File: rfi-elec.txt Dated: February 2, 1996 ARRL Technical Information Service EMI - Electrical (Electrical Interference) Rev.: November 1993 This information package was prepared as a membership service by the American Radio Relay League, Inc., Technical Information Service, 225 Main St., Newington, CT 06111 (860) 594-0214. Email: tis@arrl.org (Internet). "Reprinted" from: November 1991 QST, "Basic Steps Toward Tracing and Eliminating Power-Line Interference" and September 1992 QST, "A Line Noise Sniffer That Works" Copyright 1993 American Radio Relay League, Inc. All rights reserved. Thank you for requesting the following information from the ARRL Technical Information Service (TIS) or the ARRL Automated Mail Server (info@arrl.org). ARRL HQ is glad to provide this information free of charge as a service to League members and affiliated clubs. For your convenience, you may reproduce this information, electronically or on paper, and distribute it to anyone who needs it, provided that you reproduce it in its entirety and do so free of charge. Note!: This information package provides introductory information only. Additional information on this subject and related topics can be found in back issues of QST and the following books: Radio Frequency Interference: How to Find It and Fix It (# 3754) Interference Handbook (# 6015) The ARRL Handbook for Radio Amateurs (# 1719) Transmitter Hunting: Radio Direction Finding Simplified (# 2701) A bibliography of articles on this topic is also available from ARRL HQ. To request this listing, send a short note with an SASE to the Technical Department Secretary. If you have any questions concerning the reproduction or distribution of this material, please contact: Michael Tracy, KC1SX, TIS Coordinator American Radio Relay League 225 Main St., Newington, CT 06111 (email: mtracy@arrl.org) ---------------------------------------------------------------------------------------------------------------- Basic Steps Toward Tracing and Eliminating Power-Line Interference You don't have to live with power-line interference problems; you can solve them. Here's how to turn that bzzzzzz into a sssshhh! By Max Trescott, K3QM 3405 Ridgemont Dr Mountain View, CA 94040 Power-line noise: You probably heard it before you heard about it. On a radio, it sounds like a hummy, buzzy sizzle or roar that may be punctuated by pops and crackles. It can be weak, strong or anywhere in between. Sometimes its strength varies. Power-line noise may intensify during wet weather. It may be stronger in a particular direction; it may appear to come from everywhere at once. What you've probably heard about power-line noise varies from the overly dismal (there's next to nothing you can do to cure it and keep it cured) to the dangerously wacky (you can locate noisy poles by walking around town with a sledgehammer and portable radio, listening for a change in the noise as you bop each pole's base.) This article explains the causes of power-line noise, how to locate power-line-noise sources, and what to do to make the noise go away and stay away. Power-Line-Noise Types Power-line noise comes in two flavors: corona discharge and spark-gap noise. Corona occurs when the strong electric field associated with a high-voltage conductor ionizes air near the conductor - particularly at sharp edges and projections. The ionized air may glow blue and crackle audibly. Corona also rips apart diatomic oxygen (O2) and produces triatomic oxygen (O3, ozone), a pungent, corrosive gas that destroys power-line hardware and endangers human health. Corona also generates broad spectrum electromagnetic noise. Generally, the higher the voltage involved, the stronger the corona. Corona also increases with humidity and precipitation because these make the air more conductive. Corona-induced power-line noise is generally worst during periods of rain or snow, when precipitation hangs as droplets from the bottom of transmission lines. Trying to locate these sources of foul-weather power-line noise is rarely worth the trouble because little can be done to eliminate them. Spark-gap noise causes most power-line interference. It occurs whenever a spark jumps between two conductors. This occurs when sufficient potential difference exists between the conductors to ionize the air in the gap between them. Ionization decreases the air's resistance. When the air's resistance drops enough to support conduction, a spark jumps the gap and current flows through the ionized air channel. The ionized channel's resistance varies considerably, causing current variations that can be induced into and travel along power lines. The spark also directly radiates noise across a broad frequency spectrum. Under some conditions, sparks can trigger trains of successively weaker pulses. The resulting damped waves contain strong harmonic energy and can cause severe interference well into the VHF region. [1] Spark-gap noise generally weakens with frequency (see reference for Fig 1), [2] a characteristic that can be very useful as you track interference. One exception to this rule occurs when the power lines connected to the noise source resonate at a particular frequency or frequencies. The noise may peak at these frequencies (see reference for Fig 2). Much power-line spark-gap noise occurs when sparks jump across spaces between the hardware pieces used to mount cross-arms, insulators, transformers and other equipment to power poles. In most cases, such noise can be eliminated by the power company. Unlike corona, spark-gap noise is usually a fair-weather phenomena; it may disappear during rain because precipitation short-circuits inter-hardware gaps. Because residential power lines carry 60-Hz ac, the voltage on them passes through two maxima (one positive, one negative) each cycle and through zero twice each cycle - 120 maxima and 120 zero crossings per second. Corona and spark-gap noise follow this pattern, generally starting and stopping 120 times per second. This gives power-line noise its characteristic hum or buzz. It usually appears continuously across a