Locating power-line noise quickly is crucial to solving these problems in an economic and timely fashion. Mike Martin, an interference investigator for RFI Services, has twenty-five years of experience in the business of tracking and handling power-line noise complaints. Mike now shares some of his interference locating secrets with us.

An Overview 

Once we receive an RTVI (Radio and Television) complaint form, we'll proceed to the complainant's neighborhood. We then track down each of the power-line interference sources we find in the neighborhood. We'll typically find several sources and list them on a repair request and send it to the line department for repairs. 

Sound familiar? If so, your process of locating & dealing with power-line generated (RTVI) problems is similar to over 75% of the power companies with which I work. As you've probably already discovered with this technique, the process of locating RTVI sources can be extremely expensive if not handled correctly. 

Here's a quick pop quiz. During a 24-hour day, there is only one time to investigate an interference complaint. Know what it is? If not - read on. My goal is to provide you with the basics for a sound and economic approach to resolving interference issues. I'll be answering this question and a lot more during the remainder of this section.

The First Step 

First, all on-site investigations begin the same way - eliminate the possibility that the interference source is located in the complainant's residence. The majority of power company RTVI complaints I investigate are actually caused by equipment inside a customer's residence. Since interference gets worse as you get closer to the source, it makes sense to start your investigation with the complainant. (If the source was closer to someone else they would most likely be complaining.) 

Fortunately, as we previously discussed, eliminating a particular residence or locating an inside source is a very easy process. It's simple enough that many complainants can actually perform this test as a prelude to the formal RFI investigation and without your utility's involvement. For your convenience, a simple step-by-step procedure handout, plus instructions for "locating inside sources" and "locating the residence" can be downloaded from www.rfiservices.com. This information is similar to the initial power-line noise test we discussed in the previous section. Providing it to your complainant as a first step can reduce your on site investigations by as much as 65%. 

Let's now consider a case in which the customer is unable to perform this initial test -- or you've determined the interference source is external to his residence. A site investigation is obviously required. Again, start with the initial test as a first step. Even in cases where the customer has already performed this procedure, it's always a good idea to verify his results. 

Once you've eliminated the possibility of an internal noise source, always start the RTVI locating process with the customer's equipment each and every time it is investigated. Whether it is a television interference (TVI) or radio frequency interference (RFI) complaint, always start by monitoring the customer's equipment while the problem is active. It is difficult, if not impossible to locate an RTVI source that doesn't exist. When you arrive at interference site, always view the problem as experienced by the customer with his or her equipment.

Finding The Source

Once you've confirmed the problem is active we're ready for the next step -- locating the source. Attach our Defect Direction Finder (DDF) receiver to the customer's antenna. See Figure 6. This specialized equipment will enable us to monitor the symptoms as received by the customer's antenna. Our DDF setup should include a broadband AM receiver that covers the frequency range affected by the problem, an oscilloscope (scope) and an attenuator or RF gain control to adjust the RF signal level. With these tools we'll be able to monitor the sound and pattern produced by the RTVI source(s). 

As an example, let's now consider a TVI complaint on television channel 4. While viewing channel 4 on the customer's TV, we should see dots and lines in two horizontal groups slowly moving upward on the screen. As one group goes out of view at the top of the screen a second group re-appears at the bottom. (This characteristic of power-line noise has already been previously discussed in this section.) Tune to 67.25 MHz, the frequency of channel 4, with our DDF receiver. We should now be able to view the noise as displayed on our scope and observe it's unique noise signature. 

Scope patterns show many important facts about the source(s) affecting the customer's equipment. They can reveal the number of simultaneous sources, determine which source is the strongest, and even provide an indication as to the size of gap across which the spark is occurring. When working with TVI complaints, the scope can show which source is having the most impact on the TV picture. Don't be intimidated by this instrument. It is a very powerful tool and a simple process to use. Once it is set up, a scope rarely needs adjustment.

The Signature or Fingerprint Method 

Each sparking interference source exhibits a unique pattern. By comparing the characteristics between the pattern taken at the customer's residence with those we find in the field, we can now determine which is an offending source from the many sources we might encounter. It therefore isn't surprising that a pattern's unique characteristic is often called its "fingerprint" or "signature." See Figure 7 for an example. 

