A Smarter Approach to Resolving Power-Line Noise

Sep 1, 2004 12:00 PM
By Mike Martin, RFI Services; Riley Hollingsworth, FCC; and Jody Boucher, Northeast Utilities

Imagine you have received a complaint from an amateur radio operator. The rules are still pretty much the same as with the TVI complaint:

The source must be active at the time of our investigation.

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, to the offending frequency. 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 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 spirally away from the house to find the noise, you can focus searching in one direction.

Another important clue can be obtained by tuning the DDF receiver to higher frequencies. 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 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 near — perhaps within less than a quarter-mile radius. If it diminishes around 4 MHz, however, the source can be more than one mile away.

An Important Rule

By now, you can see a tremendous improvement in noise locating efficiency. Perhaps the most difficult hurdle to overcome in this process is to ignore those noises not affecting the customer's equipment. An important rule for efficient and economic RFI troubleshooting is to locate and repair only the source causing the complaint.

Locating the Utility Source

Head in the direction from which the antenna indicated the noise was the strongest. After a few blocks, you might expect to receive a noise with the exact pattern as the one recorded at the complainant's house. Now, reduce the signal level on the DDF receiver. In most cases with a modern DDF receiver, simply turn the RF gain control down to 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.

If the signal level increases, you are approaching the source. Continuously adjust the gain to accommodate changes in the signal level. The importance of this rule cannot be overstated.

Directional Antennas

With an omni directional or whip antenna, you must move to determine the direction of the higher signal level. If you use a handheld or vehicle-mounted Yagi (directional) antenna, you 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. A handheld Yagi works at VHF and UHF within a specified frequency range. Not only are VHF and UHF antennas typically smaller, but direction headings are more reliable. An attenuator is required between the antenna and the receiver if the receiver does not have one (Fig. 7).

Pinpointing the Source

The investigator must be able to pinpoint the source on the structure down to a component level. An investigator also can use a hot-stick-mounted device to find the source. An ultrasonic dish is useful for pinpointing the source of an arc. An unobstructed direct line-of-sight path is required between the arc and the dish. It is only useful for pinpointing a source once it has been highly localized and is ideally suited for pinpointing the arcing hardware once the offending pole has been isolated (Fig. 8).

Common Source and Locating Misperceptions

Note that transformers are not listed among the most common power-line noise culprits. Despite their reputation, only a small percentage of transformers are actually found to be the cause of an RTVI complaints. Many times transformers are replaced because they are believed to be RTVI sources, when in reality, the transformers' loose hardware merely needs to be tightened. Sometimes, locaters are fooled by the hardware associated with a transformer pole. A transformer pole has a driven ground conductor, lightning arrestor, and often a down guy or other hardware that can act as an antenna to radiate noise. This can cause a high level of noise at the pole, but it is actually being generated by another source.

Corona discharge also gets a bad rap as another RTVI source when it rarely, if ever, is a source of power-line noise. Corona discharge is defined as the partial breakdown of the air that surrounds an electrical element such as a conductor, hardware or insulator. Corona typically is nothing more than a minor annoyance, as corona noise is usually confined to lower frequencies. This noise does not propagate far from the source because it is a low-current phenomenon that does not couple into adjacent wires. Hence, corona cameras are not recommended for locating RTVI sources.

Another type of equipment that has little directional locating capability is thermal/infrared detectors. Ultrasonic detectors, on the other hand, are very useful but often misunderstood. As previously discussed, they are not practical for finding the general source location, but they can be of great assistance in pinpointing the exact noise source once the structure is located.

How to Fix

RFI repairs on the utility system usually involve eliminating an arc of some type. Arcs can occur due to loose hardware, cracked insulator, tracking, corrosion between two pieces of metal, or a loose tie wire.

Long-term repairs eliminate arcing by replacing the offending part, tightening hardware, or cleaning to prevent tracking. Freezing and thawing can cause hardware to loosen, especially in colder climates. Helical spring washers added to the bolts can absorb the expansion and contraction of wood poles and maintain hardware tension to prevent gaps from forming.

New products and materials for line construction are constantly evolving in the industry. For example, polymer construction of various types of post top insulators, dead ends and fused cut outs provide higher Basic Insulation Level, lighter weight and are less prone to stress cracks. Vice top polymer insulators are far superior to wire ties that can arc when loose, especially in cases involving covered wire.

Acknowledgments and Resources

The authors would like to thank Mike Gruber and Ed Hare of ARRL Laboratory, and Terry Rybak of General Motors for their contributions to this article.

RFI Services provides locating and consulting services. Visit its Web site at www.rfiservices.com for additional information. Help is also available from the ARRL at http://www.arrl.org/tis/info/utility.html.

Mike Martin owns and operates the RFI locating and consulting firm called RFI Services. He's been locating interference sources and training power and telecom companies for more than 20 years and solves an average of 500 interference complaints a year. Martin tests all RFI locating equipment and makes recommendations to the manufacturers for improvements.

mike@rfiservices.com

Riley Hollingsworth joined the Enforcement Program of the Compliance and Information Bureau (formerly the FCC's Field Operations Bureau) as legal advisor for Enforcement in 1998 and became special counsel for the new Enforcement Bureau formed in late 1999. He is responsible for enforcement in the Amateur Radio Service and interference resolution in the Land Mobile Service. He has a master's degree in political science from the University of South Carolina and a law degree from Wake Forest University Law School.

rholling@fcc.gov

Jody Boucher is the test department supervisor at Northeast Utilities and has 25 years experience in the electric utility industry locating, correcting and developing solutions to RFI problems.

bouchjm@nu.com

Common Household Items That Cause Interference