broad frequency range, although it may come and go. --Sidebar: SOURCES OF POWER-LINE SPARK-GAP NOISE Most power-line noise comes from sparks that jump gaps in the hardware used to mount cross-arms and other hardware to power-pole tops. Even if sufficiently tightened at installation time, this hardware can loosen as wood expands and contracts with temperature and humidity changes. Sparking can also occur across the film of corrosion that builds up on power pole hardware as it weathers and ages. Power companies constantly evaluate new hardware that better resists loosening and reduces spark-gap problems. Power line insulators may break down under the aging influences of sun, precipitation, dust and pollution. As a result, carbon tracks form across insulators, causing power-line leakage that may generate intense spark-gap noise. Power utilities combat this by periodically hosing down insulators, and through continuing research in insulator materials with greater breakdown resistance. Perhaps surprisingly, pole transformers rarely generate power-line noise. Investigation usually reveals that noise from a transformer bearing pole comes from sparking between pieces of the hardware used to mount the transformer to the pole. --K3QM --End of Sidebar --- How Power-Line Noise Travels Power-line noise can be transmitted by conduction, induction or radiation. Interference can be conducted through the power lines, and into a receiver power supply. It can travel by induction when the power line carrying the interference is close enough to an antenna or some other part of the receiver circuit to couple the interference into the receiver, or into another power line. It can also travel by radiation: power lines may act as an antenna. Conduction and induction are usually responsible for only lower-frequency interference because the conducted current decreases rapidly with frequency. Above 100 MHz, for example, conducted power-line interference is usually only a problem if the distance between the noise source and the receiver is less than the distance spanned by six to eight power-line poles. [3] Power-line interference in the VHF and UHF ranges almost always occurs because of direct radiation from a spark-gap noise source. Although cases have been documented of objectionable interference traveling up to 30 miles by radiation, most power-line noise sources can usually be found within one or two miles of the receiver. Radio amateurs are more likely to pick up line noise from farther away because hams generally use better receivers and antennas than the general public. Tracking Interference from Sources Inside Your Home After you've decided to track down the source of spark-gap noise, make sure the interference isn't coming from your house. (Quite a few power-line noise complaints stem from sources within complainants' homes!) The easiest way to check this is to turn off your house's power at its entrance panel. Before you cut the power, get a battery-powered receiver - a portable AM radio should work just fine - and tune in the interference. If the noise goes away as you cut the power, the noise source almost certainly exists in your own house and is not a power-company problem. If cutting the power does not stop the noise, turn the power back on and skip to "Tracking Interference from Sources Outside Your Home." If cutting the power reveals that the noise comes from an in-house source, turn the power back on and confirm that you can once again hear the noise in your monitor receiver. Then turn the main power off again. Next, turn off all the individual house circuits. (Keeping the MAIN switch off while you do this is safer than doing it with the MAIN switch turned on.) Then turn the MAIN switch back on. Next, isolate the noise-generating circuit(s) by turning the individual house circuits on one at a time. When you activate one that brings back the noise, turn it off again and note that circuit. Work through the remaining circuits likewise to determine if any more are noisy. Once you've worked through the entire service panel, turn everything back on except the noisy circuit(s). You should hear no line noise in your monitor receiver. Turning any suspect circuit(s) back on should make the noise return. After you've determined the circuit(s) involved, identify which room(s) that circuit powers. Once you've identified the room(s) involved, identifying the offending appliance(s) is easy. Unplug--don't just turn off!--all appliances, and switch off all lights, in the suspect room(s). [4] Plug them back in and switch them back on one at a time. When you activate one that brings back the noise, turn it off and note that appliance or light. Work through the remaining appliances and lights likewise to determine if any more are noisy. Once you've worked through everything in the room(s) served by the noisy power circuit(s), power everything back up except the noisy circuit(s). You should hear no line noise in your monitor receiver. Powering up any suspect lights or appliances should make the noise return. The sidebar "Possible Line Noise Sources in the Home" lists some of the more likely suspects. --Sidebar: Possible Line-Noise Sources in the Home Determining that power-line noise comes from somewhere inside your house is one thing; finding the device responsible is another. Here are just a few of the common household devices that can cause power-line noise when functioning normally: electric motors, including those in saws, vacuum cleaners, garbage disposals, hair dryers, clocks, water pumps, furnace blowers, microwave ovens, model cars and trains, air conditioners, sewing machines, shavers, mixers and blenders, thermostats, including those in air heating and cooling systems, electric blankets, and water and fish-tank heaters, air ionizers, purifiers, humidifiers and dehumidifiers, fluorescent lights, TVs, computers and TV games, TV booster amplifiers, doorbell transformers, neon signs, light-dimmer switches, scanners, gas- and oil-appliance igniters. Anything that uses ac power may generate power-line interference if it