SDR Newbie Question

Discussion in 'Software Defined Radio (SDR)' started by KN4SLJ, Oct 25, 2020.

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  1. KN4SLJ

    KN4SLJ Ham Member QRZ Page

    I recently got an RTL-SDR v3 and have been playing with it. Reception was hit or miss until I turned on "Direct Sampling (Q Branch)." Almost the entire band is now lit up like a christmas tree. I've tried two antennas: A Tram 1480 Dual Band Ham antenna (for my vhf/uhf radio), and a ~50ft random wire slung horizontally across my yard, about 5 feet off the ground. I get the best overall reception with the wire by a wide margin, but I'm curious what's going on. I'm seeing AM broadcast stations all the way up around 144 mhz now. Dozens of them. There is a consistent "Fox News Radio" program in the bunch somewhere (syndicated?) but the other stations are all an assortment of semi-local and DX AM broadcast. I'm seeing basically the same thing across the 10-11 meter bands as well. Are there really broadcast stations in those parts of the band or could I be picking up some sort of harmonics because of my (obviously) inefficient antenna? I'm also seeing a religious program at 1.5ghz FM of all places, butI am assuming that it's somehow related to a religious broadcaster located about 1/4mile from my house. (10+ dish antennas out there) I may be (and probably am) wrong. I just want to understand what I'm seeing.
     

    Attached Files:

    N2HUN likes this.
  2. N6YWU

    N6YWU Ham Member QRZ Page

    You are likely seeing intermodulation distortion and/or clipping harmonics caused by the AM station's signals being far too strong for your RTL-SDR's input AGC and limited 8-bit sampling range. To test this, perhaps first try a much smaller antenna, maybe only a few inches of wire stub, and see if the multiplication of AM stations into other bands stops with a much weaker RF signal input.

    You can fix this kind of front-end overload problem with an AM band-stop or high-pass broadcast blocking filter, which will attenuate the power in the AM band RF spectrum before it overloads your SDR, but still allow signals from the higher bands into your SDR.

    Stong nearby FM broadcast stations (or any nearby high-power transmitter) can also cause this kind of problem, in which case an FM band-stop filter might be the solution.
     
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  3. VE7BPB

    VE7BPB Ham Member QRZ Page

    Q-branch is only for use below 24Mhz. Above 24Mhz you must disable it.

    You didn't mention what SDR software you are using. There are a lot of options but the most versatile ones are SDR Console, with it's ability to do multiple virtual receivers, and SDR# for it's multitude of useful plug-ins.

    Enjoy the dongle. You can do surprisingly well with them.

    regards, Roy
     
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  4. RETROLISTENER

    RETROLISTENER QRZ Member

    Hi all I have an ALINCO DX-R8E conected to a windows 10 PC using the ERW-7 USB cable I can use the clone software no problem but having difficulty using KGSDRE it does not fully function any one else having the same problem
     
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  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    This is a good one for Version 3.

    http://www.hdsdr.de/

    Direct sampling is not used for VHF or UHF. It is mainly for HF.
     
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  6. KN4SLJ

    KN4SLJ Ham Member QRZ Page

    Thanks for the advice! Seems you were right about MW and FM overloading the front end. I've ordered all the parts needed to build band reject filters for both bands. I'll probably build those into altoids cans or something. I'd like to put both filters in the same can, but I'm wondering if they would interfere with each other. Anyway.. I've been using SDRSharp but am open to trying others. I was curious what the direct sampling/quad sampling was actually for. I've seen manufacturers bragging about direct sampling and just mistakenly assumed that it was somehow overall "better." So thanks for clarifying that. Sounds like I just need to find some reading material. After playing with it some more, I'm actually impressed with how well it works. I've listened to a good bit of 10-meter SSB DX and shortwave broadcast from a wire antenna jammed into an so-239 connector. I would imagine that one could do worse. I'm looking into a wideband discone to replace the Tram 1480 vhf/uhf antenna up on the roof too, so I can't wait to see what comes from that.
     
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  7. G0GSR

    G0GSR Ham Member QRZ Page

    The RTL dongles contain a tuner chip (down-converter) intended for domestic VHF/UHF reception and have a lower frequency limit of about 24MHz.
    Later incarnations of the dongles allow the antenna input to be switched directly into the ADC, bypassing the tuner chip. This is direct sampling mode.

    The BIG drawback with this is that there is NO INPUT filtering so the ADC sees everything from DC to light.
    A set of switched bandpass filters would help the ADC cope but this brings the cost similar to the more expensive SDRs.
    E.g. This one.
    A tunable HF preselector might be a good option.

    Frank
     
    Last edited: Oct 28, 2020
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  8. N2HUN

    N2HUN Ham Member QRZ Page

    Try lowering the RF gain on the software you are using for your SDR dongle. I have the same RTL-SDR as you but I use Nooelec's HamItUp upconverter for Medium wave and HF in tandem with the RTL-SDR dongle. That works much better. Still, you need to adjust the RF gain from low to high depending on the band. You can also purchase filters for the FM and AM broadcast band that might help. Commonly used software is SDR# and HDSR. Here's an excellent site for more information:https://www.rtl-sdr.com/

    Enjoy your SDR dongle and keep the questions coming. I've used dongles for almost 10 years and they are lots of fun to use.
     
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  9. N4MU

    N4MU Ham Member QRZ Page

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  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    The version 3 is nice.

    But keep in mind that it requires a bit of power, That some USB ports can not provide.

    Some powered hubs can introduce noise. But a extra amount of power is required if you use the T voltage.

    I have 3 Version 3 models, And the performance is different between all of them, When put under a RF Lab inspection. But they all meet specs.
     
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