This is a very powerful technique and a real money saver. Even though you may encounter several different noise sources in the field, this method helps identify the sources are actually causing the interference problem. You need only correct the problem(s) matching the pattern affecting the customer's equipment. You can accomplish this method one of three ways:

• Record the pattern on the scope by drawing it on a note pad.
• Take a photograph of the pattern. You can then compare it to patterns found in the field.
• Or, use a receiver with a built in scope and the ability to store the pattern. This is the most modern method.

Interference locating receivers, such as the Radar Engineers Model 240 previously shown in Figure 6, have a built in oscilloscope display and waveform memory. They are ideal for the third method described in the previous paragraph. This is now the preferred method used by professional interference investigators. It provides the ability to toggle between the pattern saved at the customer's house and those from sources located in the field.

Once armed with the customer's noise fingerprint, you are ready to begin the hunt. Start your search in front of the customer's residence. Next, travel in a circular pattern around the customer's house, block-by-block, street-by-street, until you find the noise pattern matching the one recorded at the customer's house. Use VHF or UHF if you can hear the RFI at these frequencies. The longer wavelengths associated with the AM Broadcast Band, and even HF, can create misleading "hotspots" along a line when searching for a noise source. At these frequencies, you may find that the noise peaks at certain poles with different types of hardware mounted on them. As a general rule, only use the lower frequencies when you are too far away from the source to hear the offending RFI at VHF or UHF. Work at the highest frequency on which the noise can be heard. As you approach the source, keep increasing the frequency. See Figure 8.

By now you're probably beginning to see the value of having the correct equipment. Once you've matched the pattern obtained at the customer's house with one in the field, you're well on your way to locating the structure containing the source. The process now gets a bit interesting, but let's first go back for a moment and change the scenario a bit.

An Amateur Radio Complaint 

Recall that the original complaint was TVI on channel 4. In this case we tuned our DDF locator to the frequency of channel 4. Let's now consider a complaint from an amateur radio operator with RFI at 21.4 MHz. The rules are still pretty much the same as with the TVI complaint:

• The source must be active at the time of our investigation. 
• As always, observe the symptoms on the customer's equipment. 
• Start the investigation by verifying the source is not located in the customer's residence. 
• Connect the DDF receiver to the customer's antenna before investigating the area outside his house.

In this example however, tune the DDF receiver (while connected to the customer's radio antenna) for a frequency of 21.4 MHz. Again, observe and record the noise pattern for future viewing. Once ready to begin the hunt, start traveling in a circular pattern away from the customer's house until you find the matching noise fingerprint. If however the customer has a rotating antenna, use it to your advantage. Determine the direction of the noise source from the customer's house and reduce travel to a minimum. 

Whether the complaint is TVI or RFI, a rotating antenna is always helpful. Instead of traveling in spiral away from the house to find the noise, you can reduce your search to only one direction. You need now travel only in this direction toward the source. Obviously, you can ignore any noise patterns that don't match our recorded fingerprint and concern yourself only with the offending source(s). 

Another important clue can be obtained by tuning the DDF receiver up in frequency. Listen to the noise at VHF and UHF and make note of the frequency at which it starts to diminish. This frequency can provide an important clue as to the proximity of the source. The closer the source, the higher in frequency you can receive it. If the noise can be heard at 440 MHz, you can expect it to be relatively close by -- perhaps within less than a quarter mile radius. If it diminishes around 4 MHz, however, the source can be over a mile away. 

An Important Rule 

By now you can easily see a tremendous improvement in our noise locating efficiency. We can now quickly locate the direction of an interfering signal and match its pattern with any number of suspect noise sources. Using this fingerprint technique, we can more easily locate the structure containing the source. 

Perhaps the most difficult hurdle to overcome in this process is to ignore those noises not affecting the customer's equipment. Whenever a suspect noise pattern doesn't match the recorded one, you must ignore it. Whenever attempting to locate the source of an interference complaint, you may encounter many power-line sources and other interfering signals. This is normal and to be expected. If however you were to repair all of them, the task of locating and solving RTVI complaints would become more difficult. As a result, the repair cost would quickly become unacceptable. An important rule for efficient and economic RFI troubleshooting is to locate and repair only the source causing the complaint.

During the Hunt 

Let's now pick up where we left off a few paragraphs ago -- on the trail of an interference source. Thanks to the customer's beam antenna, we had a good idea of the direction to start the hunt. We started out heading in the direction from which the antenna indicated the noise was the strongest. This cut our travel distance down considerably since we didn't need to travel in a spiraling path away from the complainant's house. After a few blocks, we start receiving a noise with the same exact pattern as the one we recorded at the complainant's house. Let's now determine the actual structure containing the source. 

