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Basic steps toward eliminating telephone RFI

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Many radio amateurs live with telephone interference or "solve" it by going off the air. Don't be one of them! The solution may be simpler than you think.

Telephone radio-frequency interference (RFI), the interception of radio signals by telephones, can be every bit as irritating as television interference (TVI). Telephone RFI may become more than just a bother if it interrupts an emergency phone call.

Many hams seem to believe that telephone RFI is unsolvable. Don't believe it! Every case of telephone RFI can be solved somehow - and I don't mean by going off the air! This article discusses telephone-RFI basics and basic steps you can take toward curing telephone RFI.

Factors in telephone RFI

Telephone RFI may vary in strength from one phone to another within a house. The strength and nature of interference depend on a number of factors, including the telephone type; your antenna type and its location relative to the phone system; your transmitting mode, frequency and power; and the condition of the telephone-system ground and wiring. Let's examine these factors in more detail.


Telephone-industry deregulation means that more consumers are buying and installing phones than ever before. Home telephone systems may include phones ranging in age from new to several decades old. This means that the wide design differences among a system's phones may cause RF susceptibility to vary from instrument to instrument.(1)

Most new phones provide better telephonic communication than their predecessors. But they also tend to include additional circuitry that renders them more susceptible to RFI than older phones. That circuitry - Touch-Tone pads, LCDs, LEDs, memories, clocks and calendars, lighted panels, audio amplifiers with speakers, electronic ringers and so on - gives telephone users far greater convenience than possible with their rotarydial, plain-vanilla (or, more in keeping with their standard color, licorice) ancestors.

Added features mean additional solidstate circuitry - circuitry that, like the basic telephone circuitry it enhances, receives its operating power directly from the telephone wiring. As if this isn't enough of an RFI menace, feature-laden phones include additional semiconductors - diodes and metal-oxide varistors (MOVs) - to protect the devices that provide added features!

As is all too common in consumer electronics, new telephones usually include little or no designed-in protection against RF fields. Home-brew attempts at RF-proofing new phones with old-phone anti-RFI techniques, such as brute-force application of bypass capacitors in the phone itself, may reduce audio signal levels and ringer current, and disrupt the special features you may have bought the phone for in the first place!

We can conclude that the state of the home-telephone art is unfavorable from an Amateur Radio viewpoint. Nonetheless, curing telephone RFI is not a lost cause. We just need to keep RF from reaching the telephone.

Does your antenna system promote telephone RFI?

Antenna placement can be a major factor in telephone RFI. Generally, the closer your antenna is to your house, the greater the likelihood your telephone(s) will suffer telephone interference. Those of us who install antennas near, on or in our houses create our own devil, so to speak.

By definition, transmission lines are supposed to radiate relatively little RF energy. Feed lines can radiate, however: Coax may support outside-of-braid currents, and two-wire (ladder, twin-lead or open-wire) line may radiate if the currents in its wires are unbalanced. Such radiation can induce RF current in telephone wiring. Probe your transmission line with a field-strength meter if you suspect feeder radiation. Outside-of-shield current flow on coax can be cured with shield-choke baluns.(a) You may need to modify your antenna system for better balance if open-wire-feeder radiation is a problem.(b) It's safe to say, however, that telephone RFI is usually not caused by feed-line radiation. Although antenna-radiated RF is usually the culprit - as you would expect - this article will not suggest reducing transmitter power as a telephone-RFI "fix"!

Your phone wiring: one big antenna

Even a relatively small home may contain hundreds of feet of telephone wire. We must expect that telephone wiring, bathed in the strong radio field of a nearby transmitting antenna, intercepts enough of that energy to conduct significant RF current in addition to telephone-related audio and dc. The differences in phone-system configuration from one house to another can explain why your neighbor's telephone RFI may be stronger or weaker than yours. Reducing phone-system wiring's ability to induce and conduct RF currents is most of the telephone-RFI battle. - WA8KZH

  1. L. Wolfgang and C. Hutchinson, eds, The ARRL Handbook for Radio Amateurs, 1991 ed (Newington: ARRL, 1990), pp 16-9 and 16-10.
  2. For antenna-system aspects that may affect feeder balance, see D. Newkirk, "Why Worry About Feeder Balance, Anyway?," the third sidebar in Z. Lau, "A Relative RF Ammeter for Open-Wire Lines," QST, Oct 1988, pp 15-17, 20.

