerial cables in your area. Winding a conductor around a core of air or iron, can increase this property for many good and just reasons, such as in a transformer with both secondary and primary windings.
Back to our voice frequency example: when aerial wire is subject to not only the adjacent pair of wires, but if placed on a joint-use pole with alternating current power distribution primary or secondary lines above it, additional currents can be added, aggrevating the crosstalk problem to further unpleasant state of affairs.
What's the solution? In our little discussion example, it might be to twist the wire pairs at pre-determined locations along the line so as to break up this interference and "coupling." We might have a wooden bracket lead, where we simply jumble the brackets in different locations in a continuously disjointed--but organized--pattern along the pole line, sometimes called barrel transpositioning. This is used in Europe to a high degree along British open wire and continental line routes.
We, in North America, don't subscribe to this technique--although it is effective. Here, we devised a pattern of transpositioning which also fit our application of the horizontally mounted crossarm design. Barrel transpositioning is quite favored when you have vertically mounted steel brackets on a pole instead of arms. Europe succumbed to this style simply because of their physical line design methods.
One of the first big experiments in telephone long distance technology realized this problem very quickly when AT&T built a major toll lead in the East. Upon "heating it up" the line produced basically garbled nonsense and it was quickly deduced by J. J. Carty and his engineering staff that mutual induction was to blame.
It didn't take long to figure out that while the problem could be remedied, it was not going to be simple. But there was a golden lining to this problem. Figuring that mutual coupling could be both a burden and an opportunity, early engineers devised a way to utilize this "coupling" to create the "phantom" circuit, thus creating the first economical savings in long distance transmission getting more speech, with fewer wires to be strung.
Transposition schemes began to be devised by AT&T and other telephone companies for their gradually extending networks. At first, multiple wooden pins in sets were devised to construct on double-armed poles, to hold the new two-piece transposition insulators to make highly effective "point" transpositions. Later, brackets were designed to break the induction with two spans, such as how a "drop" bracket was applied to rural, subscriber, exchange and toll lead projects.
Yeah . . . I know . . . a little mathematics has to be introduced here to achieve some intellectual breadth to our story. You probably ask, "Why not just transpose the pair equally throughtout the length of the line througout its length and let it go at that?" Well, the problem with open wire--and even more troublesome with insulated multi-pair cable--is that you have . . . neighbors! And, lots of them! On a 50-wire toll lead of the 1930s and 1940s, you have pairs above, below and beside your favorite pair. Then, to make that more complicated, you have on the other side of the highway a 3-phase a.c. power distribution circuit operating at 7.62/13.2-kV and occasionally there are some 161-kV transmission lines cutting across the highway on their way to a nearby substation or from a generating plant. All of this induces substantial voltage on your favored pair of wires for your conversation to carry.
We know how Young Frankenstein did it, so . . . here's how telephone engineers did it.
The inconsistencies to suffer along a 40-wire, 100 mile long toll lead:
All circuits on an openwire communications lead have these charactistics in common either in one or in many circumstances. When an "unbalanced" condition occurs, such as in inductive coupling for voice frequency exchange circuits or capacitive coupling for high frequency carrier toll circuits, transpositioning is undertaken. Telephone and telegraph circuits are designed for low loss, low leakage design. The transference of this crosstalk voltage from one wire of a pair to the other and from the neighboring pairs is extremely small. The communications engineer back in those openwire days would compute the current and voltage of the inducing circuit as though no currents whatever were induced in the other neighboring circuits and then computing the inductive effect of these currents and voltages on each adjacent circuit. The caveat here is to do this with the assumption that no current would flow in any of the other circuits.
So, we get a little more complex. Let's say we have a 40-wire four crossarm toll lead. We need to figure out the inductive coefficients between all these wires and circuits! Actually, it's pretty clear how to do this: instead of waiting fifty years for computers to come about to compute these highly complicated factors on a 110 mile lead, the engineer at the time simply made the computation more practical by taking a short portion of the line under construction and measured.
Now, we have our figures and can compute with some approximation, what the inductive effect between any two short lengths of the lead, could be. However, electricity's physical properties add some complexity to this simple comparative computation. With a lead over a hundred miles, we have the added problem that the disturbing current and voltage are different in magnitude and phase angle along each portio of this line. Because of losses, such as wire resistance, poor insulators, pin leakage, and the like, can vary significantly. Add the phase angle changes which can be up to 10% degree increase or decrease per mile and the propagation of this induced voltage/curent from terminal to terminal cannot be perfectly balanced. In short, no line is perfectly formed, and thus whatever transposition arrangement you create, cannot perfectly remove this nagging problem of crosstalk entirely.