At this point we want to reduce our signal level on our DDF receiver. We can do this in one of two ways. Typically, in most cases with a modern DDF receiver, simply turn the RF gain control down to the point we can achieve a minimum signal level (as indicated by the receiver's signal strength meter) and still have a clear noise pattern on the scope. If the receiver does not have an RF Gain control, an attenuator between the antenna and receiver can be used to reduce the signal level at the receiver's input. 

We'll know if and when we are approaching the noise source by observing its signal strength during the hunt. The pattern amplitude increases as the signal gets stronger and we get closer. Alternately, if we find the signal getting weaker, we'll know we are going the wrong way and may lose the signal. 

One secret to effective Defect Direction Finding is to maintain proper signal levels at the receiver. As indicated previously, we control this level by either an RF Gain control or an attenuator. Always maintain the minimum signal level necessary to observe the signal. As we approach the source, the signal level will increase. We must continuously adjust the gain to accommodate changes in the signal level. The importance of this rule cannot be overstated. Improper gain settings can make it extremely difficult to determine the direction of the source. In extreme cases, weaker signals may no longer be detected and stronger signals can produce abnormally high noise levels. 

As we previously discussed, always ignore those patterns not consistent with the customer's complaint. As you approach the source and reduce your gain, the number of sources you want to ignore will decrease due to the receiver's reduced sensitivity. The receiver will no longer be able to hear weaker signals. Always maintain the lowest level possible when viewing noise signal patterns.

Directional Antennas

As previously discussed, the process is much easier when starting with a directional antenna at the customer's house. You can also use this same method while on the street. With an omni directional or whip antenna, you must move the vehicle to determine the direction of the higher signal level. If we use a handheld or vehicle mounted Yagi (directional) antenna, we can follow the direction of the strongest signal to the noise source. This will greatly reduce the amount of time and travel distance required during the hunt. 

Radio Direction Finding (RDF) techniques typically offer the best and most efficient approach to locating most power-line noise sources. It is often the primary method of choice used by professionals. A hand-held Yagi works at VHF and UHF but must be used within its specified frequency range. Not only are VHF and UHF antennas typically smaller but also direction headings are more reliable. An attenuator is required between the antenna and the receiver if the receiver does not have one. Use as much attenuation as you can in order to minimize the area of search. As before - you'll need to add more and more attenuation as you approach the source. See Figure 9.

Pinpointing The Source

Once you know the structure containing the offending noise source, the next step is obvious. You must find the source on that structure. The RFI investigator, even if not a lineman must be able to pinpoint the source on the structure down to a component level from the ground. Alternately, an investigator can instruct the lineman on the use of a hot stick mounted device used to find the source. Regardless of your particular situation, both methods are similar. 

The key to success, just as with locating the structure, is the gain control. Hot stick mounted locators and the tools used from the ground can work very well provided you maintain minimum gain after initially detecting the noise. If the source appears to be at more than one location on the structure, reduce the gain. In part, this will eliminate any weaker noise signals from hardware not causing the problem. See Figure 10. 

An ultrasonic dish is a useful tool for pinpointing the source of an arc. While no hot stick is required, an unobstructed direct line-of-sight path is required between the arc and the dish. This is not a suitable tool for locating the structure containing the source. It is only useful for pinpointing a source once it has been highly localized. An ultrasonic dish, for example, is not useful for locating the pole on which a noise source is located. It is however ideally suited for pinpointing the arcing hardware once the offending pole has been isolated. See Figure 11.

Common Sources 

The following list contains some of the more common power-line noise sources I've encountered over the years. They're listed in order from most common to least common. Note that some of the most common sources (listed first) are not connected to a primary conductor. This in part is due to the care most utilities take to insure sufficient primary conductor clearance from surrounding hardware:

• Loose staples on ground conductor
• Loose pole top pin
• Ground conductor touching nearby hardware
• Corroded slack span insulators
• Guy touching neutral
• Loose hardware
• Bare tie wire used with insulated conductor
• Insulated tie wire on bare conductor
• Loose cross arm braces
• Lightning arrestors

A Common "Non-Source" 

Note that transformers don't even receive honorable mention in the list of most common power-line noise culprits. Despite their reputation, only a very small percentage of transformers are actually found to be the cause of an RTVI complaint. 

