Shortwave stations mixing

Discussion in 'General Technical Questions and Answers' started by KX4O, Nov 27, 2019.

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  1. KX4O

    KX4O Ham Member QRZ Page

    Last evening I was tuning up the HF band with my SDR. Lots of stations to listen to. As usual I hear 4840 kHz good and strong. Later I ran across the Ampegon stunning signal from WBCQ on 9330 kHz. Later I found a wavering, but often moderately strong AM signal (much stronger than any other ham transmission) at 14170 kHz with both stations' audio mixed together nearly perfectly. No surprise 4840 + 9330 = 14170;.

    A friend made me aware of the Luxemburg–Gorky effect...


    ...but that suggests you hear the mixing on one of the two original frequencies rather than the mixing sum frequency.

    Anyone have any idea what might be causing this effect? I haven't ruled out my receiving setup either, but others are invited to look for this effect when 9330 fires up.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd suspect non-linearity and mixing right in the front end of your SDR's receiver in the presence of at least one and likely two very strong signals. Do you have any band filtering on the front end of your SDR or is it wide open?

    A simple test would be to add one or more fixed attenuators (assuming you have some) in the antenna path of the SDR, that or use a minimal antenna. If the phantom mixed signal goes away then it's almost certainly front end overload non-linearity.
     
    W6KCS, K0UO and KX4O like this.
  3. KV6O

    KV6O Ham Member QRZ Page

    Any mixer will give you the sum and differences. A diode can act as a mixer. A rusty bolt can act as a diode. That said, the mixing is probably happening in your radio.
     
    KK4NSF and KX4O like this.
  4. KK4NSF

    KK4NSF Ham Member QRZ Page

    It is almost certainly occuring inside your receiver. Some SDR Receivers are prone to do that sort of thing. My old RSP1a was notorious for it, while my RSP2 Pro does not do it at all. I heard all sorts of things like that before I realized I was overloading the front-end. Even a slight overload will cause all sorts of heterodynes, mixed signals, and out of band reception. BUT it is easy to deal with by simply reducing the RF gain, and/or the IF gain if you are using SDR Console.

    If it's still a problem, you might try a magnetic loop antenna so you can rotate the null to one of the signals, and thus block it from getting into your radio. You can also use the narrow bandwidth as a filter to keep it out. If you are really serious about SWL, you might want to consider a good pre-amp /preselector ahead of your SDR Receiver. They do a great job of reducing signals where you don't need them, and boosting signals when you need some gain.
     
  5. KH6AQ

    KH6AQ Ham Member QRZ Page

    Is there a mixing product at 9330-4840=4490kHz?
     
  6. KX4O

    KX4O Ham Member QRZ Page

    I'll check soon. I just saw it again tonight this time with 4840 and 9385 mixing to 14225.

    Tonight 9385 is almost 0 dB on the signal strength scale of SDR#. I am not at all sure how this is calibrated. If it is really dBm (will test with my signal generator tomorrow), I think we have our candidate... namely my antenna plus AirSpy HF+ is a little too hot for its own good. It's been a lot of fun so can complain too much. Even if not dBm, it's very strong.

    The controls in SDR# for the AirSpy HF have have the attenuation slider grayed out. The Preamp radio buttons work, but don't really appear to change anything in the measured values unless the screen cal changes along with it. No effect on the mixing products.

    Learning the limitations of your gear is part of the fun right everyone? Thanks to all for their comments and suggestions.
     
  7. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Screenshot_20190629-161609_Chrome.jpg
    WBCQ has a new 23db gain antenna and a new 500,000 watts TX. Transmitter is a Continental 500,000 watt it is the first ones they made in 40 years, They had six Engineers crawling over that thing for about 2 months.
    Timtron has a real station now!
     
    Last edited: Nov 28, 2019
    KX4O likes this.
  8. KX4O

    KX4O Ham Member QRZ Page

    Visited there last November to see it. Cold and snowy. Quite a sight and worth the pilgrimage. Run by former pirate gone honest, the older shortwave station and this new Ampegon/nicer-building monster are an interesting clash.

    The new 3-phase power line along the road was the first evidence something big brewing.
     
    K0UO likes this.
  9. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Big 23 DB gain curtain distributed fed 20191127_210725.jpg
     
  10. K4SAV

    K4SAV Ham Member QRZ Page

    Any nonlinear device that sees signals can be a mixer and will create multiple mixing products. It can happen in your receiver or it could be due to something else.

    Intermod is a continual problem for guys that operate 160 meters. The AM stations are usually the big contributors. DXers on that band know to not operate on even number frequencies like 1810, 1820, 1830, etc, so that they won't be on a birdie frequency in someone's receiver. Many people use high pass filters to attenuate the AM broadcast band and that may fix a receiver or preamp that generates intermod, but that won't fix everything.

    Example: I have a very good high pass filter to eliminate the AM signals but there were still intermod products present. I tracked the worst one to an intermittent connection between the mast on my tower and the tower itself. A strap between the two killed it. That was radiating the signal to my receiving antenna 300 ft away. After that I discovered other weaker intermod products on the same frequency. On of them was caused by a rusty fence in my back yard. Fixed it, but there were still 3 other sources on the same frequency, weaker still and from different directions (also intermittent). There are many rusty fences in rural areas.

    Jerry, K4SAV
     

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