malfunctions. --K3QM --End of Sidebar If you can't locate the faulty appliance, track down the interference source by carrying the receiver around the house to find where the noise is strongest. You may be able to save time by using the receiver's medium-wave antenna directivity to get an idea of the noise source's direction (see the sidebar, "Direction-Finding with a Portable Radio"). --Sidebar - Direction-Finding with a Portable Radio Use a portable radio, preferably one that covers AM, FM and shortwave, as your primary tool for locating noise sources. Starting at the lowest frequency receivable on the radio, find a clear spot on the dial and listen for the noise. Switch to higher-frequency bands until you determine the highest band on which you can still hear the noise. If you have a choice, listen in AM or an SSB mode. FM detectors generally reject or greatly suppress AM noise, which power-line noise is, so avoid listening in the FM mode if you can. * A directional receiving antenna is helpful, though not essential. Most AM broadcast-band receivers contain a ferrite-bar-based antenna, and such antennas are very directional. (You can determine the alignment of your set's antenna if you know the location of a local AM station's antenna relative to you. Tune in the station and rotate the radio for maximum signal strength, and note which of the radio's dimensions [front or side] points toward the station. This same alignment will prevail when you rotate the radio to maximize the sound of the noise you're tracking, and will give you an indication of direction in which the noise source is located. [1]) Most AM/FM/SW portables use an extendable whip antenna for shortwave and FM. Although these antennas are omnidirectional when vertical, they. can be somewhat directional when horizontal. To use this characteristic, turn the radio on its side so that its antenna is horizontal, hold the radio close to your chest, and point the antenna in front of you. Turn slowly until the noise signal peaks. The noise source should be directly in front of you. --K3QM * Pinpointing a noise source is easier at VHF, but most AM/FM/SW radios, and 2-M Amateur Radio gear, afford only VHF FM reception. Snag: FM radios don't receive line noise (a form of AM) well. Unsnag: An air-band (108 to 136 MHz) receiver may help you end run this problem, because air-band stations use AM to avoid FM's capture effect suppression of the weaker signal[s] when two or more stations transmit simultaneously. In the August 1991 Potomac Valley Radio Club Newsletter's "A Line Noise 'Sniffer' That Works," Bill Leavitt, W3AZ, describes his success in using a "$20 aircraft radio receiver that Radio Shack makes" (apparently the Jetstream AM/VHF-Air, #12-601). Leavitt cautions that the radio's relative unshieldedness requires that it be mounted in a metal box with its whip antenna removed to allow the attachment of a Yagi for directional reception, and that switchable input attenuation is necessary to keep the set's AGC from broadening peaks and nulls. --Ed [1] - Because such antennas are bi-directional, taking a single directional bearing with an AM set's antenna can't tell you whether a noise source is behind or ahead of you. So, when tracking noise, move a hundred yards or so and take another bearing. Imagine where the bearing lines intersect to determine the approximate location of the noise source. --Ed --End of Sidebar Even with everything in a powered circuit turned off and unplugged, it's possible - though highly unlikely - that faulty wiring may continue to power the noise source. For example, a lighting switch installed in the fixture neutral lead (incorrect and dangerous wiring) instead of its hot lead may allow a hot-to-ground gap in the fixture to receive ac even with the switch turned off (see reference for Fig 3). Tracking Interference from Sources Outside Your Home If your investigation suggests that the line noise probably comes from a source outside your house, quieting the noise will involve help from the power company. Many power companies have field-investigation units that track interference sources. There's a stronger incentive for doing such work than helping power customers achieve quiet radio or TV reception: Fixing spark-gap problems early on can keep more serious problems, such as burned poles, from developing later on. In considering calling the power company for help, consider these alternatives: (1) stop tracking the noise and call the power company to ask for their help in finding the noise source and (2) investigate further to find out more about the noise before calling the power company. Alternative 1 requires less work on your part (and is your prerogative as a power-company customer, of course). Alternative 2 is arguably the better of the two, though, because the more you can tell the power company about the noise and its possible source, the faster they'll be able to fix the problem. (Telling your power company that strong spark-gap interference emanates from, say, "Pole 2156" gets a repair crew off to a considerably better start than a vague "I'm getting some interference" call.) This is especially so in cases of intermittent interference interference that, according to Murphy's Law, (naturally) falls frustratingly silent when the repair crew arrives and interference that does not occur during regular business hours. (Power crews can track noise during off hours, but may need management approval to do so.) Tracking interference can also lead to self-improvement: It hones your direction-finding skills, valuable in foxhunting or tracking a jammer. Whether or not you've located the interference source, the power-company personnel who answer your call will probably ask you questions about the interference. What kind of device is affected? What does the interference look and/or sound like? When does it occur? At what frequency? Be prepared to give as much information as you can, because the person recording the information may be merely completing a form and may know nothing about radio interference. Also be ready