This page gives information about fixing Radio Frequency Interference, the unwanted reception of radio signal. You'll find out how to determine what is at fault (the transmitter or the receiver) and what you can do in either case.

Interference from licensed radio transmitters being defective is so incredibly rare that they do not even discuss it. Radio transmitters do not transmit at audio frequencies, therefore radio transmitters are never at fault when a telephone receives a radio signal. The following is a " Bell Systems Practice" on telephone RFI.

RADIO SIGNAL SUPPRESSION FOR TELEPHONE SETS

Radio interference frequently occurs where a radio station is located near telephone facilities. Generally the radio signal is picked up by the wire which acts as an antenna, and demodulated (changed to audio frequency) by non linear components, such as varistors, transistors and diodes in the telephone set. However demodulation can also result from corroded connections and terminations.

Before ordering a telephone set modified for radio signal suppression, to the Express Electronic Lab, check for the following:

Corroded connections (inside and outside plant)
Loose wire terminations (including set wiring)
Abandoned drop wire still connected to line
Inside wire connected but not used
Foreign attachments

Installing a modified telephone set on the customers premises will not be effective if demodulation is caused by any of the above conditions.

Telephone set components that may act as demodulators are:

Speech equalization varistors in networks
Transistors, ICs in amplifiers of some networks and handset
Varistors and transistors in Touch-Tone dials
Diodes in polarity guards
Click suppression varistors across receiver units
Carbon transmitters

The cause of radio signal demodulation in networks has effectively been minimized. In This networks the speech equalization varistors have been replaced by resistors and a strapping option that provides speech equalization.

Where demodulation is attributed to components in a telephone set, or associated voice coupler, in addition to changing to a modified set it's recommended that a 40 BA capacitor be placed at the protector and a 1542A inductor be added to the connecting block. The capacitor will bypass, to ground, radio signals picked up by the drop wire while the inductor will tend to attenuate radio signals picked up by the inside wire.

40BA Capacitor Schematic
          0,25 uF      0,25 uF            0,25 uF     0,25 uF
|------------|  (-------------|  (--------o----------|  (------------|  (--------------|
|                                         |                                            |
|                                         |                                            |
T I P                           GROUND                            RING

CAUTION : Only authorized service technicians may internally modify telephone instruments. Only telephone company personnel may filter outside telephone lines.