improving SDR reception with BANDPASS filter

Discussion in 'Software Defined Radio (SDR)' started by KN4TOU, Nov 11, 2020.

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  1. KN4TOU

    KN4TOU Ham Member QRZ Page

    This guy is using a RTL SDR in direct sampling but the reception is very poor, after he connects a BPF Amazing difference

    AI3V likes this.
  2. WA7F

    WA7F Premium Subscriber QRZ Page

    I think Bandpass filters helps any radio. A BCB filter is also a huge help for SDRs.
    KK4NSF and KB3UWC like this.
  3. W6UV

    W6UV Premium Subscriber QRZ Page

    Depends on where you live. I live far from any high-powered BCB or SW transmitters and don't have these issues with any of my SDRs, even ones without any kind of RF front-end filtering.
    N6YWU likes this.
  4. KK4NSF

    KK4NSF Ham Member QRZ Page

    I use a bandpass filter, and an LNA on a directional 5' square non resonant loop antenna system.... and my receive results are outstanding. Below is a pic of my receiver system while still under construction.

    The bandpass filter board has 16 channels from 1.5 to 30 mhz, and the LNA is ~12db. Both are old surplus parts....and I must say that those Gov't guys certainly do have some nice toys! The power supply is heavily filtered and shielded... that's why it's so big. The SDRplay is an RSP-DX, controlled by a Dell R710 which is also very RF quiet.

    So.... my opinion of Bandpass filters? For really good reception, they are a necessity. Of course some radios have good ones built into the front end. Others are not so good.... and adding one for each band you operate on will really help you.

    By the way... a cheap pre-amp is not only useless, but actually decreases your signal / noise ratio. BUT a real LNA is a whole different animal, and will really help you pull in weak signals when combined with a good bandpass filtering system.

    Last edited: Nov 13, 2020
    WA7F and NQ8J like this.
  5. N1IPU

    N1IPU Ham Member QRZ Page

    Having difficulty with lack of info on which LNAs to use or where to find them. Lot's of conflicting info out there.
  6. KK4NSF

    KK4NSF Ham Member QRZ Page

    Yes there is. I use old mil-surplu units, but these are hard to come by nowadays. I've heard a lot of good things about DX engineering's RPA-2 Receive Preamplifier, though. If I were looking for a new one, that's what I'd buy.

    A few notes:
    1- A LNA (or preamp) will boost all incoming signals, so a good bandpass filter is a necessity. Preferably, it should be a narrow one for just the band of interest.
    2- You'll need to use the OFF button quite often. While helpful on weak signals, it can also cause overloading from nearby strong signals.
    3- they seem to work best with directional antennas- loops, arrays, yagis and so forth. Omni directional antennas pick up noise from all directions which may be problematic
    4- to use an LNA / Preamp, you MUST have a T/R switch which grounds the RX while transmitting, and preferably a seperate RX antenna. If you don't, you will kill the LNA very quickly.
    N1IPU likes this.

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