to provide a daytime phone number so a complaint investigator can follow up with questions. You may also be asked if you're willing to demonstrate the interference. Tracking Power-Line Interference Determine the general direction from which the interference comes. If your antenna is directional, rotate it to peak the noise. Naturally, the narrower your antenna's beam width, the better, but even an antenna with relatively broad directivity (such as an HF tribander) can provide a useful bearing. Rotate the antenna 180 degrees from the noise peak and check again to see if the signal is louder from that direction. It's entirely possible to get a strong reading off the back of a beam and start searching in the wrong direction! (If your antenna isn't directional, or if your directional antenna is not rotatable, don't despair. You may still successfully locate the noise; keep reading!) Tune in the noise on your portable radio. At first, you may not be able to hear the noise on the portable, especially if you originally located the interference using your ham-station gear and a high-gain, directional antenna atop a tall support. If the portable's whip or directional AM antenna doesn't let you hear the noise at ground level, try enhancing reception with a small, portable Yagi (such as a three- or four element 2-M antenna). If you don't have such an antenna, or you still can't hear the noise despite receiving system enhancements, follow the power lines in your area in all directions until you start to hear it. Most power-line-noise sources cause interference only within a mile or two, so you should be able to pick up the noise signal relatively quickly. But expect to get some exercise! If you consider driving instead of walking, keep in mind that a car may be more of a liability than an asset if it generates electrical noise that's stronger than the noise you're tracking. Having to shut off a car while taking noise-strength readings may be such a hassle that tracking the noise on foot may be easier and faster if your receiving setup can be carried easily. Once you pick up the noise on your portable radio, keep following the power lines. Periodically switch to higher receive frequencies to see if the noise is now audible at higher frequencies. Because spark-gap noise strength decreases with frequency, you should be able to hear the noise at increasingly higher frequencies as you near its source. An increasing line-noise S-meter reading at a single frequency may indicate that you're getting closer to the source, but can be misleading because conducted noise is generally stronger at corner and transformer poles. [5] --Sidebar: Don't Bang on Power Poles! Ham folklore holds that banging hard on suspected power poles can sometimes fix power-line noise. This is wrong: Although it may momentarily stop the noise or change it, such "tapping" doesn't actually improve anything that can go wrong on a pole. The noise always returns! Banging on power poles is dangerous: It can damage a pole, make a pole-hardware problem worse or even knock out power! And banging on power poles is illegal: Power-company hardware is neither your property nor your responsibility. Tracking power-line noise is one area where power company and ham expertise overlap, and most power companies will welcome your direction-finding input. But when the time comes to evaluate and repair power-company hardware, leave the job to the experts. Don't hit -or climb!- power-company property. --K3QM --End of Sidebar If you can hear the noise at 2 M, a small, four-element Yagi antenna can be very helpful in locating the noise source because it is sensitive to polarization in addition to being azimuthally directional. A Yagi noise receive antenna usually produces the loudest noise signal when held horizontally. When you're within 100 feet or so of the noise source, however, vertical polarization may provide a stronger signal. Thus, be sure to shift antenna polarization at intervals to determine which produces the strongest noise. As you get extremely close to the noise source, you may notice the noise peak in strength, and then decrease as you pass a particular power pole or wire span. VHF reception with a Yagi is particularly useful in such cases because sharp antenna directivity can help you determine exactly which pole generates the noise. Try sweeping the beam vertically. You may discover that the signal is loudest with the antenna pointed at the top of a particular pole. Time and effort spent localizing the noise is worthwhile even if you can't identify a specific pole as the source. In addition to being grateful that you've narrowed the search to a particular area, your power company's repair crew will be able to find and fix the problem all the faster. Although you'll probably find your local power company quite responsive to your request for help, don't expect them to fix the problem immediately. A few weeks may pass before a crew can be assigned to quiet even a well-defined noise source - perhaps longer if the noise source has yet to be identified. So, don't wait until the week of an important contest before calling the power company for help! (The sidebar "If the Power Company is Unresponsive" suggests what to do if your diplomatic requests for help meet with little or no response.) Sidebar --If the Power Company is Unresponsive Friendly, intelligent interaction between you and your local power company is one more vote for good relations between power utilities and Amateur Radio as a whole. In the unlikely event that your local power company doesn't respond positively to your request for help, you can file a complaint with your state public utilities commission. Such commissions, usually based in state capitals, mediate consumer complaints in addition to reviewing and approving the utility-rate structures that determine how much we pay for electricity. Recourse to your public-utilities commission is a last resort that you should take only after you've done your best to work productively with your local power company. --K3OM --End of sidebar After