"Telephone RFI" that isn't

Some telephones, especially those with old-fashioned electromechanical bell ringers, can detect and demodulate the changing electromagnetic field that surrounds power transformers and motors. This form of foreign-electromagneticfield telephone interference sounds like a rough ac hum and can affect all phones in the house. Moving the affected phone(s) a few feet farther from the equipment responsible usually solves the problem.

Home phone wiring commonly runs physically close to ac power wiring. Like motors and transformers, ac lines produce electromagnetic fields that vary in strength with current flow. The text describes conditions under which such fields can couple into phone lines and can cause hum that varies with line current. The differing duty cycles of the appliances concerned may make such hum seem random or, at worst, continuous.

Keying or voice-modulating an amateur transmitter, however, can superimpose an intelligent pattern on foreign-electromagnetic-field hum - even if the transmitter feeds a dummy antenna - because a transmitter's ac-power demand generally varies with modulation. The source of the resulting intelligent hum modulation may be easily identifiable to nearly anyone using a telephone!

Is your Amateur Radio equipment at fault in such a case? No - and the hum isn't caused by RFI. That's why your first step in solving a seemingly RF-related telephone-hum problem should be to check if the hum still occurs when the transmitter feeds a well-shielded dummy antenna. If the hum persists whether or not you use a radiating antenna, RFI is almost certainly not the cause.' - WA8KZH

* It's remotely possible that RF leaked into an ac line by a transmitter might couple into a phone line. You can disprove the existence of this unlikely effect by confirming the absence of ac-cord RF with a field-strength meter while the transmitter is generating RF. - Ed.

Be sure it's really RF interference

Telephone audio has never been noted for high fidelity or outstanding signal-to-noise ratio. We're all accustomed to hearing random clicks, pops and hums when we're on the phone. Your house's telephone wiring may cause some or all of these noises in the absence of RF.

Hams often refer to telephonic communication in general by the name of part of its hardware: twisted pair. The pair is telephone-industry jargon for the two wires - the talk circuit - in a one-phone-line telephone cable that actually carries the telephone signals. The pair's two wires are labeled tip and ring - names that date from the days when switchboard operators depended on three-conductor-phone-plugequipped patch cords for call routing. Single-line telephone-wire color coding calls for the relationship ring = red, tip = green, and ground = black - but phone systems are as likely to violate these standards as not.

Ideally, the twisted pair actually is twisted, because close-spaced, twisted wires offer protection against external (foreign) electromagnetic fields (EMF in telephone jargon, not to be confused with the EMF [electromotive force] of physics and electronics).(2) Untwisted telephone wire - twisted wire that's lost its twist, or consumer-installed untwisted wire - can allow ac wiring and appliances (especially those that draw considerable ac power) to induce hum and noise into a phone system. Foreign electromagnetic fields are the likely culprit when telephone hum changes loudness or timbre as appliances cycle on and off, or as you move an affected phone or its handset. So, if you hear hum in your telephone(s) when you're not on the air, RFI - at least from your station - isn't the cause. (Foreign electromagnetic fields can be responsible for one type of hum that occurs only when you are transmitting. See the sidebar "`Telephone RFI' That Isn't.") To eliminate such noise, you may have to find and replace untwisted wire, or correct the installation fault(s) otherwise responsible.

