The image above shows the output of my SSB receiver tuned to 7.000 MHz LSB. The trigger is the AC line. The image is stable with this trigger indicating that the noise is synchronized with the AC line. Further, the noise peaks are about 8 ms apart, in sync with the half period of the AC line (8.333 ms). Note, the scope can save images to a 3.5 inch floppy disk, but I don't have a computer that can read them right now. So, you get cellphone pictures!
Below is a view of the noise waveform from the audio editing software Audacity.
and this is what it sounds like.
Here's more information on power line noise.
Here's a closer look at the power line noise at the receiver output. Most of the noise seems to be coming from a single phase of the power line though there may be noise from
other phases causing some of the noise between the large noise spikes.
The first image above shows a capture of a couple of the noise bursts. The next image shows us zoomed in on the first noise burst. Note the peak amplitude is about 400 mV. The next image is the noise between the noise bursts. Its peak amplitude is about 50 mV. The final image above shows the second noise burst. Its peak amplitude is, again, about 400 mV. This puts the power line noise bursts about 18 dB above the background noise (daytime 40 meters). Calculations below put the power line noise at about S6 (AGC=118.2, input level = -92 dBm). If we could eliminate the power line noise, the remaining ambient noise would be -110 dBm or S3. The receiver equivalent input noise appears to be about -118 dBm.
The plot above shows the receiver (tuned to 7 MHz LSB) AGC voltage (actually, AGC data captured on the EIA 485 bus between the radio and the control head) over a 24 hour period (Sun Feb 04 16:06:43.103 2018 through Mon Feb 05 18:25:37.946 2018). At Mon Feb 05 15:43:41.673 2018, the antenna was disconnected to see what the reading was due to receiver noise. With the antenna connected, the average value was 118.2. With the antenna disconnected, the average was 10.2. See below for the relationship between AGC voltage and receiver input level. The fact that the noise is continuous over a 24 hour period seems to eliminate the source as grow lights or most household appliances. It could still be something that is powered continuously, such as a doorbell transformer, but current efforts are concentrating on power line insulators.
An ARRL power line noise mitigtation form is available here. A report of the noise was submitted on 2/10/18. On 2/26/18, I was visited by a tech from Xcel Energy. We rode around in his truck. He identified an improperly installed lightning arrestor on a pole a couple blocks from here. I could not really detect the noise with my MFJ-856 noise receiver, which receives at about 135 MHz, but he got it on his 300 MHz receiver. He could also hear the noise with his parabolic ultrasonic microphone. He could see that the ground on the lightning arrestor was improperly installed. A crew should be out to fix it in the next week or so. It's quite possible that this is only the first of several problems, so we'll see what happens to my noise level and if it is not fixed, we'll see what's next.
There's also a set of high voltage lines going through a park a few blocks from here. In the past, I could hear what sounded like arcing from these lines, but it appeared to be mid-span instead of at insulators on towers. I could not hear it yesterday. Both the MFJ-856 and the 300 MHz receiver showed strong signals in that area. We'll see what happens after the lightning arrestor is dealt with.
We talked about other problems he's found. One was a doorbell transformer. Turning off power to it made the noise go away. Turning it back on caused the noise to come back after about 5 minutes. Another problem is an electric fence. The owner would not let them turn it off for a test, so the FCC is being contacted on it.
Update 3/26/18 - Just got a call from the power company tech. They cleaned up two poles (at 53rd and Independence and at 52nd and Garrison) today including the one mentioned above with the bad lightning arrestor. Unfortunately, the noise level here did not decrease. He will be out again next week to check further.
Update 4/4/18 - The tech visited again and said he's found more poles with leaking insulators. Apparently road salt and other debris settles on the poles and insulators causing them to leak and, in some cases, catch the poles on fire . He'll do more investigation next week.
Update 4/20/18 - The tech was working on this yesterday and today. He identified an area around 51st and Iris that seemed noisey. He turned off the power to two houses that were suspect, but the noise here remained. There is possibly a problem on the pole feeding these two houses. Connecting his EMI reciver to my antenna, he was easily able to receive the noise on 7 MHz and capture a trace. However, connecting it to his mobile antenna, he was not able to receive the noise. Using his 300 MHz receiver, he did find noise at the intersection mentioned above. I have not really been able to find the source using my MFJ-856. The antenna on the MFJ-856 is really broad, so you have to look for a null off the side. The direction of the null seems to be different on the left and right side of the antenna, further adding to confusion. I took a VHF AM receiver (aircraft receiver) around trying to determine where there was the most noise. However, in many locations it was overloaded by FM broadcast stations. I did find high noise levels at some high voltage towers with broken gound wires. But, I don't know if missing grounds on these towers would cause RFI. You can hear the noise at test.hallikainen.org. This is the output of my HF receiver which will be tuned to 7 MHz LSB unless I am using it for something else.