They Fix the Problem When you report a power-line-noise problem to the local power company, ask them to notify you when they solve it. (Of course, if the line noise is severe, you'll probably know anyway!) After you're notified that the problem has been fixed, check to be sure the interference is gone. In most cases, the noise will be totally gone. You may still hear noise, however, for a couple of reasons. First, the original source may still be generating some noise because the power company may be unable to totally eliminate it. Don't assume that this is the case, however. If you still hear noise, contact the power company again. Another possibility is that an additional noise source, now no longer masked by noise from the cured source, may be audible. Again, if you consider the noise a problem, contact the power company. You may not be able to eliminate all the line noise you can hear, and may opt instead to go after only the most severe problems. In my case, I eliminated two major, fairly constant S8 noise sources. Since then, I've heard what I believe is power-line noise occasionally peaking at S6, from several other directions. These sources are usually considerably weaker and disappear altogether in the evening. If they become severe - and therefore worth the effort to find them - I'll be out tracking them down! In the meantime, it's comforting to know that something can be done about power-line noise and that, in most cases, it can be fixed! Special thanks to Deep Gupta, N6XHY, of the Electric Power Research Institute, and Wally Hanifin of Pacific Gas and Electric, for providing input and reviewing this article. (Fig 1 caption) - Spark-generated interference generally decreases in strength with rising frequency. The text describes how this characteristic can help you localize an interference source. (Fig 2 caption) - Spark-generated interference may not generally decrease in strength with rising frequency when power lines associated with the noise source resonate and peak the noise at one or more frequencies. (Fig 3 caption) - Correct light-switch wiring interrupts the hot connection between the light fixture and the ac line. Incorrect wiring leaves the fixture and much more of the wiring connected to the hot ac wire whether the switch is on or off Turning the switch off in the second case can not quiet line noise caused by hot-to-ground sparking in the light fixture. Notes: [1] - Damped waves weren't always our enemies; we once communicated with them! But the horrendous interference caused by damped wave emission compelled amateur and commercial radio users alike to adopt undamped emission - continuous waves, or CW - over the spark transmission commonly used during radio's infancy. --Ed. [2] - W. Nelson, Interference Handbook (Wilton, CT: Radio Publications Inc, 1981), p 24. [Available as #6015 from The ARRL Bookshelf, this book, written by a power-company investigator with over 30 years of experience, is a must-read for anyone interested in or beset with electrical radio frequency Interference. --Ed ] [3] - T. Gonen, Electrical Power Transmission System Engineering (New York: John Wiley, 1988) [4] - The power switch in some consumer devices disconnects the device circuitry from its power supply while leaving the supply connected to the ac line. Power-supply components can generate noise, so merely turning off such a device leaves a potential noise source connected to the line. --Ed. [5] - Interference Handbook, p 59. See note 2. ========================================================================== A Line Noise Sniffer That Works - Tracking down power leaks by radio is easier with a receiving setup designed for the job. Here's how to build one. By William E. Leavitt, W3AZ 1411 Laurel Dr Accokeek, MD 20607-9611 As mentioned in an editor's note in Max Trescott's "Basic Steps Toward Tracing and Eliminating Power-Line Interference" (November 1991 QST), details of Bill Leavitt's line noise sniffer were reported in the August 1991 Potomac Valley Radio Club Newsletter. Prompted by requests from noise-bedeviled hams nationwide, Bill describes his system for QST. I had a serious power line noise problem. It was more than a minor annoyance; it had reached the point where it was interfering with my normal operation on the 14, 21, 28 and 50-MHz hands. To make matters worse, the noise was intermittent and appeared to come from two directions. I tried to find the source by using an AM broadcast receiver, but I could only determine a general location. My AM radio picked up the standing-wave power-line noise over a substantial distance along the line. In addition, the directional characteristics of the receiver's built-in antenna were insufficient to pinpoint the source. There had to be a better way! I found that the noise source could be defined with greater accuracy at higher frequencies - VHF in particular. Directional antennas are much smaller at VHF as well, making highly directional antennas practical for a hand-carried unit. I began experimenting with various components, and the result is an accurate, inexpensive noise sniffer. With sniffer in hand, it's much easier to locate noise sources. Construction The sniffer consists of three parts: an antenna, an attenuator and a receiver. A block diagram of the sniffer is shown in Fig 1. You'll notice that the antenna is connected to the receiver through an attenuator. The attenuator is very important to the proper operation of the sniffer. When you're in the vicinity of a strong noise source, the receiver AGC attempts to keep the receiver output at a fixed level. Since the AGC tends to smooth out any fluctuations in the signal strength, it's nearly impossible to pinpoint the noise location. By using the attenuator, even a very strong signal can be reduced sufficiently to provide an obvious peak indication when the antenna is aimed at the noise source. The receiver volume control should be set to maximum at all times. A signal-strength meter would be a handy addition to the sniffer, but it isn't necessary. To keep things simple, the sniffer