My XYL and I lived at our current QTH for just over 16 years. At first I ran HF mobile so never really put together a solid home station. Now that I'm no longer mobile, I work from home and thought it was time to put some effort into having that solid home station.
I've always experienced very high noise levels; 160M was totally out of the question due to living about a mile from a local AM broadcast station KFMB 760 AM. It's an odd station, running 5KW during the day but 50KW at night. Most non-clear channel AM stations do the opposite with their output power. 80M through 10M bands were plagued by 20 over 9 from some kind of broad-band RFI. I thought it was related to living within about 250' from a 69KV substation. I wasn't about to troubleshoot this noise problem thinking there wasn't much I could do about all this, but decided to give it a shot anyway.
My main station antenna is a full-wave 40M loop, that's fed with 450 ohm ladder line. The tuner is a home brewed balanced line tuner. The ground system consists of two, 8' ground rods about 10 feet apart. One inch braid connects all equipment to the grounds.
I got started by doing some research on RFI. Found a few really good papers that are attached below. Once that was done, I put together a plan of action. The first thing I tackled was the AM station. I built up an AM broadcast band (BCB) high-pass filter inside an old low-pass filter case. When tuned to 760 Khz without the filter, would pin my meter on my radio. It was so strong it would cause images to appear on other frequencies than 760 Khz.
Installing the BCB filter pretty much wiped out the signal! I was concerned that being in the near-field of the broadcast towers, it would simply overload the receiver right through the case. Luckily, that's not happening. Matter of fact, I can barely hear the host speaking. I can't hear any other AM stations at all. Simple PI filter was all it took to reduce that huge signal to almost nothing.
KFMB 760 AM signal strength
Now came the hard part, tackling the RFI. My initial noise research involved taking some videos and used an audio spectrum analyzer to "see" what I was hearing. Here's the setup:
I started out with qualitative baseline noise measurements. I don't have access to calibrated, professional test equipment, so I improvised. Here's the initial audio spectrum of the noise:
Initial RFI spectrum analysis
I adjusted the inputs to the soundcard and software to set the noise floor as near zero as possible. The tall peaks displayed is the broadband noise the receiver is experiencing. These peaks of noise continue on through a wide range of frequencies too:
Even more RFI
As you can see, this noise is pretty much everywhere and strong. Very interesting that some of the peaks were multiples of 60 Hz. Cheap full-wave rectifiers poorly filtered and shielded will show peaks at 120 Hz, with multiple harmonics too. Here's a video of what I'm hearing:
The RFI is rather extensive.The first step is to see if something in my own home was generating this RFI. So on a day when no one was going to be home, I turned off the main circuit breaker to the house, then all individual circuits too. I ran the radio and laptop on battery while troubleshooting. The good news is, the noise I'm experiencing was somewhere in my house!
Why is this good? It's good because it's something I can control. If the RFI was coming from the substation or a neighbor, those sources could be difficult to impossible to fix. To determine which circuit was broadcasting this RFI, I turned the main breaker back on, then turned each circuit back on one at a time until the noise returned. The noise was coming from a circuit that was labeled garage and rec room, right where I have all my radios and other equipment. This is when the fun began.
I started by walking through the house with a Grundig Yachtboy 220 portable AM/LW/SW radio. The RFI was very loud on the AM band specifically.
I followed the noise around the outside walls, the noise was possibly coming from our motion sensor based halogen light, motion sensor porch light, GFI outlet on the porch. RFI was present on the DirecTV RG-6 cables and was very strong coming from the attic. I turned that circuit breaker back off and got my ladder out to reach the driveway light. While looking up to place the ladder, I noticed the little motion detected LED was flashing. No problem, that takes it off the suspect list and saves me a trip up the ladder. Not finding anything else, I focus on the cable TV coax in the attic. Lots of noise coming off those. My home is about 40 years old and there's coax going every which way. All of it old and not connected to anything. I start pulling up the old coax, probably had 200' of the stuff up there, plus a few splitters. I suspecting there was a cable TV distribution amplifier up in the attic but there wasn't. After pulling out all that old coax, I hear my wife yelling for me that our greyhound managed to get out and is racing up the trails behind our home. I didn't realize just how hard it can be to get out of an attic quickly! After about 20 minutes, my son and I get the leash on her. It's very difficult to see a nearly all black greyhound running in the dark.
I got back to the task of finding the source of this RFI. It was still present but with much less intensity. I was checking out the hot water heater and noticed two corroded grounds. Not sure that could be the cause but cleaned them anyway. RFI stayed the same. Found the old solar water heater controller too, but that was unplugged. That went into the recycle pile, it had some good hardware to remove. I removed all the old, unused TV coax from the garage overhead.
By this time, I've covered the entire home and the RFI is still there. A good 20 dB less but still present. I turn off the the breaker, noise is gone. Thing is, I don't see anything turned off that should be on. It's been turned off for a week and still we don't know where it goes, or what is causing the noise. Right now, the solution is to keep it off. Maybe I can repurpose that circuit and dedicate it to the shack. Can always use another 20 amp circuit dedicated to the hobby.
End result was a major reduction in RFI, here's a video on 160M AM:
Now, I could hear one more noise - static discharge. Those occasional crashes that reach S7-8, drive the AGC nuts. What to do? I tried the 1M carbon resistor off the feedline to ground. That sort of helped, but still present. I knew that AM broadcasters would use a shunt inductor straight to ground at the base of the antenna. AM broadcast antennas are verticals, but my antenna is a loop. Thinking about it, why not put two shunt inductors, on for each leg of the ladder line, straight to ground? Figuring I needed about 230 uH of inductance to still provide enough impedance on 160M to be ignored, they were going to be huge. I ordered 128 feet of 16 AWG enameled wire. I used 2.5" PVC pipe and wound enough turns for an 8 inch long coil. Using the AADE Inductance meter, got close enough.
I made two of these coils:
My antenna feed-through panel is made from a large aluminum sheet that's covering the opening where an old air conditioner used to be. The panel is grounded using 1 inch braid to an 8 foot ground rod that's 4 feet down. I mounted the coils, one on each leg of the ladder line where it enters the shack. The opposite ends were bolted to the antenna panel:
I installed them at these funny angles thinking it would help reduce any mutual coupling. Here are the results. You may have to listen to the videos a few times.
In the final video, you'll notice the static crashes are gone and the S meter reading is much steadier with the shunt inductors in place.
It was a challenging project but well worth it. The next evening, I was spinning the dial on 40M and heard 4X4WN around an S3. Also heard ZS3DW from South Africa. I couldn't work them with 100 watts, but just the fact that I could hear them made this RFI troubleshooting well worth it.
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