Observation 1617402: NOAA 18 (28654) terrestrial interference on its frequency

Regarding Observation 1617402

Lately I’ve been getting horrible decodes of NOAA 18 like this one. Looking at the waterfall it looks that there is another signal being received at the same frequency. So I pulled up sdr# and sure enough, there’s another signal at 137.912M. Its not another satellite though. Any ideas on what terrestrial source would be transmitting at this freq?

I want to go back to getting decodes like before…

Looks like noise, rather than an outright signal. You might have to hunt it down, and even then, there may be nothing you can do about it. :frowning:

On SDR# its clearly a modulated signal of some sort. I ran SDR# for a long while and the signal never got weaker or stronger so its definitely not a LEO satellite. It wasn’t there two weeks ago but its been interfering with my NOAA 18 observations for several days now. At first I thought it might be another LEO coincidentally in view when NOAA 18 was making its current passes, but its not. Are there legal terrestrial transmitters at this freq?

Doesn’t look like it. Supposed to be only space to earth in that area according to the FCC.

Could be the Fixed or Mobile part.

Quite often you find switching power supplies are terribly noisy around 130-150 MHz. They can be a pain to track down.

A good starting point is to switch off essentially all electrical devices you have control over, to the point of cutting power to your house entirely. It sound like you’re set up to receive off the antenna with a separate receiver (airspy + laptop?) so you should be able to see if the noise disappears when power is cut.

If the signal does disappear, then you can switch items on in sequence to find the culprit. If it doesn’t disappear, then it’s likely from a neighbour, which can be a lot more painful to track down. A 3-element yagi antenna does work well for direction finding however.


So if the source is from a neighbor or other source that I can’t do anything about, my only recourse is to ditch the QFH and switch to a more directional antenna with a rotator setup, correct? Once you mentioned neighbor, it does occur to me that the house across the street has been empty until about two weeks ago when the seasonal renters showed up for their Winter escape.

I would first check your own location for the noise (shut off the main breaker and run your SDR off of battery power). The inverse square law is in your favor here (with regards to the neighbors noise).


Good thing for me that the wife is working tomorrow so I can switch off the house w/o all that yelling that would normally happen.

Raining here today, so taking my laptop outside to run the rtlsdr dongle wasn’t an option. I unplugged every wall wart I could find and flipped off every breaker except for the ones powering the RPi, my router, and my desktop downstairs so I could run rtl_tcp and SDR#. No luck. I’m pretty convinced this is an external source. Oh, I also swapped out the power supply for the RPi with a brand new one and tried removing the LNA from the receive chain. I had only originally complained about the interference at 137.9 but there’s another signal at 137.0 that is interfering at that end.

Could it be from the laptop or RPi itself? My shack computer has a birdie at 144.39, which makes APRS work a little more difficut. Perhaps the same thing going on with your station? Aside from that, maybe it’s time for a fox hunt (when it stops raining).

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When it stops raining long enough, I’ll swap the RPi out with another one. The computer is downstairs and the antenna with the RPi at the base of the mast is outside on the two story deck. I also have a spare rtlsdr dongle I can try swapping out.

Fox hunt sounds kinda fun but first I’ll have to make or buy a yagi.

It was the RPi.



RPi and their powersuplies are a problem. I had S9 noise around that frequency. I put each of the 3 RPis in a aluminum box with a fan. I grounded the RPi cases and the rtl-sdr cases. Then I got rid of the switched mode 5V powersupplies and replaced them with a good old inefficient analog powersupply. It gets a little warm but noise gone. Look at my stations id-568, id724 omnidirectional QFH antennas

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I’ll have to look into something similar. I know this is just a hobby but if I do something I want it as optimized as it can be. Are those aluminum boxes waterproof?

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No, they are perforated to allow the heat to escape.
I believe if you were to put the RPi in a sealed box, the heat generated by the main processor would destroy the RPi in a short time.
I recently had to add another external 12VDC fan to my RPi stack (3) when we had these really hot days. The rtl-sdr V3s get quite hot too.
The 12V DC fan runs nicely and quietly on 5V. All good now.
BTW the RPi fans available on Fleebay don’t last very long when you run 24/7, they become noisy after only a few months.
The slightly bigger fans which still fit inside the RPi are no better. I believe with a large (8 x 8cm) ex computer power-supply fan no internal fan will be needed.

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Any fix for this? Having this issue on my Pi4 station.

I posted en extensive fix for the noise problem some weeks ago.
basically earth the cases of the RTL-SDRs and the metal cases (fleebay) for the RPis. This gets rid of most of the noise. Check for continuity of the rtl-sdr case to ground.
Clip on ferite cores on all leads. Replace Switched mode power-supply with an analog power-supply if it’s noisy.
8cm x 8cm 12V fan running on 5V cools 3 RPi3s and 3 rtl-sdrs and is very quiet.

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Interesting fix. I wonder if I disable the HDMI if that might remove it. (Rather figure out the actual cause in the Pi then just basically do try everything fix. Weirdly this only appears on my Pi 4. This signal doesn’t exist on my Pi3 when I use it for a station.

I couldn’t get anything out on the HDMI port, perhaps the RPi image i downloaded doesn’t provide HDMI support or it is disabled as we only talk to it via PuTTy?

Could be. I haven’t looked at hdmi lately. I just remember it never use to be disabled and I was able to save some power by disabling it. I will get my good rtlsdr out and hunt around for what area of the Pi it’s coming from.