Some telephone-wiring errors do not stop telephones from operating. Provided the pair reaches a telephone unbroken, that phone's users may never be aware of improper or deteriorating circuit conditions. The conditions most likely to promote foreign-electromagnetic-field phone interference are:

If you suspect interference from foreign electromagnetic fields, check the physical integrity of as much of the system as you can. Starting at the system protector (a terminal block, usually enclosed in a removable box, containing a ground connection and carbon fuse to protect the house wiring and phones from lightning damage) at the house end of the drop (the pole-tohouse telephone cable), move to the service entry/interface and continue to the connector block.(3) Then check all accessible phone-cable runs and the conditions inside each phone jack. Be suspicious of any physically weak or corroded connections, the sudden appearance of unknown wires in the circuit, and any conductors that appear to be something other than telephone wire.

A good ground - or the lack thereof

The same principles we radio amateurs learn about grounding our radio gear are just as valid with home telephone equipment. Verify that your telephone-system ground is sound. Correctly installed telephone wiring uses the power-company ground, or a known-to-be-conductive coldwater-pipe ground, at the point where the drop connects to the house. Look for the ground wire outside (at the protector box) or inside (at the service entry). If you see evidence of an improper ground installation, contact the your telephone company. Local phone companies will generally correct ground-installation errors at their expense. (If you're unsure about who pays for a particular phone-system repair procedure, ask. If you think you're sure, ask anyway.)

Once you've confirmed that the system ground connection is good, check for ground continuity within the system's wires and jacks. Too often, amateur investigators assume that the original wiring is correct and grounding is adequate. Take the time to verify for yourself! Proper telephone wiring includes a continuous, uninterrupted conductor from the system ground to each telephone jack in the house.

Residential-system telephone wiring generally relies on one of two common interconnection methods (Fig 1). One of these, straight cable runs, depends on dedicated cables that radiate from the service- entry area to each telephone jack. You can identify such a system by confirming that the number of cables leaving the servicearea connector block equals the number of telephone jacks in the house. In the other method, loop series wiring, a single cable from the connector block connects to the nearest phone jack and continues to each of the system's phones in turn. Excepting the jack where the wires terminate, wires enter and leave each jack location.

Fig 1
Fig 1 - Residential telephone systems usually depend on straight cable runs (A) or loop series wiring (B) for phone interconnection. Your system connects to the telephone company via the pole-to-house drop (which may run underground in some installations); a grounded, fused protector that minimizes lightning damage to your wiring and telephones, and the service entry - the terminal block at which the phone wires actually enter your house. This drawing shows possible insertion sites (X) for the RF filters discussed in this article; not shown is the ground wire that goes from jack to jack in a properly installed system. Tip and ring are phone-company nomenclature for the talk-circuit wires. See text.

Open each phone jack and check to see that the ground wire is connected to the jack. In loop circuits, a typical wiring error made by installers is to connect the pair and ground wires to the first jack, and to continue the pair, but not the ground, to the remaining jacks. This deficiency may not be apparent until RF is present.

If you find this condition, correct it by connecting the ground wire that leaves the jack to the jack ground terminal and the ground wire that enters. Do this with every jack that's wired incorrectly.

While you're at it: Good telephone-wiring practice calls for grounding all unused conductors in active phone cables. This is easy to do at the service entry. Simply locate all unterminated wires and attach them to the system ground. Exception: In a straight-cable-run system, entirely disconnect, rather than ground, all wires of cables to unused jacks.

Phone-line filtering

If the interference remains after you've ensured your phone system's physical integrity, RF currents - usually, common-mode currents (Fig 2) - exist in the system. Your next job is to reduce or stop that RF flow with filters.

Fig 2
Fig 2 - Audio, control and power signal energy appears in phone wiring as differential-mode signals - signals impressed "across" the Iwo wires of the pair. At any instant, differential-mode currents in the wires flow in opposite directions, with the pair serving as source and return. (The system ground wire - not shown here - is for protective purposes and does not serve as a return for talk-circuit signals.)
RF energy travels down the pair in common mode, in which both of the pair's wires act as a single wire. Common-mode RF signals may return to ground via multiple paths, or they may not need a ground return at all if the system wiring acts as a balanced antenna.

