Is this normal?

[SpaceForceCmdr]

Just got a portable radio, and I notice when I tune in a weakish signal, sometimes it sort of pulsates. It'll get stronger, weaker, stronger, weaker. It almost sounds like when I walk around with my handheld scanner and the signal gets stronger or weaker as my position changes. However, in this case, my SW radio is just sitting in one spot, and the antenna is not moving.

 

[K4EET]

The short answer is yes, it is perfectly normal for an HF transmission to pulsate between a strong signal and a weak signal with the receiver and receive antenna stationary. Sometimes the received signal will also sound as if it has gone off frequency momentarily before coming back on frequency. Welcome to the wonderful world of HF propagation. I’ll see if I can find a good online article with pictures that introduces you to HF propagation with a top-level explanation of this phenomenon.

 

[K4EET]

Here are two resources:

Fading - Wikipedia

en.wikipedia.org

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001RS002495

 

[SpaceForceCmdr]

Thanks! That's good to know. I just wanted to make sure my radio wasn't faulty or something.

 

[Boombox]

it's the same sort of phasing and fading you'll notice on distant AM band stations at night. Same principle. Except with SW it can happen during daylight hours, too, depending on the frequency range.

 

[SpaceForceCmdr]

To add to my original post, I'm also using an SDR to listen, I hear the same thing on certain freqs. Here's a short video to give you an idea of what I'm talking about. The thing I find especially annoying is the low freq hum that grows and weakens. Going by the display in the receive window on the right side, it appears to be around 125 Hz. At the end, I turn on some noise reduction, which helps a little. Is this low freq sound caused by the HF propagation as well? If not, what else could be the cause? I don't mind a little hiss and whatnot, but this kind of thing just makes a signal unlistenable to me. It's really grating on the ears.

 

[Boombox]

The China Radio International broadcast you may have been listening to was from Cuba, and Cuba's transmitters have sometimes had problems.

If this same noise occurs on other stations, too, it could be RFI from somewhere nearby your home, and possibly a switching 'wall-wart' power supply from some nearby equipment.

 

[SpaceForceCmdr]

Thanks. I have noticed it on some other stations, too, but not all. I may try hooking up to my laptop and taking it outside, and see if I hear the same thing. I think I was hearing the same thing on my portable as I walked around throughout the house.

 

[littona]

That buzzing sound likes a jammer?

 

[SpaceForceCmdr]

It sure is annoying, whatever it is.

 

[Boombox]

That buzzing sound likes a jammer?

Not sure it would be a jammer. Who's going to jam a Chinese broadcast, in English, to the US? It sounds more like RFI of some sort, to me.

 

[SpaceForceCmdr]

Here's another recording. Same freq as the earlier vid, but it happens on multiple stations. For instance, if I tune to 4840, I seem to get a similar thing. If I walk around my house with my portable radio, I also hear the same low frew pumping sound. If I go outside, it still happens, but if I stand in certain spots in my yard, it seems to go away. So, I guess this is some kind of interference? Just not sure where it's coming from...

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

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

streamable.com

 

[K4EET]

@SpaceForceCmdr, in order to determine if the source of the interference is from within your home, place your portable radio somewhere near your main circuit breaker panel where it is receiving the interfering signal and so you can hear the interference and can tell if it goes away. Then, one by one, trip each circuit breaker to see if the interfering signal goes away. If it doesn’t, turn that breaker back on and trip the next breaker. Repeat until the interfering signal goes away.

If you go through all of your circuit breakers and the interfering signal never went away, the source is most likely not in your home. I say “most likely” because it could still be a bad connection somewhere on the primary feed up to the circuit breaker panel and the individual circuit breakers themselves but that requires opening up the breaker box exposing lethal voltages. If you are not familiar with how to work on a breaker panel, it is best left to an electrical contractor.

If one of the tripped breakers causes the interfering signal to go away, then you now know what household circuit the culprit is on. All you have to do now is unplug everything one by one until you find the culprit. If you have gone through everything on that circuit and the interfering signal never went away, then you have obviously missed something on that circuit.

