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