Why are they blamed so often for noise they do not actually cause? Let's take a closer look at a typical scenario for some insight: 

A customer calls with an RTVI complaint. He'll typically say he has looked long and hard for the cause the problem. He'll also add that he found the source on a transformer pole and that he believes the cause is the transformer. When power company investigator comes to start his investigation, like the customer, he finds the highest level to be at that pole. He too may then conclude the problem is the transformer. The transformer is changed and the problem is gone. Problem solved! 

You may now ask the obvious question, "If the transformer wasn't the source, why is the noise gone? The actual reason may be that the source was only loose hardware. The hardware was tightened when the transformer is replaced. Obviously, it is far more economical to only tighten the loose hardware and not change the transformer. There is also added hardware associated with the transformer pole. Remember, the pole will have a driven ground conductor, lightning arrestor, often a down guy and other hardware that can act as an antenna and radiate noise. This can cause a high level of noise that fools the investigator into believing he has found the source structure. He hasn't found the source of the noise, only a better antenna to radiate it. 

The Final Step 

After you have successfully located and repaired the source, always check back with the customer to verify the complaint has been entirely solved. You may need to check for additional sources. If necessary, repeat this process until all the sources are corrected.

Some Final Tips and Comments 

Let's now review and highlight some of the key points we've discussed in this section.

• Always visit the customer before an RTVI investigation.
• Always eliminate the complainant's residence first.
• Maintain minimum receiver gain once you've detected the offending noise pattern.
• If the noise appears to be coming from more than one source or direction, reduce the receiver's RF gain.
• Ignore any noise patterns not seen by the customer's antenna.
• Locate and repair only the source causing the complaint.
• Every antenna works best at one frequency. Use that frequency when using that antenna.

Equipment 

By now you may be wondering about some of the DDF equipment we've discussed. Let's take a look at some professional grade instruments that might be used in a typical noise-locating arsenal: 

Receiver 

A good DDF receiver should be broadband and cover the AM broadcast band through at least 250 MHz and operate in the AM mode throughout this entire range. In addition, it must have an attenuator or RF gain control, and signal strength meter or indicator and preferably the option of battery power. The Radar Engineers Model 240 receiver was previously shown in Figure 6. 

Antennas

You'll want several antennas to complete your DDF arsenal: 

• A 7 MHz or lower frequency amateur radio antenna. This will help narrow down the area of sources that affect the AM Broadcast band as well as the lower amateur radio band frequencies. Such antennas are readily available from Amateur Radio suppliers. They are used for mobile HF operation and can be easily mounted on a vehicle with a multi-magnet base. See Figure 12. 
• A 140 MHz-150 MHz or higher whip antenna used to locate the source structure. This antenna can be easily installed on a vehicle with a magnet base for this purpose. While this antenna does not have directional capabilities, you can use it for monitoring relative signal strength as you approach the source. See Figure 13. 
• A directional Yagi type antenna. This antenna provides the capability to determine the direction of a noise source. Because it is larger and must be rotated, this is not an antenna that can easily be mounted on a vehicle. It can however be hand carried. Look for a directional Yagi antenna suitable that covers a frequency or frequency range somewhere between 140 and 500 MHz. See Figure 14.

Ultrasonic Dish

My ultrasonic receiver/pin pointer is great for pinpointing a noise source from the ground, once the structure it's on is located. Even though takes some experience to discriminate between corona and sparking sources, I wouldn't leave the office without one. See Figure 15. 

Hot Stick Sniffer 

A hot stick mounted sniffer is "indispensable" to say the least. This is one tool without a substitute. It can be used to locate all power-line noise sources once the structure on which they are located is known. See Figure16. 

Equipment of Minimal or No Defect Direction Finding Value 

Thermal/ Infrared detectors and corona cameras are not recommended for the sole purpose of locating RTVI sources. It is rare that an RTVI source is detectable using infrared. In some cases, thermal vision and corona camera techniques can actually cause RTVI. These are not useful tools for locating power-line noise.

Answer to Pop Quiz 

We've now covered a lot of ground in this section. I hope I've been able to provide you with some insight as to how better handle interference complaints in an economical, effective and timely manner. Have you been thinking about the answer to the pop quiz at the start of this section? By now you may think you know the answer. 

Answer: When it's active. 

And now let me end with this final and important comment: 

Remember to always visit the customer first. Be sure to view the symptoms before locating sources in the field that may not be related to the complaint.