Loading: Open Wire Versus Cable Applications
Rare photo of storm-guyed structure sporting top arm loading coils (black cases). Note the use of break-irons to dead-end the loop pairs to each metal-cased coil assembly. Original coils were placed in wooden cases and surrounded by air, unlike these above, whose internal parts were oil insulated and cooled. Wood was thought to mitigate the stray magnetic effects leading to possible energy losses. Photo date: shortly after World War I or early 1920s (note automobile style in background). Photo source and location unknown.
Many of you have seen the very large cases or “tanks” attached to a pole on a cable route. These are separated from each other by considerable distance. At inspection, the ground viewer will see an exposed cable stub leading from the main aerial lead- or suspended plastic-sheathed cable spliced into the pigtail of the tank-like device. These are “loading coils” and are of various sizes. Some, as were placed on the “A&B” cable between Chicago and Omaha on the transcontinental route, were the size of 50- and 100-kVA electric pole transformers. Their cases were mounted on a two-pole structure with two steel “I” beams between poles. The earlier models were square; the later 1920s and after, round. They could be as small as a couple of inches in diameter and a half foot long.
Loading coils were not placed in series with cable pairs for decoration or to reduce the power to customers. Instead, they were “induction devices,” as regular A. C. distribution transformers, but did several things in regards to telephone D. C. circuit health:
- Cable circuits undergo passage through a loading coil which causes a “bandpass filter” effect.
- Each installation increases series resistance and inductance of the conductors (pairs).
- Placing loading coils into cable circuits increases inductance, thus reduces the speed of voice and carrier currents. In fact, for voice frequency circuits, loading is applied liberally. Unfortunately, for carrier, it deteriorates signals where they operate higher than the cutoff frequency of the loading design.
Now, why talk about cable instead of open wire, for introducing this topic? The principles of loading on telecom voice frequency circuits are well understood today. However, the only load coils seen today are those on cable–aerial, buried or underground. After the mid-1920s, loading was removed from open wire even though initially applied in the early days on some long routes. Today, cables are frequently loaded in many situations. However, the effects of an inductance device such as a load coil on open wire circuits opened significant “cans of worms,” to their efficient operation and evolution. So, while we see multi-pair copper cable lines loaded (POTS), to open wire it is a “No, No!”
Let’s go back in history and review why open wire saw such initial promise with these coils but was quickly removed, and instead, became the standard of quality cable pair operation.
Let’s “Load” Up on the Basics of Attenuation
What is “attenuation?” This, in general, is a five dollar word for interference. Interference on open wire is much less than when confined within a sheath as cable pairs. Cables were instrumental, even in the early days, of allowing open wire signals to be transmitted into central offices (C.O.s) and various repeater huts. Railways, their civilian counterparts in telecom, and other transportation organizations, used cables because they were less exposed to the elements, allowed use of limited clearance and space, and could be buried or placed in underground conduits. Clearly, the cable had a major role to play not just into the future, but even in the early days of open wire applications.
Open wire toll plant, by 1880, had expanded to nearly 50 miles, while its rival, cable in 1883, had expanded to over one quarter mile in Boston. This major initiative was surpassed swiftly by a ten mile cable route between New York and Newark in 1902.
Open wire was rushing west, as an 1893 open wire toll lead furnished daily service between Chicago and New York. This nine-hundred mile link was groundbreaking, but heartbreaking as well, since this appeared to be the longest stretch of line for which speech satisfactorily could extend without impacting fatal electrical losses.
With the application of U. S. Patent 0-652-230, filed on December 14, 1899, by Mihajlo. I. Pupin, the loading coil became practical, however, Pupin only refined the thinking of an earlier scientist, Oliver Heaviside. Heaviside was a brilliant mathematician who advanced calculus to analyze circuit elements. By examining telegraph cables in use, he found there could be ways of increasing inductance, thus emphasizing further efficiencies of operation.
