I have spent some 50 hours over the last two weeks hunting for power line noise with my modified MFJ-856. I’m learning it’s not quite as easy as it sounds and there are some tricks of the trade that one picks up along the way. Today I will share some of my experiences and things I’ve learned so far.
It is very important the noise not overload the meter when you get close to a source. One needs to be able to see peaks and nulls while rotating the unit. As noted in a previous post, switchable attenuation is an absolute must in my area. Our distribution lines are on the high voltage side at 13 kV or so. I also have two 46 kV transmission lines to contend with. Perhaps these high voltages contribute to the need for attenuation. I have not had the opportunity to try it in an area with lower line voltages.
Walking works best. I’ve found the best way to hunt noise is on foot. From a vehicle, it would be very difficult to manipulate the 856 to take readings. I find it necessary to rotate the unit very often, not only around the points of the compass but also up toward an overhead power line.
Keep the yagi moving. I found walking under or nearly under the power line is usually a good idea. I aim the yagi at each pole as I pass, preferably varying the angle as I do. I rotate the yagi so the elements are parallel to the overhead wires, and also try perpendicular. I point at the top of the pole (where all the stuff is!) at an angle before I reach it, while passing in front of the pole, and after I’ve passed it. I note any noise heard, its relative strength, and any unique sound or “signature”. Pointing the yagi toward the horizon is a good way to locate noisy poles far away, but I don’t walk along with it in this position. Doing so can lead to hearing noise from afar but missing a noisy pole that I walk right past! Sometimes it is useful to stop and do a full 360 degree “sweep” of the horizon to see what directions might be noisy. This can lead one toward problem areas. I try this with the yagi horizontally polarized (elements parallel to the ground) and vertically polarized (elements in a plane perpendicular to the ground). Sometimes one polarization works better than the other.
Finding the source pole. Once a small area has been confirmed to contain a noise source, it is time to start taking measurements to find the source pole. Stand at least 20 feet away from the power line (more is better) an equal distance from two poles. Point the yagi at one pole and take a signal strength measurement. Do the same for the other pole. Whichever pole has the higher reading, move to a point between it and the next pole and repeat the process. Keep going until you find one pole with a higher reading than all the others. This is sometimes very difficult with the MFJ due to its limited S meter resolution.
Use attenuation. If I’m getting full scale readings on the meter from more than one pole in a small area, I increase attenuation to knock the signal down. This often helps isolate which pole the noise is actually coming from.
Use the nulls. There is a deep and relatively sharp null directly off the side of the yagi. It is much narrower than the peak off the front and can be used to help verify a noise source. Think of orienting the yagi so that the driven element itself is a pointer that you are using to “point out” a spot to an audience. Noise will be minimum when the end of the driven element is pointed directly at the source. When I find two adjacent poles that seem to have about the same noise level (yagi pointed at them), I can often stand half way between and use this technique to determine which pole is actually responsible. Of course it could be both!
Be aware of obstacles. When pointing toward the horizon to home in on a distant source, I found it is important to be aware of obstructions. Sometimes the signal will seem to get weaker as I walk along. This may or may not mean I have missed the source. Is there a hill or rise in the direction of the noise? Buildings? Forest? These can all attenuate the signal. I learned to try going around, over, or through such obstacles (as appropriate) and see what noise readings I find on the other side. I was going crazy trying to figure out what was happening in one area until I realized this!
Conducted noise and re-radiation. Even at 135 MHz I found sometimes noise can be conducted along the power line for considerable distances, then radiating from many poles in an area! It can be quite a challenge to home in on the real source in these cases. So far these seem to be the more intense noise sources. In general the noise gets stronger closer to the source, but anomalous peaks can happen at poles where lines cross or which have more or different hardware than neighboring poles. Is this pole noisy in its own right or is it simply a good radiator for noise generated elsewhere and conducted along the power line? Good question! Sometimes it impossible to know. I try to concentrate on the sound of the noise. Is it a buzz? Hum? Frying sound? Does it sound different at this pole, or is it the same sound I’ve been homing in on? If it sounds different, there is a good chance there is some additional problem on this pole. If not, it may be just that this pole is a good antenna for radiating noise “sent” to it over the lines from somewhere nearby.
Alternating peaks at three to four foot intervals. Sometimes while walking along with the yagi pointed ahead or upward toward the power line I find noise peaks roughly every three or four feet, with very distinct nulls between. This is a good indication of noise (RF energy) being conducted along the power line. A half wavelength at 135 MHz is roughly 3.6 feet, which accounts for these peaks and nulls. Recall there will be current maxima and hence maximum radiation at half wave intervals along an antenna (or a feed line operating with standing waves on it).
The “sea of noise”. I found some areas where noise is so intense I need to use high attenuation to keep the meter from being saturated. The area may encompass several poles. It is entirely possible that lesser noises will be completely missed in such areas because the strong one masks them.
Visual inspection. Take a long hard look at the hardware on poles identified or suspected as the source of noise. Walk around it, if possible, and take a really good look. Sometimes you can see evidence of the problem. When I found this noisy pole I didn’t see the obvious – until days later when the power company troubleshooter showed me a picture he had taken. It’s so obvious I can’t believe I missed it! I went back and took this picture. Look at the wooden cross arm with a hole burned in it from sparking and heat! Click the small picture to enlarge it. This pole is not only generating RFI but could eventually become a safety and down-time issue for the company.
I find the modified MFJ-856 to be an extremely useful tool, but some ambiguity as to the specific source of noise can still exist. I am seriously thinking about getting an ultrasonic detector for my line noise tool kit. That should help isolate the specific source in areas where the 856 leaves some question.