Telephone Filter Instructions

Modern electronic telephones are potentially susceptible to radio-frequency interference [RFI] because they contain many silicon diodes which act as crystal rectifiers. The crystal rectifiers convert inaudible, RF energy into audio-frequency energy which can be heard in the earphone. This turns the telephone into a crystal-set/radio-receiver in the presence of moderate to strong RF signals. Electronic telephones also contain transistors which can amplify RF signals. This increases the RF-sensitivity of the telephone. The antenna for this unwitting radio receiver is the telephone wiring in the walls and attic of the building where the phone is located. The net effect is that people who live near an AM radio station may hear music over their telephones. People who live near airports may hear pilots talking with the control tower. Those who live near an amateur radio station may hear garbled speech in their telephones. A properly engineered telephone will not allow RF-energy to enter the telephone circuits which contain the components that act as crystal rectifiers. Including an RF-energy filter in the design of a telephone would increase the retail price of the telephone by less than $1. Just before he retired, Senator Barry Goldwater authored an act [PL-273] that would have taken care of the problem at its source, the manufacturer. Telephone manufacturers don't want the Federal Government telling them how to design their products. So far, lobbying pre$$ure on Congress from telephone manufacturers has prevented the FCC from acting in the public interest and implementing the Goldwater Act. Ironically, a more trouble­free product would benefit the manufacturer. Hubris is a terrible thing. Currently, the FCC is trying to gain authorization to make telephone manufacturers label the RF­susceptibility of their products. Naturally, the telephone manufacturers' lobby is hard at work trying to thwart the FCC.

There is considerable variation in RF susceptibility between different models of telephones from the same manufacturer. I don't know of any brand that is RF-proof without adding an RF­filter or filters. Unfortunately, a few models of telephones are very susceptible to RFI and are sometimes virtually impossible to RF-proof with an external RF­filter. If you recently purchased a highly RF-susceptible telephone, and you saved the receipt and all of the packing material, don't hesitate to return the telephone. Tell the store manager that telephones are not supposed to pick up radio signals-and ask for a full refund. If the store manager tells you that the interference is the fault of the radio station, tell him that he needs to talk to an FCC engineer. This is the only way that the manufacturers will get the message.

Radio Shack® has a desk/wall-phone that is much more RF-resistant than the average telephone. The current price is around $30. In many cases, they can be operated without a filter if minor interference can be tolerated. With a RF­filter, they are often RF-proof.

RF­filter Installation

A telephone RF­filter will perform best if it is placed inside the telephone, close to the modular input jack. The input and output ends of the filter should not be placed next to each other. They need to be separated or the filter's ability to attenuate common-mode RF will be reduced.

When opening a telephone case, it is advisable to place the push-button side of the phone down on a table so that the buttons will not fall out when the case is opened up. If the telephone's input modular socket is wired to a printed circuit board, it will be necessary to cut the two traces on the circuit board. The filter may be installed on the foil side of the board. The filter may be mounted on the component side of the board by drilling #55 - 60 holes near the cut traces. Mechanically, mounting the filter on the component side of the board is best. Electrically, it makes no difference.

If you want to install the RF­filter outside the telephone, the filter can be inserted into the modular cord between the phone and the wall outlet. To identify the polarity of the wires, mark one side of the cord at the area where it will be cut. Make the cut about 6-inches from the modular plug at the telephone end and solder the filter in series with the two ends, taking care not to reverse the polarity. Exposed conductors should be covered with plastic tape or shrink tubing.

Some pulse-dialing electronic-telephones will not tolerate much capacitance across the telephone line. In such cases, one or more of the shunt capacitors across the telephone line end of the filter can be eliminated.

Wall-mounted telephones can often be RF-proofed by installing a RF­filter in the wall outlet.

The pair of 470µH inductances {the components with the green-body and axial leads} work on common-mode RF. Sometimes, it's necessary to use a 2-section filter to increase the attenuation of common-mode RF. To do this, connect two inductors in series, per side, with a third capacitor across their midpoints, as shown below. If a 2-section filter will not fix the problem, the telephone may be acting like a self-contained RF-detector. To completely eliminate RFI, such telephones can be placed inside a shielded metal box, along with the person using the telephone. This is not very practicable, so the best solution may be to discard the telephone. One way to test for such a telephone is to see if it can detect the presence of RF without being plugged in! If this is the case, the problem is self contained and an external RF­ filter is not likely to help. Reportedly, a wooden stake and a hammer cures the problem every time.

Telephone answering machines and portable telephone base units may also require a ferrite split-core RF­filter choke on their power cords. This is done by wrapping at least 4­turns of the power cord on the core. More turns are usually better. Ferrite split-core chokes are sometimes useful as an external helper-filter for a telephone that still has a small amount of RFI after the internal RF­filter has been installed. Ferrite split core chokes {two per package} are available from Radio Shack® as Snap-On Chokes, p-n 273-104.

Since it is possible for one RF-sensitive telephone to cause secondary interference to all of the other telephones on the same line, it is advisable to test each telephone individually for RF-susceptibility with all of the other telephones unplugged from the line. This approach will help sort out the telephones that need help from the ones that are OK. This test should be repeated on each offending telephone after RF filtering is installed.

Princess and TrimLine telephones are usually more difficult to RF-proof because the ringer is in the base unit, the dialing circuit is in the handset, and they are connected by the coil-cord which can act as a loading-coil/antenna. Thus, it may be necessary to install a filter on the telephone line input wires near the modular socket on the base unit, and install another filter in the handset. Desk type telephones, in which the dialing and ringer circuits are in the same enclosure, are usually easier to RF-proof.

Loosely twisted telephone wire splices in the wall or attic can contribute to RFI. The fix is to solder the splices or coat them with silver conductive paint.

If you have a telephone that receives interference on a particular band, even with a 2-section filter, the telephone may be at a RF-voltage-maximum in the telephone wiring. It may help to RF-ground the telephone wires at the wall outlet with a pair of 50pF to 300pF equal-value capacitors. The capacitors are for tuning out the inductive-reactance of the ground lead on the troublesome band. The optimum number of pF must be found experimentally. The best RF-ground is NOT a ground rod driven vertically into the soil. This is the case because HF energy can not penetrate more than a few inches into the earth. A better RF-ground is a horizontal conductor that is on or very near to the surface. All ground system connections that are subject to moisture should be soldered with 5% silver/95% tin solder. Often, an elevated 0.2­wavelength insulated counterpoise makes the most effective RF­ground. However, if such a counterpoise is connected to a ground rod at its far end, it becomes a high-Z RF choke.

Dealing with Sociopaths

Approximately 5% of the U.S. population are sociopathic [One who is affected with a personality disorder marked by aggressive, antisocial behavior.]. Trying to help clear up a telephone interference problem with such a person can be trying. In some cases, such individuals have a secret agenda of not wanting to have the problem fixed. They would rather have something to fight about. Thus, one must be careful not to display anger while dealing with such a person. Try to concentrate on fixing the interference problem. Such persons are often skillful baiters. It is imperative that you not take the bait. When dealing with a sociopath, it may be helpful to have a telephone that you RF-proofed. With such a telephone, you can conduct a convincing A-B comparison test with the complainant's telephone.

------ The ARRL has an information packet on telephone RFI. 203-666-1541

Rich, AG6K-