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I am just beginning to learn about SDR. I purchased a "NooElec NESDR Mini 2 SDR & DVB-T USB Stick (RTL2832 + R820T2) with Antenna and Remote Control"

I plugged it into my Mac, downloaded CubicSDR, and was quickly able to listen to local FM radio stations. I also searched around and found a weather station as well. So I know it works, and I know how to identify frequencies in the software interface. For example, a local radio station is 93.3 WMMR and I can easily find it.

While exploring, I found that on 1.355 MHz, using FM modulation, I am hearing a hockey game. At a very nearby frequency I can also find a country music station. But (from what I can tell with some web searches) FM radio is supposed to be located within 87.8 MHz to 108 MHz. Which makes me wonder: Why am I hearing entertainment on 1.355 MHz?

To recap: Why am I hearing these hockey games and music stations at a much lower frequency range than normal FM radio? (I am hearing these things at and near 1.355MHz.) By "why" I mean: what are likely reasons that someone is broadcasting at such a low range? What are helpful resources I could use to figure this out? It's just curiosity. This is tonight's mystery as I explore SDR.

(Please see screenshot to see the frequency on the screen. Am I misunderstanding something? The reason I have taken extra time to explain the tech I am using is that I want to validate I am not misunderstanding something about the interface. Am I for sure listening to 1.355 MHz? It seems I am. But is there something I am not considering? Or, alternately, could sports and radio stations be on this low frequency for some reason?)

1.355 MHz

UPDATE:

I found a different example with much more detail:

Instead of the 1.355 MHz original signal, I have now found another example. In this example, 21.452 MHz and 88.5 MHz share the same signal. Details follow:

On the advice of comments, I listened for the call letters. I wasn't able to find the station from last night, but I easily found another one this morning where a signal on 21.452 MHz is also on 88.5 MHz. And here is what I found. I heard the call letters WXPN. A web search revealed it is 88.5FM in Philadelphia (where I live.) The announcer said it's a University of Pennsylvania Radio station which is available on these frequencies:

88.5 WXPN HD1 Philadelphia

88.7 WXPH HD1 Middletown

91.9 WXPJ HD1 Hacketstown

90.5 WKHS Wharton

and streaming at xpn.org, "member supported radio from the university of pennsylvania"

So as these two screenshots show, I can hear the same station at both 21.452 MHz and 88.5 MHz. Of course with each, I can move a little up or down and still hear it. The signal is more clear on the 88.5 MHz.

Does this additional information provide additional help or insight?

Screenshot 1 is the station on 21.452 MHz:

21.452 MHz

Screenshot 2 is the same station on 88.5 MHz:

88.5 MHz

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    $\begingroup$ I'm not a radio guy, so forgive me if this is just obviously dumb, but I notice that 88.5MHz/1.355MHz ~= 65 is suspiciously close to a power of two. Could this simply be aliasing? Can you hear similar things as you progressively double the frequency (i.e. go to 2.71MHz, 5.42MHz, 10.84MHz, 21.68MHz, etc.)? (Notice how close 21.68MHz is to the 21.452MHz you said you can hear something at...) $\endgroup$
    – Daniel Wagner
    Commented Jan 1, 2022 at 4:37
  • $\begingroup$ Seeing similar, so confused $\endgroup$
    – Randy L
    Commented Dec 18, 2022 at 5:26

4 Answers 4

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Don't trust everything you see when using hardware well outside its design specifications. The R820T2 tuner chip inside the NESDR Mini 2 was meant to be a TV receiver, and its datasheet says it works between 42 and 1002 MHz. SDR dongles push it beyond those limits, but even NooElec only claims 25 - 1750 MHz "approximately". The signals you're hearing aren't actually at the frequencies they appear on the waterfall, and you can confirm that by listening for a while until you hear a station ID.

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  • $\begingroup$ Thank you very much. I will listen for call letters and report back. $\endgroup$
    – Jack Amoratis
    Commented Dec 31, 2021 at 14:56
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I figured it out! I examined this chart provided by the government to see what the 1.355MHz frequency (and nearby frequencies) were allocated for: https://www.ntia.doc.gov/files/ntia/publications/january_2016_spectrum_wall_chart.pdf

I see now, they are allocated for AM radio. As I mentioned, I am new to this, so I have always known AM radio stations to have numbers such as 1210 on the AM dial, or 1060 on the AM dial. I had never associated AM radio with MHz before. Hence, I did not even suspect it.

For those who are very familiar with radio topics and frequencies, this might be an eye roller, but for me as a new explorer, I didn't know this - and I am glad that I do now! Thank you.