relies on the sound level from the receiver speaker to indicate the maximum-signal direction. The Antenna The sniffer frame uses a piece of 1- x 2-inch lumber (actual dimensions 3/4 x 1-1/2 inches) to support the antenna. A second piece of 2 x 6-inch lumber - in the shape of a rifle stock - is attached to the antenna support. The antenna is a 130-MHz three-element Yagi with 1/4-inch copper refrigeration tubing elements. Copper tubing can be easily straightened as necessary without breaking. Drill holes through the wood boom to accommodate the elements. (Choose a hole size that provides a snug fit for the copper tubing you use.) Insert each element half-way through the boom. I wrapped a small amount of electrical tape around the director and reflector elements on each side of the boom to hold them in place. Epoxy, or another suitable adhesive, can be used if you want a more permanent installation. As shown in Fig 2, the driven element is soldered to a small metal clamp that's fastened to the boom with #6 machine screws. This clamp - obtainable from a plumbing supply store - provides a means to connect the braid of the RG-58 coaxial feed line to the center of the driven element. The driven element uses a simple gamma match. The antenna element dimensions were derived from a scaled W2PV design: [1] Reflector 45 inches Driven element 44-1/16 inches Director 41-1/8 inches The element spacing is as follows: Reflector to driven element 12-1/2 inches Reflector to director 25 inches Make the gamma match from a piece of #12 wire using the dimensions shown in Fig 3. A 50-pF variable is fine for the tuning capacitor. A 25-pF variable should also be adequate. I was fortunate to have a means of adjusting the gamma match using a bridge and a signal generator. If you don't have access to test equipment, I suggest that you build the gamma match using the dimensions shown in Fig 3. Adjust the capacitor for maximum signal strength using a strong noise source. Make sure to use the attenuator to reduce the receiver output as you make the adjustment. If you're in need of something noisy, try your computer, a fluorescent light, television set or anything else that provides a decent signal. You'll find there are plenty of noise sources to choose from! The Attenuator The attenuator (see Fig 4) provides 6, 12 and 24-dB switch settings, allowing you to increase the attenuation in 6-dB steps up to a maximum of 42 dB. A photo of the completed box is shown in Fig 5. The box is a Radio Shack 270-239 and the switches are Radio Shack 275-652 units. The resistors should be 1/4-watt carbon types. A piece of double-sided printed-circuit board is mounted inside the cover and held in place by the switches. A shield partition of double-sided PC board is also soldered between each switch. A 1/4-inch hole in the partitions allows the wire to pass from one switch to the next. The ground side of each resistor is soldered directly to the PC board. The circuit board is connected at each end to lugs immediately beneath the BNC input and output connectors. The enclosure for this attenuator is available in kit form from FAR Circuits. [2] The Receiver An AM receiver must be used for best noise reception. One low-cost receiver in the VHF range is the Radio Shack Jetstream AM/VHF-Air receiver (12-601). Unfortunately, the Jetstream receiver lacks shielding. This is a serious liability when it's used in the vicinity of a strong noise source. To solve the problem, I placed the receiver in a 5-1/4 x 3 x 2-1/8-inch minibox (see Fig 6). Slots are cut into the box for the tuning and volume controls. A cutout is made in the top for the dial and small holes are drilled in the cover where the receiver speaker is located. The width of the receiver had to be reduced slightly - through the gentle application of a belt sander. Remove the headphone jack and the whip antenna. This operation can be difficult and must be done with the greatest care. Route a wire from a ground point on the receiver PC board at the headphone jack to the shield box wall. This serves as the RF ground. Mount a BNC connector on the box wall and attach a lead from the center conductor to the whip-antenna connection point. If a larger box is used, a more elegant installation is possible. The receiver dial can be tuned to a quiet frequency around 130 MHz and the volume control set at the maximum level. The receiver can be secured to the box cover with holes drilled above the speaker area. If you use this approach, bring the positive lead from the receiver battery out to a switch mounted on the box wall. This allows you to easily turn the unit on and off. The ground lead from the box to the receiver can be connected to the outer conductor of the headphone jack. (Keep this lead as short as possible.) A BNC connector can be mounted on the box wall close to the retracted whip antenna. You could connect the center conductor of the BNC connector to the antenna with a short piece of wire, or by using a small clip. Tracking Down the Noise As you begin hunting power-line noise on a regular basis, you'll want to do some background reading to familiarize yourself with the subject. Hams have been grappling with this problem for decades and quite a bit of information is available to help you understand the causes and cures. [3] Before starting your noise hunt, be sure that you've eliminated every possible culprit in your own home. Also, make sure the noise isn't originating from one of your neighbors houses. About 80- 70 % of the noise complaints received by power companies are found to originate from sources within houses - not on power lines. [4] Prior to taking your sniffer into the field, you should determine the direction of the noise using your station equipment. If you're equipped with a beam antenna, you can use it to do this. Point your beam in the direction of the strongest noise. Move the beam until the noise drops to a convenient reference point on your receiver S meter. Note this reading and the beam heading. Now, turn the beam in the opposite direction, past the peak point, until the