Locating Power Line Noise

Just as I was basking in the success of finding and resolving the interference that wiped out 160m and 80m, another source of RFI raised its ugly head.

Power line noise!

RF interference from power lines is caused by arcing as high voltage electricity bridges a gap to a lower "ground" potential on the power pole. Corroded or loose connections, cracked insulators… almost all of the hardware on a power pole can be the source of arcing.

I have an intermittent source of power line noise that affects 15 and 10m, usually in the late afternoon. This appeared a couple of weeks ago as our seemingly endless "wet season" began to show signs of running dry and the weather began to warm up.

The first time I heard the RFI (about S7 without the noise blanker inserted in the receiver) I was able to peak the direction of the noise using my HF yagi. So I grabbed my portable HF radio (an FT-817) and went for a stroll a down the road in the direction indicated by the yagi.

The stroll was short! On both HF and VHF I could easily isolate the noise to one of the poles by my house. But I wanted more! I wanted to figure out where on the pole the interference was coming from – all from the safety of ground level and complete isolation from 13 KV electricity!

Electric arcs generate ultrasonic audio – usually around 40 KHz and so way too high for human hearing. One of my friends, Ira K2RD mentioned that some years back he had started accumulating the part for an Ultrasonic Arc detector that had been written up as an article in the April 2006 issue of ARRL's QST… and would I like the parts???!

The answer was an emphatic – yes please, thank you, I'll be right over! So thanks to Ira, I got a kick start and serious motivation to get going.

I put the unit together yesterday and now just waiting for the RFI to reappear to put the unit together.

Here's a link to the QST article – note that Far Circuits sells a PCB for the design which was the basis of the unit I built. Once I had all the parts together, it takes about 6 hours to build including all the mechanical work for drilling the box, building the holder for the ultrasonic transducer etc.

For the curious… here are some pictures:

View of the parabolic dish with the transducer and mount.

Rear view showing the control box mounted on the PVC pipe

Control box: L-R, Volume, Tuning, Headphone jacks

I decided to add a second head phone jack wired in parallel – this way I can feed the audio from the detector into a video camera or digital voice recorder as a record. I think a short video showing the pole radiating on HF and VHF plus the arc detector makes a pretty compelling collection of evidence to present to the utility company.

My remaining addition to the detector is going to be a high intensity green laser pointer aligned with the focus of the dish so I can see exactly where the dish is pointed – the beam width is pretty narrow so I think I'll be able to isolate to the offending area on the pole with the laser. Thanks to Rick N6RK for this suggestion!