Apparent mixer creating noise around my signal

Discussion in 'General Technical Questions and Answers' started by KE0VTF, Aug 8, 2020.

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  1. KE0VTF

    KE0VTF Ham Member QRZ Page

    When I transmit, it there seems to be something mixing and reflecting my signal. For example, when I transmit on 14.300 MHz, I see signals above my transmitted signal at at 14.500, 14.700, 14.900, etc. and below my signal at 14.100, 13.900, 13.700, and so on. I tested on another frequency in twenty meters and saw the same pattern, with both tests radiating signals with frequencies "reflected" around 14.400 MHz.

    This sounds like some issue with my signal getting unintentionally mixed with some other signal. The weirdest thing is that I still see at least the primary reflection---e.g. the 14.500 MHz signal when I transmit on 14.300 MHz---when I'm using a dummy load.

    I even see noise signals being reflected around this 14.400 MHz point. What could possibly be doing this, and how can I track it down?
  2. KA0GKT

    KA0GKT Ham Member QRZ Page

    It's amazing how many things can act as a mixer. Here are a few that I have come across in 45+ years in Broadcast Engineering.

    1) Rusty connections in rain gutters and downspouts.

    2) TV antenna preamplifiers.

    3) The front end of AM broadcast band radios.

    4) Rusty HVAC ductwork.

    The rust forms a tiny P-N junction and becomes a diode mixer, the preamplifiers are Gutless Wonders and have very little filtering, the same goes for the AM radios, specially some car radios.

    It could also be the front end of some other amateur receivers in your shack.

    Tracking it down can be tricky as the signal mixing with yours may be a mixing product in and of itself. For instance, I remember one time when I was doing a proof of performance on an AM directional array, I was driving to a monitor point and the modulation on the AM station went away, but I could hear it with poor fidelity if I tuned off a bit. It turned out that the AM's sister FM station was mixing with another colocated FM and another AM located half a mile away from the FMs and coming out on the directional AM station's frequency. Happily, the interference was only for about 100 yards along a rural gravel road with cornfields and a high tension line along the road. We detuned the high tension towers and Voila! no more interference. That area is now $500K+ houses.
    KO4LZ and PY2RAF like this.
  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    How exactly are you monitoring this? Is it with an inexpensive SDR dongle?
    KL7SG likes this.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    You may have a faulty synthesiser or a low-frequency oscillation somewhere along the transmitter signal path of your radio.

    It is quite common that spurious or noise sidebands are generated from such faults.

    If the spurious signals are there even when transmitting into a dummy load, it rules out any "external mixing".

    AI3V and KO4LZ like this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Seems nearly impossible for any external-to-the-radio device would become an unintentional mixer when you transmit into a shielded dummy load.

    Never seen that.
    KO4LZ likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    My post above (#5) "doubled" with Karl-Arne's post (#4) but I sure agree.

    If this happens with a shielded dummy load connected, something's going on in the transmitter (or whatever receiver is being used to detect this could have a problem). If something in the transmitter has a 200 kHz oscillation modulating the transmitted signal, ooops.
  7. KE0VTF

    KE0VTF Ham Member QRZ Page

    I'm detecting this with the cheap RTL-SDR dongle on a different antenna in the same room. There's definitely a possibility that this distortion is being created in my measurement setup.

    The dummy load isn't actually shielded, I built it from a kit. Upon saying this, I realize it's not exactly surprising that I'm seeing signal out of it. I put it at the end of my transmission line to see if signal could be radiated from the line.
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn’t be surprised if the anomalous signals are an artifact of the RTL-SDR Dongle. It doesn’t sound like simple overload since they appear with a dummy load and the receiver in another room, presumably with a short antenna (unless there is a huge amount of leakage somewhere). I believe the cheap ones require an outboard up-converter to use on HF, and if that’s the case with yours it may be responsible for the problem.

    What happens at lower power settings from the transmitter?

    What happens if you tune the SDR to a known clean HF signal, say WWV, since you are close to them in Ft. Collins?

    You could use another receiver to check your radio for the spurious signals, such as a standard portable SW receiver, or a buddy’s mobile HF, or a spectrum analyzer if you have access to one.
    Last edited: Aug 9, 2020
    KL7SG and K6CLS like this.
  9. KE0VTF

    KE0VTF Ham Member QRZ Page

    I'm hearing WWV at 13.800 MHz with pretty good clarity on my SDR but not on my other receiver. It seems pretty convincing that this is an artifact of the SDR and not any actual problem with my setup.

    And hey, guess what I just found in the users guide for my SDR now that I'm looking for it:

    Thanks so much for helping me narrow this down, folks.
    K5WY, N0TZU and N1OOQ like this.

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