The Loading Coil


This is a VERY brief summary of the common loading coil, 88 mH (milli-Henrys), which is 0.088 Henry. It is best if you read about loading coils and loading in “Principles of Electricity”. They were invented by Mihajlo Pupin, look him up in Wikipedia.


Loading coils have one purpose in life, series inductance cancels shunt capacitance. That means that add a loading coil in a long telephone line and the loss that would normally comes with a long line is reduced. Too good to be true? Yes and no. Long lines do have a lower pass band or frequency response.


The longer the telephone line, the higher the loss is at high frequencies, not too noticeable in the voice range, but still there. Too long a line, without any amplification, and this loss becomes noticeable. Adding an amplifier, in days past, was expensive. To overcome this loss one or more loading coils were used. The effect was less loss at the voice frequencies where the power (speaking volume) was around 800 Hertz, and higher loss at the upper range of the voice, were the sounds made a voice more distinct and recognizable. Overall, it was a good design at the time.


Along came digital transmission, at first T1s, and then DSL or ASDL. Now the high frequencies were needed and loading coils had to go on these lines. Central Offices were improved and could provide some amplification very easily and cheaply.


Load Pots, containers of loading coils were common. These were steel container about a foot in diameter and about 3 feet high with a cable coming out of one end. Out in the country you would see these on telephone poles, now you rarely see them.


A loading coil is a little under an inch and a half in diameter. It consists of two coils wound on a torrid like coil form. A torrid looks like a doughnut. In the case of a loading coil it is flattened out. They are spaced about every 6,000 feet on a long line, or just over a mile. This would be on your standard dial tone line if you were several miles from the Central Office. This type of loading is H88, there are other types of loading. Loading is used only on subscriber loops exceeding 18,000 feet in length. When I lived in the Los Feliz area of Los Angeles, I had the circuit below on my telephone line. I was served out of the Normandy office and one SXS line and one XBAR line.


The total inductance of the coil is 88 mH, with each winding being 22 mH. The inductance of 22 + 22 is doubled when you put the coils in series. It could be used as a 1:1 transformer, but it never is. In that case it would be a 135 Ohm to 135 Ohm transformer, not too useful.


For more information go here: a white paper about a new smart coil is at Coil Technical Service Bulletin.pdf



Three coils are shown above. (1) The large Western Electric Type 120 Repeat Coil, (2) The small 1:1 Transformer (similar to the TU-016), and (3) A round Loading Coil, which is about a 150 Ohm/150 Ohm Repeat Coil, but not designed as one. A House Key is shown for size comparison.


For technical folks the impedance (not considering the DC resistance) of the load coil, is 2 Pi F L. Where Pi is 3.14, F is 1000, and L is .088, then add in the resistance of the coil. Z (impedance) is about 135 to 150 ohms in actual practice. This is for one winding.


Loading coils are not easy to find, I have a small supply which I hoard. I bought them 5 for $1, but now that would be about $1 each. The last time I used some was to make a Music on Hold device, using 3 coils. The Radio was on the roof of a 5 story building and the switch room was on the 1st floor. Each device then had a low impedance music source, isolated from the other devices.