Looking for HF BPF for Receiver

Discussion in 'General Technical Questions and Answers' started by KK5JY, Jul 19, 2017.

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  1. KK5JY

    KK5JY Premium Subscriber QRZ Page

    I'm looking for a kit for an HF band-pass filter that can be put in front of an RTL-SDR running in direct-sampling mode. The filter would pass only HF bands to the receiver. Ideally, it would pass 160m through 10m, although that might be unrealistic for a single filter, so I am also interested in BPFs that can cover 80m through 10m filters, or LPFs that pass frequencies below 30MHz.

    Since this is only a receiver filter, the power handling of the components need not be large, as with a transmit filter.

    I found several kits for BPFs that cover only one band, but I was hoping to find a single filter that could do some amount of preselection when the RTL-SDR is running on the HF bands.

    I'm happy to build a kit, or from a schematic. My first thought was to use a few stages of RC filter, in LPF/HPF pairs. But if there is a better design floating around, I would like to try that first.

    Thanks & 73!
    AF6LJ likes this.
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    Pick up one of the Transmit LP Filters that in the past were almost required to keep down interference from the TX to the TV bands. They can be had used for $15-20. Most cutoff in the 30-40 MHz range. Example is the Drake TV-3300. Although designed to cut down second harmonics on Tx they also work on Rx.
    KK5JY likes this.
  3. VE7BPB

    VE7BPB Ham Member QRZ Page

    sheesh.... back in the CB days everybody had a filter to keep their evil signals out of the neighbors TV. They would work just fine in reverse, doncha' think?

    Must be thousands of them just lying around in peoples closets, heh.


  4. VK3YE

    VK3YE Ham Member QRZ Page

    CB LPFs pop up frequently on ebay and at hamfests. Eg http://www.ebay.com/itm/low-pass-filter-1000-watt-amd-smaller-one/382137573523 or a new one http://www.ebay.com/itm/LPF-300W-1-8-54MHz-low-pass-filter-1-pc-for-1-band/151916768969 Another option is this kit from SOTABeams http://www.sotabeams.co.uk/low-pass-filter-kit/ or Hans Summers http://shop.qrp-labs.com/ULPF

    You could try adding on something to attenuate frequencies below 1.5 MHz. Off the top of my head that requires inductors around 5 uH and capacitors around 1500 or 2200pF. I think there's websites that assist in working out the values online.

    The 'gold standard' is something that passes only the band of interest to prevent overload from extraneous frequencies. A preselector could be rigged up with a variable capacitor and toroid or other coil. That's single section - but a sharper response is possible with two. Hans Summers sell BPF kits either individually http://shop.qrp-labs.com/BPF or as a set http://shop.qrp-labs.com/BPFSET
  5. KP4SX

    KP4SX XML Subscriber QRZ Page

    For a receive filter its pretty easy to wire up something with a few toroids and caps.
    Look online for the little design program Optlowpass at www.tonnesoftware.com
    When I 'broadbanded' my 40m QRP rig I made up some plug-ins using these filters. Big caveat...use good quality capacitors and toroids of known quality. If you have an LC meter to verify values that's even mo' betta.

    In this photo, the filter on the right is LP for the xmtr. On the left is a simple peaking filter for the rcvr.

  6. VA3VF

    VA3VF Guest

    What service is causing you problems? AM, FM, LTE etc. In my case the problem was AM broadcasdting. I installed the RTL-SDR Blog filter, and the change was night and day. The only downside of this filter, if you decide to go with it, is that it blocks the 160M ham band as well. Similar filters to block FM Broadcasting are also available. I recently purchased an LTE filter, just in case.
    KK5JY likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A BPF is just a HPF and LPF connected together.

    If you build a simple 1.8 MHz HPF and a simple 30 MHz LPF, all designed for 50 Ohms I/O, and connect them in series (even in the same little box, same circuit board, etc) you have a BPF.

    The advantage to a "transmitting" type LPF like the Drakes etc. (zillions of them out there) is because they're designed to handle some power the inductors are large gauge, usually silver plated copper air-wound coils that introduce almost zero loss. Smaller parts will work fine but may introduce a little bit of loss -- not sure if you'd ever notice that except perhaps on 10m/12m where noise levels are low and signals are weak. On 20m and lower frequencies, even 6 dB loss is usually negligible.

    A 30 MHz LPF obviously won't do anything to reject strong AM BC signals.
    KK5JY likes this.
  8. KK5JY

    KK5JY Premium Subscriber QRZ Page

    That's the reason I'm shopping for a filter design -- just in case. Since the RTL-SDR doesn't have any HF filtration (that I can find) when in direct-sampling mode, I'm trying to tighten it up a bit. The switchable filters are fine, but I really just wanted to block out VHF+ and maybe the AM BCB.

    The SDRPlay has some decent filters in it, and I like that device, so I was hoping to add some minimal filtering to the RTL-SDR device, too, since my SDRPlays are headed to the swap-meet. ;)
  9. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Sounds like a plan to me. I guess I just need to surf the ARRL textbooks and work the calculator. I was just hoping for a schematic from somebody who had already built one that they knew worked well.
  10. VA3VF

    VA3VF Guest

    Unless you are sure FM, or other higher frequency services are the culprit, go with the AM BCB from RTL-SDR Blog. Best $15 you can spend. For another $15, get their FM filter as well.

    KK5JY likes this.

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