A plain telephone loop stops working at the distance where its total resistance reaches about 1,500 ohms, which is roughly 8 km on 0.5 mm copper wire and about 5 km on 0.4 mm. Beyond that, a line can be extended in three ways: thicker wire, loading coils that offset the line’s capacitance, and repeaters that amplify the signal. Each fixes a different loss, and none helps the loss it does not address.
Part of Telecommunications
Carrying a telephone conversation beyond the few kilometres a simple battery loop will serve, and the losses that limit it.
Two Different Losses Limit A Telephone Line
Resistance loss. The microphone’s current has to flow out and back through the wire. Wire Selection for Telephony gives a total loop limit of about 1,500 Ω. Past that the microphone is starved of current (see Microphone).
High-frequency loss. Capacitance between the two conductors acts as a low-pass filter, so the high frequencies that carry consonants fade first and the voice sounds muffled (see Two-Wire Circuit). This loss comes from the line’s construction, not from the DC loop.
Wire-selection tables only address the first. Reaching further needs a different tool for each.
Thicker Wire Extends The Resistance Budget
The loop resistance per km from Wire Selection for Telephony, divided into the 1,500 Ω limit:
| Wire diameter | Loop resistance | Reach = 1,500 Ω / (Ω per km) |
|---|---|---|
| 0.4 mm | 280 Ω/km | 1,500 / 280 = 5.4 km |
| 0.5 mm | 180 Ω/km | 1,500 / 180 = 8.3 km |
| 0.6 mm | 125 Ω/km | 1,500 / 125 = 12 km |
| 0.8 mm | 70 Ω/km | 1,500 / 70 = 21 km |
| 1.0 mm | 45 Ω/km | 1,500 / 45 = 33 km |
The source rounds the first two to 5 and 8 km. The reach is for the whole loop, both conductors, at the temperature the figures assume. Copper resistance rises about 0.4% per degree Celsius, so a line that fits on a cool day can fail on a hot one. At 60 °C, 40 degrees above 20 °C, the rise is 40 × 0.4% = 16%, and 8 km of 0.5 mm wire goes from 8 × 180 = 1,440 Ω to about 1,670 Ω. Leave margin. Note that the carrier article’s gauge table recommends 14 AWG for 5 to 20 km, which is a separate guide in AWG sizes and distance bands, not derived from this arithmetic.
Loading Coils Cancel Capacitance, Not Resistance
Inductors inserted in the line at intervals compensate for the capacitive shunting, as Two-Wire Circuit describes: Michael Pupin and George Campbell developed the technique independently around 1900, and for a community system it becomes worthwhile for lines beyond about 5 km. A loaded line carries voice with much less attenuation. The drawback is covered in Wire Line Systems and Carrier Systems: loading helps the voice band and hurts anything above it.
Repeaters Restore Level, At A Price
A repeater amplifies the weakened signal. The carrier article puts one every 20 to 30 km, two amplifiers per station, one for each direction of talk. This is the same idea as a telegraph relay, described in Repeater Stations, which closes a fresh local circuit so that a telegraph line can be extended indefinitely; that relay re-forms an on-off pulse. A voice cannot be re-formed that way, so a telephone repeater amplifies everything it receives, including noise. That is the subject of Amplification.
Wire Transmission plans telegraph relays every 50 to 100 km, which is a different medium and not a telephone figure.
What You Need
- A resistance budget for the route, worked as in the table above
- A measured loop resistance of the finished line with a multimeter at both ends, before any instrument is connected
- Poles and insulators as in Pole Line Building
- A plan for lightning along the whole length
Common Mistakes
- Using the room-temperature figure for a summer line. The 16% rise above can put a marginal line over the limit.
- Adding loading coils to fix a resistance problem. They do nothing for it.
- Adding a repeater without its power. Each one needs a battery that someone charges.
- Planning the telegraph relay spacing for voice. The two figures describe different signals.
Safety
Lightning on long lines
A telephone line of kilometres acts as a lightning antenna and a nearby strike induces thousands of volts (see Lightning Protection). Fit a protector and an earth under 10 Ω where the line enters every building and repeater, and stay off the telephone in a thunderstorm.
What’s Next
- Learn next: Wire Line Systems, twisted pair and loading in detail
- Combine with: Carrier Systems, several conversations on one pair