Bad idea?

[SpaceForceCmdr]

I am currently using a 25-ish foot wire up in my attic, connected to an unun, and then fed downstairs via RG58 cable to my SDR. This actually works pretty well, but I'm getting some nasty, pulsing RFI on certain stations (mainly below 10000 khz). It sounds especially bad at a station on 4840. It's a low freq, hum that comes and goes. If I go outside with my portable, it clears up. Also, if I cut all power in the house, it clears up, so it must be something in internal.

Anyway, my backyard is super small and the powerlines run along the rear edge, so a tower just isn't going to be feasible at this point. Since the random wire works well, I'm wondering how I'd do if I ran it outside along the top of my wooden fence. On the plus side, it'd be away from whatever RFI is in the house, but the downside is, there are feeder lines that run over my backyard and to my neighbors. Basically, the area right over my backyard is a mess of wires. Is it a bad idea to have my random wire out there? It would be well below them. Powerlines aside, I'm not sure how much signal I'd lose by having the wire only about 5 ft off the ground vs high up in my attic like it is now.

You can't see the fence on the side of the yard in this photo, but it's on the right where I drew the green line.

 

[rja1]

Why not track down the source of the RFI? Try opening one circuit breaker at a time to isolate what circuit is causing the problem. Most likely a wall wart (switching) power supply or even an old doorbell transformer.

 

[SpaceForceCmdr]

I tried this, but can't seem to find the source of the problem. The breaker method helped me find a device that was causing a huge spike in the AM region, but that was easier because I could see it with my SDR. With this, there doesn't seem to be any big spike, but it's almost like it's "built-into" the signal I'm receiving. For instance, if I tune in 4840 during the day when there is no broadcast, I don't hear the pulsing hum. However, if I tune in later on when there is a broadcast, there it is. This happens on other stations, too. If I tune off one, I get the usual static, but the hum seems to subside. When I have the station tuned in, I get a strong signal, but also strong, intermittent humming.

 

[WA0CBW]

Can you submit an audio clip of the "hum."
BB

 

[nd5y]

For instance, if I tune in 4840 during the day when there is no broadcast, I don't hear the pulsing hum. However, if I tune in later on when there is a broadcast, there it is. This happens on other stations, too.

I have seen that happen on a multiband SW receiver with a wall wart type power supply. No hum on battery power, hum on some frequencies on AC power, no hum when there is no signal. Moving the power cord and telescopic antenna around helped sometimes on some stations.

I assume you are running the SDR on a computer with AC power or a powered USB hub. The noise could be coming from anything the SDR is attached to that has AC power. That could he hard to locate and maybe impossible to get rid of.

 

[SpaceForceCmdr]

Can you submit an audio clip of the "hum."
BB

Sure, this is a great example here:

Watch SDR Console 2022-05-09 22h 33m 39s | Streamable

Watch "SDR Console 2022-05-09 22h 33m 39s" on Streamable.

streamable.com

Looking at the little spectrum analyzer on the right, the pulsating hum is around 125 Hz. For the heck of it, I did run a wire outside along my fence, and it doesn't seem to perform as well as the one I have in the attic. Interestingly enough, I think I'm hearing the hum on it, too. I hear the hum on both my main PC and laptop (on battery), and if I move around the house with my portable radio, I can pick up the same thing. This is why I think it's something in my house causing it, and not specific to my SDR or PC.

The only way I've been able to eliminate it is by completely turning off the power to the house.

 

[WA8ZTZ]

The only way I've been able to eliminate it is by completely turning off the power to the house.

Since the breaker method did not pinpoint it, the noise is probably coming in on the powerline and is being radiated by the house wiring.
In other words, it may not be any specific thing in the house (especially since the breaker method did not locate it).
This may seem counter intuitive at first, but think about it.
You disconnect the house wiring (antenna) from the source (powerline) and the noise goes away.