noise drops off again. Keep turning the beam until the strength of the noise falls to the same S-meter reading you noted before. Once again, note the beam heading. The midpoint of these two headings indicates the approximate direction of the noise source. This technique works only for relatively steady noise sources. Even with a steady source, it's a good idea to perform this measurement a number of times to determine the repeatability of your readings. If there's backlash in your rotator, try to take all your readings while turning the beam in the same direction. Use the main lobe of the beam instead of the sides or back; the sharper the beam, the more accurately you can determine the direction of the noise. In many cases my 6-meter beam has pointed right at the offending power pole! Beams are handy when it comes to getting an initial bearing, but what if you don't own a beam antenna? Make your best guess and use the sniffer. Begin by walking around your yard with the sniffer. --Sidebar: POWER-COMPANY RESPONSIBILITY Who should find and who should fix a power-line interference source? Most of us never consider the question. We just assume any EMI that sounds like arching with a 60 Hz component should be fixed by the power company (let's call it the utility). In fact, much of the noise arriving via power lines comes from appliances attached to those lines in a customer's home. It's important to know whether the EMI is "home made" or "imported". How can we determine which portion of the electric system is our responsibility and which is owned by the local utility? There is a well-defined point at which the change of ownership occurs, but it is defined differently for overhead and underground connections. The definition may also vary somewhat from one utility to the next. First, let's determine the type of drop. Almost all suburban homes built before the early 1970s and virtually all rural homes are supplied from overhead distribution systems. Most urban and suburban multiple occupancy buildings, and suburban single occupancy buildings erected after the early `70s, are supplied from underground electric facilities or drops. If you have an outdoor electric meter, you can determine which type of supply you have by looking at the area above this meter. Is there a cable going up the side of the house to a point either on the house side near your roof line or to a mast attached to your house? Does this cable connect to another electric (not telephone, CATV and such) cable spanning the distance from a nearby utility pole? If so, you are served from an overhead electric distribution system. If you find no overhead cable, but rather one or more pipes or cables that lead directly from the electric-meter box into the ground beneath it, your service is most likely from an underground system. If your electric meter is indoors, examine the outside of the building for an electric cable coming from a nearby pole. Overhead electric-supply cables usually attached to a wall or mast at least 16 feet above ground level and connect to another cable that enters the building. If you don't find an overhead feed as described, the electric supply is probably underground. If, after doing all this, you are uncertain, call your local utility. Their records should show which kind of electric service you have. Overhead Electrical Service On overhead electric service, virtually all US electric utilities identify the point of ownership change as the set of connectors at the upper end of the cable entering the top the meter panel. These splices connect the ends of the customer's service entrance cables to the utility service drop (coming in from the pole). The electric meter is the only utility-company equipment on the customer side of this point. (A few utilities also own the cabinet or meter socket where the meter is mounted.) Underground Electrical Service US electric utilities define the point at which ownership changes on underground service in various ways. Most commonly, this location is one of the following: 1) The point where the cables between the main breaker/fuse panel and the electric meter are connected to the electric meter. 2) The point where the underground cable crosses from public jurisdiction onto private property. This location is generally referred to as the "Property Line." If it becomes necessary to define this point more accurately, contact the local utility for help. The utility is responsible for locating and correcting interference sources on their side of the ownership change point. Responsibility for all such sources on the customer side falls to the customer. --Harry D. Thomas, KA1NH, Windsor, Connecticut (from Radio Frequency Interference: How to Find it and Fix It). --End of Sidebar If you're not receiving the noise, slowly widen your search pattern until you get close enough to the source to get a fix on its direction. With any luck, the noise will have a peculiar sound that you'll be able to separate from anything else you may hear. If the source is distant, you may need a friend to drive you around the neighborhood as you conduct your search. The AM car radio may be helpful in locating the general area of the noise. Start your hunt by checking the closest power line in the direction determined by your beam (or by your initial sniffer sweep), and work outward from there. When you think you've found the location, listen to the noise from various positions and see if the sniffer points to the same spot. Use the attenuator to reduce the signal strength as much as necessary to obtain an accurate fix. In some cases, interference propagates along power lines, making it difficult to identify the exact source. If you think that power-line propagation is causing false readings, try walking directly away from the line at a right angle. At some distance from the line you should be able to get true readings to the source if you're close enough. Now walk parallel to the power line and see if the sniffer still gets its strongest noise from the suspect pole. If so, you've found the source! Be careful of false