Heaviside sought a “distortionless” transmission line, but found this could only occur if several factors combined. These, he referred to as, “The Heaviside Condition.” Series impedence (Z), must be proportional to the shunt admittance (Y), within all frequencies. No transmission line at that time was characterized by such a perfect situation, because insulators leak (low value) whether they are glass or plastic (in cables). To remedy this situation, Heaviside considered improving inductance by:
- wide spacing of each line wire
- impregnating the insulators with iron dust
These were rather impractical during the late 1880s, so Heaviside proposed placing spaced inductance devices along the transmission line at regular intervals. This was a remarkable idea, but the powers that be in Britain, didn’t warm to the idea. Because his ideas resided in theory, rather than a practical patented device, his suggestions were politely ignored.
Around the mid-1890s, John S. Stone, an AT&T engineer offered the idea of “continuous loading” analogous to Heaviside’s theories on cable induction, but it never saw direct application. Later elements of his thinking found derivation in other forms of cable manufacture and design.
Where open wire might have stalemated, loading furnished a load of relief to telephone engineers, who now might extend the western toll routes. Soon followed some record achievements following the invention of the loading coil in 1900:
- Completion of the Boston-Washington, D. C. underground cable (455 miles) in 1913.
- Boston to San Francisco open wire lead (3,650 miles) in 1915.
As the H.M.S. Titanic’s construction and preparation for its first and final voyage was ending, 85,000 miles of open wire in the United States was loaded. Cable, too, was loaded to the total of over 170,000 miles. As we have spoken in Song elsewhere about Toll Entrance Cables, we’ll refresh the fact the majority of these loaded cables were inter-office trunk types. At the time, over 125,000 loading coils were then in use.
Use of loading coils in conjunction with the birth of the telephone repeater, made further generous loading of open wire possible. By the 1924 Presidential Election, 200,000 miles of open wire and 135,000 miles of cable were loaded. Added up, over 777,000 loading coils were in use on the Bell System (according to their records) of which half a million coils were on inter-office trunk lines. 1,240,000 Western Electric loading coils were implemented around the various operating companies of the Bell System and its Long Lines Division. This does NOT include the various thousands of Independent companies (as for example in Iowa, where companies numbered over 800 Independents!) using loading on their smaller systems where exchange and short distance toll were efficiently applied and activated. Sounded like a good cause should go on . . . forever . . . right?
So, why did loading remain on cable and removed from open wire beginning in 1926? Let’s look at the basics of cables vs. aerial wire. If you go to the section on Aerial Toll Lead Obituaries, you’ll find a cross-section of the original A&B Cable used from Chicago to Omaha replacing the original open wire in 1936. In your neighborhood, spot an aerial cable and if you cut it open, clearly exposed will be different insulated-colored conductors. Typically, such cables contain either: 19-, 22-, 24-, or 26-guage conductors. Obviously, these are quite small gauges. As they become more distant from the C. O. or RST or RT (Remote Subscriber Terminal) their size diameter increases.
However, with toll cables, where some distance passes between town to town, two pairs with identical electrical characteristics are transposed (twisted pairs) forming a four-wire group, or “quad.” They are directional; meaning one pair transmits one direction “East” while the other “West.” You’ll find it referred to as the “four-wire circuit” and carrier is most often utilized on these pairs.
Capacitors are created when an insulator separates to conductive materials and electricity is applied. Aerial copper open wire pairs are no different. The two wires (and on large toll leads) the pairs above, below and beside them, perform this effect, as conductors are separated by an insulator–air. So if the lead is a long one, capacitance increases. Great insulators lessen capacitance and poor ones result in greater degrees of capacitance.
Cable circuits suffer far greater attenuation, or interference losses, than open wire for a multitude of practical and important reasons. When you combine multiple pairs so densely packed in one sheath, higher attenuation results. Now, if we introduce carrier, with its higher frequencies, then losses skyrocket. But first things first . . .
