Telephone interference

Telephone interference

I'm offering the following comments in response to a letter to Bisnis Indonesia which was published in the "Your Letters" section of The Jakarta Post (Jan. 3, 1995). The writer of the letter complained of radio frequency interference (RFI) to telephones in the vicinity of a commercial AM broadcasting station in Bogor.

Such RFI often occurs to telephones located near radio stations, whether the stations be commercial, police, military, or amateur. In the case of the latter three, the radio transmission are intermittent and therefore not as noticeable as from a commercial station.

The telephone distribution lines and the service line into the house act as an antenna, picking up the radio signal. Modern telephones use an equalizer network to reduce crosstalk and shape the audio for best voice transmission. This network may have a pair of voltage-sensitive diodes that detect and rectify the radio signal similar to the detector and conversion circuits of a radio.

Telephone RFI can also be caused by corroded connections, loose connections, abandoned service lines and unterminated loops to other rooms in the house. It should be the responsibility of the owner of the telephone (instrument) be it telephone company or the homeowner--to make corrections to the instrument to eliminate/reduce the RFI.

Telephone RFI should be handled in two ways. Prevent the radio signal from entering the phone, and prevent it from being rectified. First, check the connections and for unterminated loops. The use of "ferrite beads" (which may be available from the electronics supply houses in Glodok) slipped over the telephone line will prevent RFI entering the instrument. If ferrite beads are not available, a 680 mH choke should be installed in each wire (of the two) in the telephone line--inside the instrument. A 0.01 mfd capacitor should also be placed across the terminals of the instrument's microphone.

JACK SPROAT

Semarang,

Central Java