So, we can build an openwire line which is operable under "type unbalance" and that while we cannot entirely eliminate the problem, we can keep it under manageable levels.
Problem solved? Not quite. There is another contributing factor to honing our openwire transpositioning technique: positioning transpositions at exactly the correct physical points along the line. Laying out any pole line is a science, but done with "hand grenade accuracy." That is, the surface of the earth, and hence your road, highway, property line, fence line, building, stream, river, canyon, is not a CAD-draughted line drawing. In laying out a new line it is sometimes possible to set poles exactly at the theoretical transposition point,and when this cannot be done the nearest pole was generally chosen as the transposition pole. But, furthermore this circumstance is frustrated, when using a drop bracket construction method; you have to have a full span to accomplish the wire revolution. Also, what if you have to cross a highway? Now, you have a curve in the line's geometry. This small action induces some irregularity in this induced voltage.
In the early days we noted how the Phantom Circuit, mentioned previously as an initial benefactor in the struggle over electromagnetic coupling, came into being as the ability to use two pairs of wires as a third or "side" circuit. This "side circuit" enabled more conversations without more wires. But this little gift comes with a significant price: if you have phantomed openwire construction, transpositioning is complicted further.
Now you have a non-physically connected circuit which must be balanced along with the physically connected conductors. You can go to the central office (C. O.) and make some changes with the terminal transformers there and other apparatus, but now you must utilize a transposition scheme which provides a skillful "balancing" act between these pairs. Additionally, there are phantomed circuits beside the existing phantoms and some above and below!
So the problem is to transpose the phantom groups. In the words of Harold S. Osborne, who wrote in the American Institute of Electrical Engineers Transactions, June 27, 1918:
It is evident that in designing the transpositions in the side
circuits it is necessary to take into account the transpositions in the phantom. For example, side circuit C would be considered balanced to side circuit F if the transposition in these two circuits only were to be considered. However, the transpositions in the phantom circuits change the coefficients of induction between circuits C and F in different parts of the section shown and therefore prevent the transpositions in the side circuit from balancing this section.
Whew . . . what a problem! Plugging a hole with a pole, plastering a pole with crossarms and throwing a phantom doesn't appear quite as simple as our little example above, does it?
How are transpositions arranged?
The "type unbalance" of which we mentioned earlier, is present in the designs of a successful transposition scheme. There is a further complication with this design when you have neighboring a.c. power circuits. This is called "exposure" and was--and still is--very common. Transpositions cannot entirely guarantee a trouble-free line because of this, but when AT&T made an offer to work with the railroads under the guise of the American Railway Association's (today called the Association of American Railroads) Communications Section Committee to alieviate or at least, dampen this problem, improvements began to occur.
It was quite clear to both sides, the ARA and AT&T, that exposure to nearby circuits of both railway communications signal circuits and AT&T openwire counterparts in nearby road and railway easements was causing some consternation. Add that with the variable of electric utility distribution and transmission facilities intruding into the communication industry's domain, and a common solution would have to be sought. So, for every two years or so, the ARA and their counterparts at AT&T and other Indendent Telcos, joined to relate common issues and resolve them under the Communications Section meetings and published Transactions.
They both commensurated over their own communications conflicts and issues:
1. Non-transposed telegraph circuits, with their residual current and voltage inductive qualities in nearby power system circuits.
2. Single phase power distribution lines nearby or crossing easements where communications lines routed.
3. Three phase power distribution and power transmission lines, where they intersected with communications circuits of both the civilian and railway signal circuits.
In regards to power systems, there were only one, two or three conductors carrying power current/voltage, whereas with telecom there were only two--and multiples of two. This lead to "inductive coordination" processes with the electric utilities in conjunction with AT&T, the Independent telcos and the railway authorities. Electric utility engineers specified a certain number of transpositions on their primary circuits, both for distribution and transmission, and the communications' designers of the railways, Independents and Bell Companies, designed rigorous transposition schemes into their most important top pole circuits to further combat the problem.
Furthermore, first class long distance toll circuits on the top arms, were rigorously transposed beyond some of the standard transposition methods, to insure an balance factor exceeding what had been originally engineered into the lead.
The electric utilities reconstructed some of their neighboring lines to incorporate barrel transpositions equal in length to one-half or one fourth of the telephone lead's counterpart transposition section. So, for an eight mile line, the electric utility would barrel their circuit for each four miles of line. For each progressive decrease in barreling a power circuit, the number of telephone transpositions could be decreased and those existing could do their job much more effectively.