You ask, “But wasn’t carrier first developed for open wire?” Yes, it was. Development of open wire carrier led to the use of impedance-matched carrier systems for cable. However, loading coils tended to block higher frequencies, as were used in open wire carrier. Also, higher speed telemetry circuits experienced problems with loaded circuits. When a long-haul open wire circuit was loaded, even under perfect conditions, the transmission velocity was substantially slowed. One major issue with these open wire long lines were the echo effects. While echo cancelers were applied later to toll cable, the advance of carrier on toll carrier essentially rendered the loading coil unnecessary on lengthy routes. Suburban and exchange carrier yet use them, but their use is decreasing.
Here’s how Conductor Size, Resistance, Frequency Attenuation and Capacitance impacted cable design:
Clearly, cables were shown to have higher losses than open wire lines. While attempting to reduce distributed resistance by increasing the size of the cross-section of the metallic pair, might work (and does), now you have a very weighty cable and fewer pairs contained within the same sized sheath. This impractical improvement has its benefits: distributed capacitance is cut by increasing space between them and in turn decreases capacitance along the whole run of the cable, too. However, instead of a 300-pair cable along an expensive ROW/easement, you’ve got a far less efficient by-way for telecom. Remember: in telecom traffic efficiency is number one priority.
For example, if loading coils had never been placed in the early years within the outside plant investment of long distance communications companies and larger diameter conductors were to take their place, it is estimated that the plant investment by 1930 would have been greater than one third of a billion dollars! Think of the open wire facilities! Using 16-gauge conductors, for example, would make crossarm mechanical stability necessary. This would require heavier pins and arms. Lots of costs pile up there.
What is the alternative to this “fewer pairs are better” arrangement? Loading. By increasing distributed inductance, attenuation is cut significantly. What loading does is convert a cable with certain negative characteristics to one with improved electrical characteristics. The word which is most important is “constant.” Loading in voice frequency cable pairs allows the entire per-unit length to be static; predictable in quality throughout. Between the various voice frequencies, there can be a maximum transfer of power, because loading is undertaken.
Now, open wire lines . . . don’t have the significant losses, or attenuation which cram-packed copper cable pairs experience. Yes, there are losses, but are quite different and vary over time. We talked about “constant” electrical characteristics of multi-pair cable with loading. But, open wire is not protected within a sheath and therein lies the problem. Exposed to the open air, these typically uninsulated conductors suffer the vagaries of climate: rain, sleet, snow (wet snow conducts; dry snow acts as an insulator), wind (which pitches the separation of pairs from one another as well as deflecting them at points of contact with insulators), pairs hung above and beneath other circuits as well as electric power A.C. potential induction and contact, simply do not allow open wire to be loaded.
Here’s another peril: loading coils are impractical to increase open wire’s inductance as an efficiency method because we know how wet insulators impact attenuation–especially at higher frequencies–and no insulator is completely an “insulator”. All have leakage losses of one form or another. This bouncing “imbalance” between electrical characteristics versus cable pairs protected within a sheath, make loading really pretty impractical.
Another feature restricting their use on open wire was the reduction in line distortion effects. To explain, loading open wire revealed the need for far better insulators at each pole attachment along with new procedures where separation between physical pairs was crucial. A loaded line worked at higher voltage than non-loaded. Because all insulators “leak,” there is a propensity for all insulators to shed a little voltage over their sides down to the pin and arm and to the pole when it rains. Other contaminates also worked vengeance on open wire, such as dust, salt spray from the oceans and industrial pollution. Steam trains were major perpetrators of the latter with their exhaust of black smoke billowing from their engine stacks and depositing upon insulators, un-insulated tie wires and crossarm pins.
When the U. S. Transcontinental (original) link was being constructed in the early teens of the 20th Century, loading open wire furnished enough “umph!” to make transmission efficiency capable from New York to Denver. However, loading open wire was substantially different than the existing cable lines at the time.
Open wire was loaded, but distances between loading coils was far closer making these aerial pairs very heavily loaded. Also, as in this chapter heading photograph, the load coil cases were large, so they could accommodate very large coils–much bigger than cable’s similarly installed units.
Open wire was much more susceptible to lightning attack than cable, thus they had a “breakdown” test strength at the early part of the last century at 8-kV. Each coil was protected with lightning arresters on each side pair.
Also open wire, which in early use, was considered far more e