indications, especially if you move into wooded areas or areas with lots of buildings. At times, noise peaks can be obtained 90 to 180 degrees from the true heading. On one occasion, I found a peak about 150 degrees from the correct direction - at an elevation of about 45 degrees! (An electric tree?) It should be emphasized that my noise hunting method is not a cure-all. It's been proven to work well for 7- and 8-kV lines where arcing causes wide-band interference. In many instances, the power company found that the noisy poles had insulators with numerous burn marks. (These poles were usually silent when it rained.) When a number of adjacent poles are buzzing simultaneously, pole identification becomes more complex. Pinpoint as many as you can and get the loudest ones fixed first. Each pole must be identified and fixed in turn until they are all silenced. Intermittent noise sources can be a real headache to find. Noise sometimes appears and disappears depending on temperature or humidity. Locating a noise source on power lines carrying higher voltages is another challenge. Noise at 130 MHz seems to propagate quite a distance over these lines, making it difficult for the sniffer to obtain a true heading. Dealing With Your Local Power Company It's important to maintain good relations with the power company and not to involve them in false alarms. Take your time and be sure of your findings before picking up the telephone. Show your power company representative how your sniffer works. You'll reinforce your credibility by demonstrating that you really know what you're doing! Before the noise problem is solved, the power company may have to make several visits. This is particularly true when the noise is intermittent and the power-distribution equipment is old. Be patient with the power company. Removing that terrible noise from your receiver is a high priority to you, but it's a low priority to the company. They'll attend to the noise when a crew is free - assuming they don't have to tangle with power outages and other messy problems! Notes [1] - J. Lawson, W2PV, Yagi Antenna Design, Table 8.22, p 8-28. Available from your local dealer or from ARRL HQ. [2] - Enclosures (only) for the power line noise sniffer attenuator kit are available from FAR Circuits, 18N640 Field Ct, Dundee, IL 60118-9269. [3] - W. Nelson, WA6FQG, Interference Handbook. This is the definitive reference for power-line noise interference. WA6FQG was an RFI Investigator for Southern California Edison Company. Available from your local dealer or from ARRL HQ. [4] - If the interference changes with weather, the source is likely to be outdoors. If it regularly changes with the time of day, it's probably an indoor phenomenon related to human activity. One exception was a pole that began buzzing at sunrise and stopped at sunset! First licensed in 1936 as W9YFD, Bill has been a ham ever since. Bill spent 35 years working for The Naval Research Laboratory doing moonbounce research and communication satellite system design. After leaving NRL, Bill was employed by Satellite Business Systems and MCI. He's been enjoying an active retirement at his Maryland home for the past two years. Figure descriptions Fig 1: A block diagram of the power-line noise sniffer. It consists of three components: antenna, attenuator and 130-MHz AM receiver. Fig 2: The driven element is soldered to a small metal clamp fastened to the boom with #6 machine screws. Fig 3: An illustration of the gamma match design. It you don't have the test equipment to tune it properly, use a nearby noise source and adjust the variable capacitor for maximum signal strength. Fig 4: Schematic of the signal attenuator. All resistors are 1/4-W, 5% carbon composition or film; J1, J2 - BNC chassis-mount jacks (Radio Shack 278-105); S1, S2, S3 - DPDT (Radio Shack 275-652). Fig 5: The signal attenuator secured to the sniffer and ready for action. Switches select 6, 12 or 24 dB of attenuation. By using various combinations of switch settings, other attenuation levels can be achieved. For example, by activating the 6- and 12-dB switches, you'll have a total attenuation of 18 dB. Fig 6: The AM aircraft receiver must be mounted in a shielded enclosure (see text). ========================================================================= ADDENDUM - Regulatory Information pertaining to power line interference The power company is responsible for any interference it generates to your station or to anyone's receiving equipment in the area. The following federal regulation applies: Title 47, CFR Part 15.5 15.5 General conditions of operation. (a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration certification of equipment, or, for power-line carrier systems, on the basis of prior notification of use pursuant to section 90.63(g) of this chapter. (b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator. (c) The operator of the radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected. (d) Intentional radiators that produce Class B emissions (damped waves) are prohibited. Section 15.13 of Title 47 of the Code of Federal Regulations states: Manufacturers of these devices shall employ good engineering practices to minimize the risk harmful interference. Additionally, you should inform the General Manager of your power company of its obligation under the above provision in a registered, return receipt letter. Keep a copy of all correspondence. If you hear nothing after a reasonable time (usually 30 days), send a follow up letter to the General Manager of your power company with a copy sent to the President or Executive Vice President of your power company. If the situation is not corrected to your satisfaction after a reasonable time, send a registered letter to the Chairman or Commissioner of your state Public Utilities Commission with a copy sent to the Engineer-in-Charge of your local FCC Field Office. The FCC is reluctant to become involved unless all other remedies have been tried. Please contact the Regulatory Information Branch here at HQ if we can be of further assistance. *eof