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Where should I put my MW filter?

Hello, I recently bought an MW filter: Distill AM ( I've also own a RA0SMS Mini Whip.
When I installed the filter before the KiwiSDR RF input, I had too much attenuation and had to take it out.
Later I read the following item on the filter description: "As a true bandstop filter, you are able to pass-through DC (bias power) when it is required.".
Can I install the filter between the antena and the antenna feeder (bias tee)? Is there where it should go?



  • When I am in KH6, I cascade two of the Nooelec filters as my QTH is very close to a MW transmitter site. The filters are between the Kiwi & the bias tee/balun as my receive loop uses a LZ1AQ pre-amp ( ) which uses CAT5 cable for a feeder so I will never need the filter to pass DC.

    Here in VR2 I am about the same distance from a MW transmitter site, but as it is not so QRO, I only use one filter. Either way, attenuation >MW is not a problem.

    My multimeter has died, so I can't check if the filter indeed passes DC - I expect it should.

    I would not put the filter downstream of the bias tee as although the resistance from the series inductance of the filter should be insignificant, in general it would be better to reduce the number of things between the bias tee & what it is powering.

    Buena suerte.

    73, VR2BG.
  • You have placed the filter in the correct position.

    If the attenuation is too high on the MW band you could try adding a resistor between the center pins of the filter input and output connectors in order to by-pass a small amount of signal around the filter. Decreasing the value of the resistor will reduce the amount of in-band attenuation.


    100 Ohms will give you 6dB of rejection
    220 Ohms will give you 10dB of rejection
    1K Ohm will give you 20dB of rejection


    Martin - G8JNJ
  • Hello, thanks for the responses guys!

    I did remove the nooelec mw bandstop filter because the attenuation was so high that I couldn't hear HAM bands (S1 level after putting the filter on 40m). This is strange since I get the OV indicator when I don't use the filter, but the filter seems to be too much.

    How about adding a LNA after the filter?

    I'll try the resistor trick @G8JNJ suggested!

  • There shouldn't be a noticeable attenuation on 40m with that filter.
  • I agree that ham bands should not have been significantly affected if the particular filter you have is making its specification. Does everything appear normal with connectors and the filter itself? Does it measure continuity through the center pin (does it pass DC)? Perhaps there is something wrong with the one you have.
  • Is there so much MW energy that the amp gets upset when looking into the shunt of the filter?
  • I doubt it. I'd expect IMD but not likely to have zero output. It sounds like a connector or misloaded board problem.
  • Hello! Thanks for your help! I tested the filter, that wasn't the problem. The problem was a faulty SMA pigtail I was using (brand new). Now the everything works prefect! Thanks guys!

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