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Do I need a Filter? - Find RF Signals In Area

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  • #61
    One of the best filters i ever had:
    You can see the result on:


    • #62
      The next day.

      I was using a non-name non-RTL-SDR usb card to measure, with generic dipole antenna. That might make the measurement funny. So, today I took a 1090 MHz antenna and RTL-SDR v3 USB receiver.
      This is the result:

      Now I see that there is a considerable noise in 1090 MHz region.
      Let'e try my second location:​

      That's more like it!

      The moral of this is - use good equipment for measuements. Preferably the one that will be used for "production".
      Now, I will have to use the filter to see if it resolves my problems.
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      Last edited by Kvisko; 2023-10-22, 07:36.


      • #63
        I did a scan and it basically looks like the entire range has noise - but surely that's not correct?scan.png

        Edit: I believe it's power supply related. I moved it further from the rPi and it's much better, but there is still noise:

        I'll grab a replacement genuine rPi power supply and retest.
        Last edited by furkly; 2024-02-07, 03:06.


        • #64

          It is likely that this type of heatmap is caused by the noise generated by RPi and picked by the dongle.

          Instead of plugging the dongle directly into RPi, use a usb extender cable to connect dongle to RPi. This will create some physical seperation between Pi and Dongle. Also the USB 3.0 ports are noisier than USB 2.0 ports. Try to plug your dongles in USB 2.0 ports. The Pi 4 has two USB 3.0 and two USB 2.0 ports.


          • #65

            Someone in Flightaware forum got similar scan and found that running the rtl_power​ turns OFF the bias-t of his dongle (RTL-SDR Blog V3). This makes his LNA to stop working, and the dongle stops getting 1090 MHz signal, and gets only noise which resulted in this sort of scan.




            • #66
              Connected the dongle via a USB extension cable and got a super weird result:

              Scan 3.png

              Went back to directly connected:

              Scan 4.png

              Ran rtl_power with the -T command per that thread, same as the other results:

              Scan 5.png


              • #67
                Are you using a LNA (RF Amplifier)?
                Failure of LNA or lack of power supply to LNA results in the LNA instead of boosting the signal, attenuate (reduces) severely. This will result in sort of heatmap scan you got.

                If you are not using a LNA, but the dongle you are using has built-in LNA, then failure of dongle's LNA chip, or lack of enough DC power to dongle can also result in the yype of heatmap you got.