Ferrite-core chokes work well in reducing common-mode HF-RF current flow on telephone wiring.(4) Properly designed, they are transparent to telephone operation, and any number of such chokes can be inserted into telephone wiring with no adverse effect on telephone performance.

Fig 3 shows a bifilar choke with inductive characteristics appropriate for phone-line filtering in the 3- to 30-MHz range. Both of the talk-circuit wires pass through the ferrite core together to block common-mode RF current on the telephone pair. Inserting such a choke at the telephone or telephone jack may solve your interference problem. (I'll refer to these chokes as filters hereafter because they filter out phone-line RF while allowing audio and dc to pass.) Try this and see if the interference diminishes. If not, don't be discouraged. A tower-guy installation technique may help solve the problem.

Fig 3
Fig 3 - Simple RF chokes like this can help stop RF flow on telephone wires. Schematically (A), the choke consists of two 1.7-mH windings (25 turns of no. 30 enameled wire wound over 80% of the diameter of a 1/2-inch OD, mix-75 ferrite core (Amidon FT-50-75A* or Palomar F-50-75**). If necessary, you can use heavier- or lighter-gauge wire; heavier-gauge wire may necessitate the use of a larger-OD (up to 1 inch) core. The author reports that filters of this design have been effective in reducing or eliminating telephone RFI from transmitters operating between 3 and 30 MHz.
B shows how the two windings shown in A are wound together on a single ferrite core. To make construction easier, you can twist the wires together (to a pitch of five to ten twists per inch) and wind them as single wire. To keep track of which winding is which, use different-colored wires or (if you use wire of one color for both windings) use an ohmmeter to check for continuity through each winding.
C shows such a bifilar (two-wire) choke equipped with terminals and connecting wires, and mounted on a board for potting as a commercial filter. The terminals on this filter (the contents of a K-COM RF-2) are available only in bulk quantities; you can use terminal strips (Radio Shack 274-688) instead when building your own filters. A dab of silicone adhesive (such as GE RTV) holds the completed choke to its carrier board. K-COM also sells phone-line filters (model RF-1) equipped with modular phone jacks; contact the author for details.
* Amidon Associates, 2216 E Gladwick St, Dominguez Hills, CA 90220, voice tel 213-763-5770, fax tel 213-763-2250.
** Palomar Engineers, Box 455, Escondido, CA 92033, tel 619-747-3343.

Breaking up the line electrically

If you've gone to the trouble of erecting a high antenna support for a directional antenna, you don't want resonance in and reradiation from the tower guys to distort that hard-won directivity. Breaking the tower guy wires into electrically short, nonresonant-in-ham-bands lengths, each of which is insulated from its neighbor(s) by strain insulators, can cure this. We can do the same thing in a telephone system. Rather than use insulators, we'll discourage RF-current flow in the system with filters.

Assuming that a filter at the telephone or wall jack did not reduce your interference to a sufficiently low level, let's move our attention to the telephone system's front end: the protector or service entry. Install a filter there and recheck the interference level.

If your interference is gone, congratulations! If not, don't give up! Even if a service-entry filter doesn't reduce or cure the RFI, installing such a filter narrows your search by disallowing RF current flow into the house from the service drop. In other words, now you know that in-house telephone wires are picking up the RF. Now, systematically breaking up your house telephone wiring with filters can greatly reduce the system's RF sensitivity.

If your telephone system uses straight cable runs, you can take either of two approaches. The first, and arguably the faster, of the two approaches may involve installing more filters than the minimum necessary to cure the interference. The second (and more methodical) approach minimizes the number of filters installed but may take longer to complete. Study both methods before trying either:

Method 1: Insert a filter at the beginning of each cable run (at the service-entry connector block) and at its terminating phone jack. This approach usually solves all but the severest telephone-RFI cases. Curing stubborn interference may require you to physically break one or more long cable runs to insert additional filters. Consider doing this only after you've filtered both ends of all runs. If you must cut a cable run to insert a filter, be sure that all cable wires, including ground, continue through the filter site. (In other words, don't install new problems as you attempt to fix old ones!)