Sometimes, in heavily concentrated easements, the electric utility would take each telephone transposition section and transpose their power circuit. This was highly effective--but costly--as power distribution costs would increase with extra sets of H-Frames, special transposition structures and/or pins, arms and hardware.
The Physical Application of Hardware to Transposition Schemes
One of the most revealing factors in the mathematical solution to the mutual induction problem was that in designing a transposition scheme, once you had figured out the 40-wire problem, adding additional wires and their "coupling" problem was easily solved. Most big toll leads of the 1920s-1950s, were 40-wire toll designs. When additional arms were added, some further study was needed. At the initial start of the line, the 40 wires basically commanded the type of transposition scheme; hence, with the fifth, sixth and seventh--even if there were eight--ten-pin crossarms, you simply repeated the first four. Hence, top arm No. 1 was to No. 5 arm; No. 2 arm was to No. 6 arm below, and so forth.
An engineer of the time reported, that in designing the early day voice frequency circuits, phantom circuits, and d. c. grounded telegraph circuits on a 40-wire toll lead of the 1920s, taken in combination of two, there could have been about 2,500 combination transposition schemes!
For a transposition scheme to actually work, there were a couple of general requirements:
First, there had to be what was termed the "Neutral Point" or "Points," whereby the communications pathway had to be arranged so that where there was maxium balance (neutral) obtained between all the circuits on a toll lead were provided where there were discontinuities, or special demands, such as angles, long spans, etc. The Bell System used "hand grenade accuracy" in determining these points, typically around 7.88 miles in toll lead length. If there was a junction point for additional lines joining or leaving the host lead, this had to be taken into account. Thus, the telephone transmission engineer of the time had various transposition designs available, but the lead had to be at minimum 12.8 kilometers in length to incorporate them successfully. If not, there were other designs for which shorter length leads could be applied.
Secondly, there was the issue of cost. Putting a transposition in a line meant a special installation of hardware, extra bolts, different insulators--in some cases--extra bracing for angles (if the transpositions were on an angle structure), heavier and stronger pole, taller pole. And also the completed transposition section was designed at the lowest possible cost without sacrificing electrical quality and speech transmission. It was a path between putting the least amount of transpositions in the line, yet managing to maintain the electrical values which represented profitable service in quality terms to the subscriber. Phantom transpositions using a butterfly bracket or other point type bracket were more expensive and thus there was an urgent need to keep the number of these as small as possible without sacrificing quality again.
Thirdly, the transposition pole numbers had to be kept down. The population of these special poles along a toll lead needed to be reduced not only on account of expense, but because othe use of large numbers of these special structures limited the minimum length of a transposition section. The use of many poles also tended to increase the induction between telephone circuits due to irregularities in the physical construction of them.
Fourthly, in the early years where openwire lines were "loaded," junction transposition structures were installed. This was to reduce the induction between these telephone circuits which were loaded--thus adding more to the cost of the construction of a line.
Fifthly, we have not entered into a long discussion of variables such as size and composition of openwire conductors: copper, copperweld, high-strength steel, alumoweld, and variations in tie wire types and splice joints. These also contributed to the complexity of building an openwire line and transposition factors.
Forms of Openwire Transposition Designs: 1930s-1950s
Three Channel Carrier Transposition Scheme
In Bell System parlance, this was the remarkable and pioneering "C" Carrier for openwire lines of the mid-1920s.
Twelve Channel Transposition Scheme
Commonly referred to in the Bell System as "J" Carrier ("K" Carrier for cable), this was an innovation of Bell Laboratories in 1937 and was first put into use on a line between Houston-Dallas and Austin, Texas in 1938.
Sixteen Channel Carrier Transposition Scheme
In 1955, the Bell System (through AT&T Labs) introduced the first 16-channel openwire carrier system: Type "O" as in "Oh". This was the last carrier system for open wire toll leads and comprised the best thinking of mid-20th Century minds for what lay ahead for the remaining open wire short haul facilities then in use.
It was considered a "dirty" carrier, able to be used on lines which had previously carried voice frequency, short-haul services, and whose characteristics primarily were found in exchange-construction design.
Type O had its counterpart in carrier for multi-pair copper toll cable: Type "N". As with the cable type carrier, the Type O 16-channel aerial wire carrier was actually in its full form, four separate carrier systems within.