Let us know what you find out with the above tests.

Cheers! Dave K4EET

 

[SpaceForceCmdr]

Thanks, Dave. I will have to give this a try because it's really got me wondering. Do you think that's it's something in my house even when I can hear it with my portable outside the house? I did just go outside, and the hum is a lot less. Also, interestingly enough, if I go down in my basement, the hum is pretty much gone on my portable.

 

[majoco]

I have a similar signal near me on 13.560kHz and it's on 24/7 - very strong, about 10dB over S9. After walking around the neighbourhood with a portable I narrowed it down to a telephone company hut a few streets over. Some more investigation showed that this frequency was one specifically for "ISM" - industrial, scientific and medical - but could also be for RFID devices. Google for that frequency and it gives lots of options - yours might do the same.

 

[K4EET]

Thanks, Dave.

You are certainly welcome 🤗.

I will have to give this a try because it's really got me wondering. Do you think that's it's something in my house even when I can hear it with my portable outside the house? I did just go outside, and the hum is a lot less. Also, interestingly enough, if I go down in my basement, the hum is pretty much gone on my portable.

The source could be anywhere. The main thing to do is start eliminating things. If it is coming from inside your home, that will make things much easier since you can control that environment. Happy hunting!

 

[SpaceForceCmdr]

I'll poke around a bit. I did tune in 4840 this morning, and there was no low hum like in the video. However, there wasn't much of a signal, either. Still, though, even if that station weren't broadcasting, or the conditions weren't right for me to receive it, wouldn't the pulsing noise still be there if it were something local?

 

[SpaceForceCmdr]

So.....some good news and bad news. Good news is, I think most of the RFI is coming from my own house. Bad news is, I'm not sure how to eliminate it. I did narrow it down to whatever is on one of the breakers. When I flip off that breaker, most of the offensive RFI goes away. Unfortunately, there are a lot of things on this break, including most of the first floor lighting. My fridge is also on this, but I don't think it's the culprit since it wasn't running.

I did figure out that some of my LED light bulbs seem to be causing some noise, so that's an easy fix. However, here's where things get weird. If I turn off the problem circuit, let's call it circuit 2, RFI goes quiet. BUT, if I flip it back on, that annoying low hum returns, even with all the lights off. I'm pretty sure I turned off everything that I can that's on that circuit, but the hum is there. It's almost as if just having the wires energized is what's causing the problem. Not sure if there's something funky going on with the wiring in my house or what.

Not really sure what to do at this point, besides turning off breaker 2 every time I want to listen, which isn't really feasible. When I take my portable outside, things are definitely better, so it would make sense to just use an outdoor antenna. Problem with that is, there's not a great place for it. I've included an attachment of my backyard. It's very small, and the power lines that feed my house and my neighbors hang over most of it. I do have a passive Youloop antenna, and also an active MLA-30+, so I guess I will play with those some more. Unfortunately, they seem to still pick up pulsing, bassy RFI.

One last thing of note, so far, it seems like this pulsing hum happens mostly on SW broadcast stations in AM mode. I'll have to listen some more, but it doesn't seem to be much of a problem on local AM, just the far away broadcasts. 4840 khz seems the worst so far. On the bright side, I haven't noticed much of an issue when listening to hams in SSB.

 

[littona]

So.....some good news and bad news.

Do you have any dimmer switches on the lights? Anything with a remote control to turn it on/off? Just trying to think of what might still be energized.

 

[K4EET]

Hey @SpaceForceCmdr, you are making good progress! Yes, LED light bulbs can be problematic. Some brands are worse than others. As for turning a circuit breaker back on with everything on that circuit switched off and the hum starts again, don’t forget that a single “wall wart” of which most are switching power supplies, becomes active if that electrical outlet is on that circuit breaker. Lots of possibilities like that exist on the circuit. Dave K4EET