Update 5/13/18 - I spent a couple weeks out of town. I took my VHF/UHF radio with me. Also, I took a POE product off line and put a streaming audio monitor online instead so I could listen to power line noise while away. When I got back, the steady buzz of power line noise was gone! On 7 MHz, I just heard hiss! However, I then put the VHF/HHF radio back on its charger and connected its external antenna. Buzz was audible again! I tried various combinations of equipment powered and not powered. I found that the POE device and the VHF/HUF radio contributed buzz to 7 MHz. The VHF/UHF radio is a handheld with a switching power supply based charger. At home, the radio is connected to an outside antenna. I found that the buzz from this radio would go away if I either disconnected the DC power to the charger or disconnected the external antenna. It appears that the switching power supply ("wall wart") is sending some common mode noise to the charger base which is then being conducted to the external antenna. Pulling the DC connector from the charger base or the connection to the external antenna limits radiation of this noise. The POE injector has an FCC label on it, but independent tests have shown it to fail class A FCC requirements. So, it looked like the problem was resolved. Perhaps local noise had masked the fix implemented by the power company.
Not so fast! - While it appeared the problem was resolved, this morning, 5/13/18, the noise was back! The noise was still there after opeining the main breaker to the house. The noise has a distinctive sound to it. However, looking at it on an audio spectrum analyzer, you mostly just see the 300 Hz to 3 kHz receiver audio bandwidth. The best place to see the noise is on a scope. In the data below, I'm capturing scope photos and other information. The scope is being powered by a UPS so it will continue to operate when the main AC breaker is opened. To ensure the UPS is not causing the noise, the UPS is shut down after the scope photo to see if the sound of the noise changes. It does not.
|5/13/18||12:43pm||7 MHz LSB. It rained overnight and is drizzly today. Photo is with AC main breaker open.|
|5/13/18||1:57pm||7.2 MHz LSB. Photo is with AC main breaker open. Note that noise is worse than at 7 MHz. We are probably seeing some power line resonance.|
|5/14/18||1:40pm||7.2 MHz LSB. Photo is with AC main breaker open. This is shortly after a thunderstorm. It's now sunny and partly cloudy.|
|5/15/18||3:53pm||No power line noise heard on any frequency. Weather is dry with temperature near 80F. Main AC breaker on. Charger and POE do not seem to contribute noise today.|
|5/16/18||3:11pm||7.2 MHz LSB. No power line related noise on any frequency. Photo shows 7.2 MHz LSB. Scope photo taken with AC main breaker closed and VHF/UHF charger powered. Charger noise may vary with battery condition which will vary load. Receiver is showing 4 bars on RSSI. Also, line related noise may appear lower in the presence of other noise that is stronger. I'll start logging RSSI levels along with scope photos.|
|5/16/18||4:37pm||3.989 MHz LSB. Power line noise due to charger visible. RSSI=5|
|5/16/18||4:37pm||3.989 MHz LSB. Charger DC cable unplugged. No line related noise visible. RSSI=3|
|5/21/18||11:45am||7.2 MHz LSB. AC main breaker open. RSSI=4. Noise is back!|
|5/21/18||5:51pm||7.2 MHz LSB. Noise appears to be gone. This is with AC main breaker closed (all house power on). RSSI=4. If you look carefully, there MAY still be some 60 Hz based noise, but it appears to not be a major contributor to overall noise.|
|5/22/18||9:01am||7.2 MHz LSB. Noise appears to be gone. This is with AC main breaker closed (all house power on). RSSI=3.|
|5/22/18||9:00pm||7.118 MHz LSB. Main AC breaker open. Noise is pretty bad.|
|5/23/18||10:36am||7.2 MHz LSB. AC related noise barely visible if at all. Main AC breaker closed. RSSI=4.|
|5/26/18||10:05pm||7.204 MHz LSB, AC mains on.|
|5/26/18||10:33pm||7.208 MHz LSB, AC mains on.|
|5/27/18||9:23am||7.200 MHz LSB, AC mains on. RSSI=3|
|6/25/18||11:14am||7.207 MHz LSB. AC mains off. RSSI=5. There was a severe thunderstorm yesterday, so perhaps insulators are still drying out. Temperatures are predicted to reach 100F this week, so interference may go away as temperatures rise. Source could not be determined with MFJ-856. A 110 MHz AM (aircraft) receiver seemed to get the most buzz in the area of 5190 Independence.|
|6/25/18||4:11pm||7.207 MHz LSB. AC mains on. RSSI=3. Noise gone! Temperature is about 85F.|
|6/27/18||7:56pm||3.998 MHz LSB. RSSI=5. Noise not audible on 40m, just 80m now.|
|6/27/18||10:58pm||3.998 MHz LSB. RSSI=4. Noise not audible on any band.|
I am very impressed with dedication of the power company tech to solving the issue. It's a difficult problem, and he's not giving up. In testing fixes, it is important to open the main breaker to make sure there is not a new local noise source!