Carrier Frequency Band in kc Openwire Line Application
OA 2 - 36 Voice-Transposed Pairs
OB 40 - 76 C-Carrier Transposed Pairs
OC 80 - 116 C-Carrier Transposed Pairs
OD 120 - 156 C-Carrier Transposed Pairs
In February 1955, Bell Laboratories Record published a succinct article outling the new transposition schemes available for this new "O" carrier scheme. Esther Rentrop, a Transmission Engineering staff member at the time, wrote an article entitled, "The Type-O1 Transposition System."
Essentially what the article explained was the use of new transposition designs and with it new physical hardware for applying this 16-channel carrier to aerial wire lines. This non-phantomed carrier operated at 156 kc for what had previously been voice channel lines. The Type O Carrier's main claims to fame were these: first, this carrier had 20 db less crosstalk allowance on its application to openwire lines. This carrier was modifiable in retrofiting former Type C (three-channel carrier lines) to carry 36 kc OA carrier (depending upon the line's length and physical characteristics). It also would "permit the use of OB carrier on nearly all existing Type C pairs without line re-arrangement.
Crosstalk measurement devices were used to determine whether or not a former Type C channel carrier pair would allow imposition of this new carrier. Because many former Type C carrier pairs had high attenuation or losses, excessive crosstalk was shown to prevent its use. Former drop bracket construction and phantom brackets--as well as point-type brackets of the conventional Type-J design, were useless in combatting this problem.
But, engineers did not give up. Instead, to use this carrier on former Type-C carrier circuits, new transposition hardware was dreamed up. New transposition schemes to accompany the "nuts and bolts" also were devised; in party, all three methods found use in rejuvenating old voice frequency and early carrier toll and exchange circuits to greater glory.
First a technical goal of achieving a minimum of 30 decibel equal-level coupling between pairs on a 150 mile line using between 2 and 156 kc had to be attained. Additionally, Type O carrier pairs had to reside on at least five crossarms per pole. More stringent needs were yet required: pair separation had to meet operational levels on former Type C pairs or on the new OA system, below 36 db.
The solution: the Type O1 Transposition Scheme. Here the evolution of communications service to the highest degree possible on openwire lines saw its summit. The system, on five crossarms, required a new method of transpositioning similar to the point-type of twelve-channel carrier in 1938, and with the removal of pole pairs, improved transmission values of quality were attained.
So, how did the Type O differ in transposition technique from the voice channel and previous carrier systems? First of all, it was not necessary to remove phantom brackets and drop brackets from existing circuits, although it was an improvement to do so. Furthermore, the carrier called for eight-inch spacings between wires and all interchanges of conductors done on new point-type brackets.
One of the issues facing the local distribution outside plant engineer, was to use this new carrier without re-building lines significantly. Essentially, the true value of this system was to add increased traffic potential to existing openwire but to simply modify the crossarm construction by maintaining a "mean" of eight inches. That is, unlike Type J, which was stringent on its restriction of span spacing, pair spacing, arm clearances from the pole pairs and point-type transpositions every other pole, this carrier made allowances. This economical carrier could be applied to standard 10A arms with the introduction of two new brackets:
- A new point-type bracket on a single rectangular base. This new 4-inch bracket was made of high-strength alloy steel and were resistant to corrosion by their hot-dip galvanized manufacturing process. Because of their new design using a single plate, they could withstand storm loads, torsional effects of angle or corner poles and fastened to the arm with a single carriage bolt. It cost one third less than the conventional 8-inch former point-type bracket.
- A new "pinch" type bracket which could be installed to create the mean of 8-inches every other pole or so. It too, was hot-dip galvanized steel resistant to corrosion and contained a bent lip, which anchored it securely to the arm and required only one carriage bolt.
- A new insulator (TS) taking the place of the CS, CSC and CSA borocilicate types. This new tempered glass insulator had two deep groves and used the short shank CS transposition bracket pins.
So the design looked like this: the usual 12-inch 10A arm spacing remained at non-transposition poles and a new 4-inch point-type bracket was used at tramp points. The spacing of those pairs not transposed at the transposition poles was reduced to four inch separation by the use of a "pinch-in" bracket. The pinch bracket did not transpose any conductors, merely pulled them in close proximity. Thus a unique tapering and expansion of wire pairs occurred at a regular interval along the Type O Carrier lead. This resulted in an average or mean spacing of about eight inches as required by the technology of the time. No major changes, and the brackets used existing pin positions on the 10A arm. Much less expensive than re-building an entire line to specification.
Transposition sections were composed of differing lengths so that existing voice-frequency and Type C carrier pairs could be coordinated properly with the new carrier. Phantom groups also were used, but many eliminated with the additional traffic channels available with this new openwire technology.