Method 2: Determine which phone suffers most from interference. (If two or more phones vie for this dubious honor, select one; it doesn't matter which!) We'll call it Phone 1. At the system connector block, disconnect the cables to all phones except Phone 1. Listen for interference in Phone 1. (If you don't hear interference - beginner's luck! - reconnect another phone [Phone 2]. Listen for interference in Phone 2.) At some point - probably when you listen to Phone 1 - you'll hear the interference. When you do, install filters at the connector-block and jack ends of the affected phone's cable run, checking for interference after you install each filter in turn. Again, you may need to cut a cable and install a filter in stubborn cases; be sure to check for results every time you add a filter. Don't install new problems: Be sure that all cable wires, including ground, continue through the filter site. Install no more filters than necessary to cure the interference.

Once you've cleared the affected phone's interference, connect and clear the rest of the system's phones likewise, one by one. As you restore the system to working (and interference-free) order, continually verify that all active phones remain clear of interference. If connecting and clearing an additional phone causes interference in a phone already cured, backtrack and experiment with additional filters until all phones are clear.

If your system uses loop series wiring, insert a filter at each phone jack and at the beginning of the run from the service entry. If you can determine the location of the first jack in the system, install a filter at that point and check results. Installing filters as you move from jack to jack along the system should effect a substantial system-wide improvement.

Concluding thoughts

Many of us don't possess the time or inclination to pursue in-depth telephone-RFI troubleshooting. Neighbors may be unwilling to allow you access to their telephone equipment. If you find yourself stymied in solving telephone RFI, ask your local phone company if they can help. The availability of parts and service for solving telephone interference varies widely from company to company. If anti-RFI parts and service are available from your phone company, ask what they cost and who pays for them before ordering work or parts. Service charges commonly apply to in-house troubleshooting.

Many hams tolerate telephone RFI needlessly, but you needn't be one of them. The basic steps I've discussed in this article should help you reduce or eliminate telephone RFI in most cases.

Safety, telephone troubleshooting and you

Be careful when working with telephone wiring. In the US, 24 to 48 volts dc appears across the telephone pair at all times, and incoming ring signals consist of 20-Hz ac at 90 volts.

Safety first! - WA8KZH


  1. The modular connectors standard on modern telephones and call-answering equipment can allow you to quickly identify RFI entry points in your phone system. Disconnect each telephone from its jack in turn, checking each time to see whether or not the interference has decreased. If it has, you've found a telephone (or wiring) that may need special attention as you work the anti-interference program outlined in this article.
  2. Twisting the wires does this because of phase cancellation: Foreign-electromagnetic-field current induced in the second 180° of a single (360°) twist cancels the current induced in the first 180° of the twist. Result: A net induced-current amplitude of 0.
  3. Photos and drawings of these and other telephone-system components, as well as detailed information on telephone installation, wiring and troubleshooting, appear in G. Lueke and J. Allen, Installing Your Own Telephones, 3rd ed (Richardson, TX: Master Publishing, Inc, 1986). This book is available at Radio Shack stores as catalog no. 62-1390A. - Ed.
  4. Some earlier telephone-RFI writings refer to Bell System items 40BA and 1542A as interference fixes. The 40BA provides capacitive RF bypassing at the service entry. The 1542A, an inductor, is also intended for application at the service entry. Both units perform best at radio frequencies below 3 MHz, and may merit your consideration if your RFI seems to be caused by 160-meter Amateur Radio or standard-broadcast signals. In cases where the 40BA and 1542A were credited with satisfactory suppression of 80- through 10-meter interference, more often than not a modified telephone supplied most of the cure.

WA8KZH, Pete Krieger.