Loading coil

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In electronics, a loading coil or load coil is a coil (inductor) that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable. Previous telegraph lines were overland or shorter, hence had less capacitance, thus didn't need extra inductance. Loading coils are archaically known as Pupin coils after Michael I. Pupin (especially when used for the Heaviside condition), and the process of inserting them is sometimes called pupinization. Submarine communications cables being particularly subject to the problem, early 20th century ones using balanced pairs were often continuously loaded by iron tape rather than discretely by load coils.

Loading coils inserted periodically in series with a pair of wires reduce the attenuation at the higher voice frequencies up to the cutoff frequency of the low-pass filter formed by (a) the inductance of the coils and distributed inductance of the wires, and (b) the distributed capacitance between the wires. Above the cutoff frequency, attenuation increases rapidly.

A common application of loading coils is to improve the voice-frequency amplitude response characteristics of the twisted balanced pairs in a telephone cable. When connected in series with a twisted pair at regular intervals, loading coils, in concert with the distributed resistance and capacitance of the pair, convert the line from an RC network into a transmission line (and, coincidentally, form an audio frequency filter) that improves the high-frequency audio response of the pair up to the cutoff frequency of the transmission line. The shorter the interval, the higher the cutoff frequency.

When loading coils are in place, signal attenuation remains low for signals within the passband of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. Thus, if the pair is subsequently reused to support applications that require higher frequencies (such as carrier systems or DSL), any loading coils that were present on the line must be removed or replaced with a smart coil.

American early and middle 20th Century telephone cables had load coils at intervals of a mile, usually in coil cases holding many. The coils must be removed to pass high frequencies, but the coil cases provided convenient places for repeaters for digital T-carrier systems, which could carry 1.5 Mb/s across that distance. Due to narrower streets and higher cost of copper, European cables had thinner wires and needed closer intervals. Intervals of a Kilometer allowed European systems to carry 2 Mb/s.

Underground and submarine power cables that carry high-voltage alternating current more than several miles or km also need loading. Their coils are generally located in large underground rooms.

A (mobile) radio antenna, shorter than a quarter wavelength for practical reasons, presents capacitive reactance to a transmission line. This can be canceled by inserting an equal and opposite (inductive) reactance in series, by means of a loading coil typically at the base or center of the antenna. Consequently the antenna presents a resistance (desirable) to the transmission line.