 

[SpaceForceCmdr]

Yeah, this could be. I just spent even more time trying to figure it out, but no luck. Super frustrating. When I turn off breaker 2, it improves things a little (at least it seems so), but the hum is still there. Breaker 2 controls many of the light switches on my first floor. It also cuts power to this mess of a junction box. It appears that the newer wiring from the basement goes to this box, and then it's tied into the old knob and tube wiring. I know the dimmer in my dining room has this same, old wiring. It looks like when they updated the electrical in the house, they ran some new wiring (looks to be mainly upstairs), and they kept a lot of the old wiring.

Anyway, not sure how much that matters, or it could just be coming in off the main lines like you said. Cutting off the main breakers seems to eliminate it, but that's not really a solution for me, haha. Besides my long wire, I have a loop antenna (the MLA-30+), and I get the hum with that one, too, but it seems to cut down on it a bit. Also, the long, random wire I have running outside also has the pulsing hum. It seems like if the interference was in the house, then the outside antenna would alleviate it.

The good news is, this humming doesn't seem to pop up on SSB in the ham bands, and that's actually what I enjoy listening to the most

 

[WA8ZTZ]

Hmmm... "old knob and tube wiring"
Nothing wrong with knob and tube if it hasn't been abused (over fused, overloaded, etc.), however, it has undoubtedly been upgraded multiple times over the years (80-90 yrs.). If somebody unfamiliar with knob and tube got their fingers in it they could have created all kinds of "interesting" situations.

 

[SpaceForceCmdr]

From what I can see of it, it looks to be in decent shape, but it is from the 1930s. I'm not sure who did the upgrades over the years, but yeah, there could definitely be some "interesting" situations. I think it is what is is for now. I plan on buying a new house at some point (if this ridiculous bubble ever bursts), so maybe that'll be a better situation.

 

[SpaceForceCmdr]

Since the breaker method did not pinpoint it, the noise is probably coming in on the powerline and is being radiated by the house wiring.
In other words, it may not be any specific thing in the house (especially since the breaker method did not locate it).
This may seem counter intuitive at first, but think about it.
You disconnect the house wiring (antenna) from the source (powerline) and the noise goes away.

Been thinking about this, and you could be right. I was experimenting with my loop antenna, which should null out the noise depending on rotation, but I've found that the hum is there regardless of orientation. This kind of leads me to believe the the RFI is coming from all/most directions in the house. Also, if I walk around with my portable, I pick up the same hum, and it fluctuates as I move, but I can't seem to pinpoint the exact source. I did discover that my basement is relatively quiet from an RFI standpoint.

 

[WA0CBW]

Maybe I missed something but you said the hum goes away when you pulled the main breaker? That would normally indicate the source of the noise to be something in your house. Did you turn each breaker on and off individually and then move to the next breaker? How did you power the receiver when you pulled the breaker? You indicated the hum was about 125 Hz which would point to some kind of power supply.
BB

 

[SpaceForceCmdr]

As far as I can tell, it goes away totally when the main is off. As soon as the main is back on, I start hearing it again. Turning off breaker 2 (which controls most of my first floor lights) seems to reduce the hum a little, but it's still there. If I flip it back on, the hum returns (even if all those lights are off).

When doing this, I powered my SDR via my laptop. However, I don't think my PC is causing the issue because it's on another circuit. When I walk throughout the house with my portable it picks up this same, pulsing hum pretty much everywhere. If I take the radio outside and get a little ways away from the house, the hum disappears.

 

[WA0CBW]

Be aware there may be something connected to a circuit that is hard wired like the door bell transformer or the furnace thermostat. Or a humidifier or HVAC transformer. Or the battery on a power tool charger feeding back into the circuit. The fact it goes away when you kill the main breaker and you can't hear it away from your house pretty much says it is something in your house.
Bill

 

[SpaceForceCmdr]

Thanks. Yeah, I think it's something in my house for sure. I'm just glad it doesn't seem to be interfering with SSB listening.