26 Gauge  --  Cable Loss

 

The most common type of cable from the Telephone Company Central Office to your home is 26 gauge cable. In very old times it was open wire. That was gradually replaced with multi-pair cables with paper insulation. Now the cable is plastic insulated.

 

All electrical cable has certain characteristics. The most common are resistance and capacitance. There is a very slight amount of inductance and insulation resistance between the two conductors, for all practical purposes, with voice circuits, these two factors can be ignored.

 

The simple explanation is that one mile of cable from the telephone company to you home has about 430 Ohms of resistance and .082 Microfarad of capacitance. If you wanted to build a mile of cable you could coil up a mile of it and put it in a box. The simple way would be to use four resistors (100 Ohms each) and a capacitor (.082 MFD 250 VDC NP) to get an approximation.

 

With a bit of work you can get this into a standard jack.:

 

 

The circuit is two way, plug one end into the phone line and the other end to a telephone. See if a mile of cable makes a difference. Or double or triple it for 2 or 3 miles of cable. I used standard resistance values, so my one mile is really only 5000 feet.  

 

If you want to be super authentic, change one of the 100 ohm resistors to 120 ohms and add a 1 MH choke in series with one of the resistors.

 

 

 

 

I have condensed a complicated explanation to just one page. Refer to “Principles of Electricity  Applied to Telephone and Telegraph Work” by AT&T, available on E Bay for only a few $$ in CD or a bit more in book form.

 

There is a loss in audio associated with this circuit and it is about 3.5 DB at voice frequencies, and more for higher frequencies.