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KiwiSDR + MLA-30

Hello,
after doing several experiments with a Mini-Whip antenna, I decided to buy and install a MLA-30 active antenna. This antenna increased the gain and reduced the noise significantly. The Mini-Whip had intermodulation issues due to high power AM transmitters a few kilometres away (excellent for rural areas, but not the best for urban environments). This antenna work better, but it picking up some noise, and I would like to know if you guys can identify the source.

Image 1:


- The noise looks about 5 MHz wide.
- The noise seems to come from the antenna / cable, since it disappears when I connect a dummy load.
- If I zoom in, the noise looks like several pulsating "carriers" separated evenly with a constant gap between them (see image 2).
- Those "carriers" fluctuate a little bit in frequency.
- When I reduce the supplied power to the antenna amp, the noise attenuates, but it doesn't disappear.

Image 2:


I don't know the source of this interference. Maybe is product of intermodulation inside the antenna amplifier. Maybe it has an electric origin.
¿What do you guys think? ¿Do you suggest doing other kind of experiments?

Comments

  • Fluctuating carriers are almost always a sign of harmonics from switch mode power supplies (i.e. cellphone/USB chargers). The oscillators of those are almost never tightly controlled (e.g. with a crystal). They use an RC network or similar. So at higher harmonics the carriers become progressively more unstable. I see that the carrier spacing is roughly 80 kHz (+/-). I'll bet that if you tune down to 80 or 160 kHz you'll see a huge fundamental carrier. You might have to rotate the loop to capture it.

    A legitimate question is why is the harmonic signal strength frequency selective and not of equal (or decreasing) strength across the band? There can be many reasons, but my guess is that the RFI is getting into your house mains wiring. And that wiring will have strange resonances that likely has peaks at HF given its dimensions. So it is being re-radiated from there. But I could be wrong.
  • edited May 2020
    Hello,
    I forgot to mention that I'm using a linear 5V power supply for both the kiwi and the antenna.
    Thanks for the response @jks. Based on your response, I added the following experiments:

    Disconnected the antenna power supply:


    I can see the same kind of interference (that I wasn't seeing before because it was under the noise) but several kHz below.

    Here is the same picture with the waterfall auto adjustment:


    Then I removed the antenna coax (unpowered) and inserted a dummy load:


    Seems like the antenna body or the coax is picking up noise. The antenna is in a balcony.
    The closet cable passes about 4 meters away (220v).

    Should I disconnect the antenna and check if that signal is being picked up by the coax?
    The coax is cheap RG58 and it's about 14m long.

    I also tried double shielded RG6, but the RG58 has less interference.

    Thanks!
  • edited May 2020
    The ML-30 has an impedance mismatch that means there is a noise spike around 5MHz with the supplied loop, DSL creates loads around 15MHz so phone lines are an issue.
    That said for a balcony install your noise level is better than many. I don't think mine 7m from the house had that few interfering signals.

    I would try another BiasTee - feed the voltage to the loop through that. The supplied BiasTee has a 5V-12V step up which does create noise. I used mine with an unregulated "6v 600ma" Linear from a DECT phone, that gives about 10V at the load presented by the ML-30. You could see a bit more noise generally but also some wanted signals may come up throught the noise.

    Also buy Ferrite beads, large sizes (11-13mm) wind all your in-home cables through those a few times while watching the waterfall. I did that unitl I could not longer see a difference.
    Concentrate on VDSL/ADSL lines and any switchmode supplies.

    I aslo found that a large FT240 size ring on the coax (as many turns as possible) realy helped, they are expensive but really do sink the QRM.

    73 Stu
    G0LUJ
  • Part of the problem is that unless you live in a remote place you probably have all kinds of RF filth coming from your neighbour's houses too.
    ChrisSmolinski
  • I purchased 2 FT240 rings based on Powenumpty's message, and they delivered an improvement. Thanks.
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