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I am attempting to hunt down many sources of RFI in and near the UHF amateur bands at my home, but I stumbled across a very specific one that I have definitely never heard before. This one sounds less like interference and more like an intentional signal of some kind. I have listened to every type of UHF signal on the http://www.sigidwiki.com and none of them line up with what I am hearing here. I set my SDR mode to LSB so that the audio would be easily discernible.

I first discovered this with my Yaesu FT-60R in AM mode on 446MHz out in the yard ~100ft from my house. I pulled it up on the SDR to easily record the signal for posting here.

Signal

I have also recorded audio of the signal in hopes it will help someone to help me ID this signal.

https://cozy.foxhollow.cc/rfi/03-Nov-2020%2018h46m01sZ,%20446%20MHz,%20LSB.mp3

If audio recordings are not allowed on this site, please forgive me. I am very new to this!

So I guess my real question is.. is this RFI, or is this an intentional signal? If intentional, I'm curious what it is. If RFI, then I need to go digging through my house trying to find the source.

73s Steve

EDIT: Apologies, I forgot to mention. I am located in the US (NE Florida) and 446MHz is allocated as the national 70cm calling frequency.

EDIT2: Here is a wider view of the spectrum around 446MHz. It appears that there is a constant spike around 445.993MHz, and then on each side there are numerous "copies" of the signal dancing back and forth that you can see in the waterfall. It seems that certain ones are dancing "backwards" from the others, as if the signal in certain spots is inverted. I officially have no idea.

Wider spectrum view

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  • $\begingroup$ that hop pattern and bursty transmission definitly looks like an intentional signal. It's pretty hard to say what that is without knowing what might have a normal allocation at that frequency. For example, if you're in the US, then amateur usage is one of two things that legally exist in that band, the other being radiolocation devices. $\endgroup$ – Marcus Müller Nov 3 at 19:15
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    $\begingroup$ In many cases the fastest and easiest way to learn about an unknown signal is to use "fox hunting" methods to discover exactly where the signal is coming from. $\endgroup$ – rclocher3 Nov 3 at 19:50
  • $\begingroup$ My bad, I meant to mention that I am located in the US and this is a ham band. I updated my question to include this info. $\endgroup$ – hairlesshobo Nov 3 at 19:57
  • $\begingroup$ @rclocher3 as mentioned in the question, I listened to every signal in the sigidwiki site for UHF. The only signal I found that even remotely resembles the one here is sigidwiki.com/wiki/Inmarsat-D(D%2B)_Downlink but the frequency is way off and the tone does not line up either. $\endgroup$ – hairlesshobo Nov 3 at 20:00
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    $\begingroup$ Personally I'm curious what you'll find, so please "ping" me (reference me by my username) if you post your findings in a comment. If you find something definitive then you should probably post your findings as an answer; no need to ping me in that case. $\endgroup$ – rclocher3 Nov 18 at 18:37
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From the wide spectrum screenshot it's evident your receiver is seriously overloaded. Note:

  • the signal around 445.982 which appears to be identical other than being "flipped", and a little stronger. This comes from IQ imbalance in the receiver.
  • additional copies of the signal all over the place. These are caused by intermodulation and harmonic distortion.

This means the signal probably isn't at 446 MHz. It might not be at any of the frequencies you see on the spectrum at all: they could all be distortion products.

One thing you can do to show it's not a real signal, but a distortion product: tune the receiver (change the center frequency of the receiver itself, not just move the cursor in your SDR software) by some amount, say 20 kHz. If the suspected signal moves by something other than 20 kHz, or it moves in the wrong direction, it's a distortion product.

Hard to say what it is. If it's overloading your receiver it's probably close. Find it by walking in whatever direction makes it stronger.

It's also possible this isn't a real signal at all, but instead an artifact of a broken or low-quality receiver.

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  • $\begingroup$ I like that answer. It's certainly like what you would hear on receive if you tune in a weak carrier and then turn the dial. And not only is it on a ham frequency, but it's on a frequency where there it's unlikely to interfere with repeaters or other operators. $\endgroup$ – Duston Nov 6 at 13:58
  • $\begingroup$ I suspect that it is a real signal, albeit RFI, because I've seen it on two RTL-SDRs, but I actually first discovered it audibly with my Yaesu FT-60R in AM mode on 446 mhz while standing out in my yard. I was walking around hunting RFI sources and stumbled on this one. I have a hunch that the RFI is coming from my home office (5 monitors, 1 desktop and 2 laptops) or my server closet (home server, pc router, solar charge controller, cable modem and ethernet switch). I hope to get a chance to do a power down soon to figure this out! $\endgroup$ – hairlesshobo Nov 23 at 19:25

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