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Telephone Interference Susceptibility Elimination (RFI)

by Phineas J. Icenbice, Jr. W6BF, Sec. Mgr. for Los Angeles
IEEE Senior Member #0835751 - -SINCE 1956

Over the past 56 years, I have been called for help about telephone interference. The first call was from the local telephone company in Deep River, Iowa, in the early 1930's. The question to me was could you help us; all of the lights on our switchboard are flashing. I told them yes and I turned off my model T spark coil transmitter.

Modern telephone systems and transmitters are different and you do not need to turn off your transmitter.

We know how to separate RF from AUDIO and we know how to design and use filters. Two types of components are used to cure 98% of all problems encountered to date. The magic components are:

Ten cent ceramic disk capacitors, and
proper ferrites (toroids).

The capacitors are to filter, or short circuit the differential mode coupling and the ferrite material is used to provide a high impedance to the common mode.

How, What and Where:

Select the correct components!

CAPACITORS: Select 600 to 1000 volt Ceramic +/- 20% 0.005 µfd (for mid-range HF)
FERRITES: Toroids, Rods or Bars - A typical selection for the core material is mix #43 but many other types specified for your frequencies are usable. Core material #43 with a µ=850 with an inside diameter of about 3/4" does provide about 6.0 millihenrys of inductance for ten turns.

At each telephone instrument solder a ceramic capacitor from the red wire to green wire. Insert the Toroid (correct ferrite) in each telephone line (use about ten turns) The exact physical position of the Toroid for best aesthetics may be near the wall outlet. The Toroid filter may be mounted behind or inside the wall box when the lady of the house is really concerned about the aesthetics.

Locate the telephone terminal box, the lightning arrestor, the red and green wires. This is where you should install a filter. This filter can take many forms. The simple case is one ceramic ( 0.005µfd) from the red to Ground and another Ceramic (0.005µfd) from green to Ground. You can add a ten turn ferrite filter in the telephone terminal box to further reduce the RF coupling to other non linear devices.
Phineas J. Icenbice Jr, W6BF, Section Manager

19323 Halsted St
Northridge, CA 91324
818-349-3186
e-mail: w6bf@arrl.org

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14 September 2001 maintained by KK6WO kk6wo@arrl.net
http://www.qsl.net/arrlsw/lax/tele.html

Phineas J. Icenbice Jr, W6BF, Section Manager
19323 Halsted St Northridge, CA 91324	818-349-3186 e-mail: w6bf@arrl.org