TECHNICAL PUBLICATION #46
SOLVING TELEPHONE RF INTERFERENCE - HELPFUL HINTS AND SUGGESTIONS
With the increasing popularity of complex and computer-aided home telephone units manufactured by numerous companies worldwide comes a corresponding increase in both the quantity and severity of telephone interference caused by local radio broadcast transmitters.
The new telephones, both domestic and imported, are the most susceptible and delicate ever built, and few manufacturers of the devices have given much consideration or effort to designs which include resistance to RFI and other forms of electromagnetic interference.
Making matters worse is the fact that ground terminal connections for telephones are rarely available at the location of an extension telephone, and telephone wiring is all unshielded and exposed.
Telephone interference is caused by radio signals produced in the immediate vicinity that are intercepted by the mass of telephone wiring in the home and outside on telephone company elevated wiring.
Signals enter the phone on different conductors, working their way through phone circuits and causing current flow, resulting in voice distortion or noise.
Most telephone interference can be remedied by the simple installation of a telephone filter that plugs simply into the rear of the phone.
These devices are designed to insert a choking effect, or loss at radio frequencies, into the phone wiring. They have no effect on the telephone operation.
About the only realistic way to resolve phone interference short of making internal circuitry changes to the phone is by choking the RF signals before they enter.There are two ports of entry that interfering signals can enter a phone unit.
The first, and most common, is through the house wiring and into the telephone set directly as mentioned above. The second is through the handset cord (cord attaching the handset to the phone body).
In nearly all cases a telephone line filter will be part of the solution to eliminate the interference. But in cases where the line filter is not completely effective another small filter device in the handset lead may be needed to bring back quiet enjoyment of the unit.
An easy way to judge for yourself if the handset cord is suspect is to make a short handset cord about 6-12 inches long with the cord and tools available at most radio parts stores.
If the interference is not present when the short cord is used to connect the handset with the phone body then the handset cord will probably have to have its own filter installed.
If the phone has a speaker then simply disconnect the handset cord and run the test with the internal speaker (and a line filter installed).
If a line filter and handset filter are both installed and interference persists, then it's time to recognize that the telephone itself is inherently hypersensitive to external electromagnetic fields.
Possible cures are replacement of the phone with a different type or brand, or internal circuitry modifications done by a local technician.
Generally in our experience the worst offenders of telephone interference susceptibility @ are AT&T and Panasonic manufactured units. The best performers are built by Radio Shack/Tandy.
If you go shopping for phone filters obtain a unit with at least 30db measured attenuation in the RF range (3 to 30 Mhz.).
If the filter manufacturer doesn't publish his figures, shop elsewhere. And get one that is designed to prevent BOTH common mode and differential mode interference.
For the most part telephone interference is the easiest type of interference to deal with, but sometimes it can be insidious. Don't be afraid to experiment with different combinations of filters, phone locations, or lead lengths to seek a final conclusion.