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    $\begingroup$ No, you're not seeing AM. For one thing, there are no AM stations licensed in the US at 1355kHz (they're all at multiples of 10kHz). For another, an AM signal is about one-tenth as wide as this, and won't make any sound at all if you use an FM demodulator. $\endgroup$
    – hobbs - KC2G
    Commented Dec 31, 2021 at 7:44
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    $\begingroup$ Thank you for this insight. I will listen for call letters. I will report back. In the mean time, I can see you feel confident I am not hearing AM. Do you have any immediate suspicions / hypothesis of what I might be hearing? $\endgroup$
    – Jack Amoratis
    Commented Dec 31, 2021 at 14:55
  • $\begingroup$ It's gone this morning (10:00 AM Dec 31, 2021 EST). It was there very strong 7 hours ago. And was a local signal with a local team (the Flyers) playing hockey as well as a country music station close by. Now nothing is here. I will keep checking back over the next day or two. $\endgroup$
    – Jack Amoratis
    Commented Dec 31, 2021 at 15:07
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    $\begingroup$ Probably because the tuner doesn't actually change frequency when you tell it to tune out of range, so what you see is dependent on what you were looking at beforehand. $\endgroup$
    – hobbs - KC2G
    Commented Dec 31, 2021 at 16:19
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    $\begingroup$ There's a subtlety you need to understand when working with SDR: there is a hardware tuner that selects a "chunk" of spectrum that gets sent to your PC, and then there's a software "tuner" in CubicSDR that selects a small portion of that chunk to get demodulated as audio. When you say you can tune between stations, that means that the second thing is working, but the first thing could have silently ignored (or misinterpreted) a command to change frequency, and sent back a completely different chunk of spectrum than the one you asked for. $\endgroup$
    – hobbs - KC2G
    Commented Dec 31, 2021 at 22:15
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I know next to nothing about amateur radio, but I'd like to float a possible explanation. If someone in your area is broadcasting an unmodulated carrier somewhere in the FM broadcast band, and if the frequency of this carrier is 1.355 MHz above or below the sports station's frequency, won't you detect an FM signal way down at 1.355 MHz due to heterodyning? Heterodyned frequencies give you new signals at the sum and difference of the two frequencies.

For example, if the sport station is at 101.1 and the unmodulated carrier is at 102.355, you'll detect FM sports programming at 1.355 and 203.455 MHz. The fact that you heard other programming near this frequency tends to support this hypothesis.

Unlicensed FM modulators that work in the 88-108 broadcast band are out there, and are recently more common due to churches and similar institutions broadcasting their worship services to cars in their parking lots during the pandemic.

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  • $\begingroup$ Thank you. That is really interesting! I am looking up heterodyne.... also I have found more information and added it to the question (see under "Update".) Do you think that the new info supports your hypothesis? $\endgroup$
    – Jack Amoratis
    Commented Dec 31, 2021 at 16:11
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    $\begingroup$ @JackAmoratis Possibly, but it needs more data. Look for an unmodulated carrier (or another station) that is 1.355 MHz above or 1.355 MHz below the 88.5 MHz frequency. Does your spectrum show something at 89.822? What about at 87.145? If so, then the 1.355 FM signal may be the beat frequency between these two signals, produced by heterodyning. (If my terminology sounds archaic, it's because everything I know about radio, I learned from Elements of Radio, 1943, by Marcus & Horton, a textbook for U.S. Navy radiomen.) $\endgroup$
    – MTA
    Commented Dec 31, 2021 at 19:10
  • $\begingroup$ Welcome to ham.SE! I like your theory, but I don't think scanning the airwaves for an unmodulated carrier signal from far away is necessary, because the radio (the SDR USB dongle) may not be well-shielded, and also there is a computer loaded with badly-shielded oscillators very close by. There are all sorts of opportunities for spurious signals, intermodulation, and other mischief. $\endgroup$
    – rclocher3
    Commented Jan 4, 2022 at 1:21
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    $\begingroup$ @rclocher3 Thanks for the friendly welcome! Your point is well taken, but the OP's instrumentation should pick up nearby computers and badly shielded oscillators in his own house too. I wasn't suggesting that there was a distant source of an unmodulated FM carrier, but possibly something like a church in the OP's own neighborhood. Cheap unlicensed FM modulators (100 - 500 mW) have a range of just a few blocks. But the "carrier" could just as easily be a noisy LED lamp, switching power supply, computer or something in the OP's own house. $\endgroup$
    – MTA
    Commented Jan 4, 2022 at 22:18
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Note that I am not a radio guy, but another possible explanation occurs to me:

The signal might really be there but not coming from the station. Rather, radio receivers typically have an intermediate frequency that's used to make the signal easier to work with. Somebody nearby was listening to the radio station you were hearing and you're actually picking up leakage of the intermediate frequency signal on whatever frequency your receiver is actually listening to (given what others have said about the frequency range it might not be where you think it is.)

They turn off their radio, the signal goes away. They set their radio to another station, you now find that other station.

(Note that this normally how receiver detectors work--listening for leakage of the signal on the intermediate frequency as they are pretty standard.)

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  • $\begingroup$ 455 kHz is a common IF frequency for FM radios, but 3x would be 1365 not 1355. Maybe you are still right though. $\endgroup$
    – AG5CI
    Commented Jan 13, 2022 at 22:35
  • $\begingroup$ @AG5CI Others have said the tuning might not be accurate as it's outside the normal operating range of the device. I can't address that part of it. $\endgroup$
    – Loren Pechtel
    Commented Jan 14, 2022 at 0:06

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